Human Impacts 2: Problems, Mitigation and Conservation PDF

CHAPTER 9 Human impacts 2: problems, mitigation and conservation Only a hundred years or so ago, it would have seemed inconceivable that anything we did could significantly affect the vast resources of the oceans, or result in large- scale alterations and damage to marine ecosystems. Now the findi...

CHAPTER 9 Human impacts 2: problems, mitigation and conservation Only a hundred years or so ago, it would have seemed inconceivable that anything we did could significantly affect the vast resources of the oceans, or result in large- scale alterations and damage to marine ecosystems. Now the findings of the ‘World Ocean Assessment I’ (http://www.un.org/regularprocess/content/first-world-ocean- assessment) indicate that the oceans’ carrying capacity is near or at its limit and that urgent action is needed on a global scale to protect the world’s oceans from the many pressures they face. We have already seen in Chapter 8, Human Impacts 1: Sea Fisheries and Aquaculture, that fishery resources are stretched to their limits. Modern technology has resulted in a huge increase in our ability to catch and store edible marine species. Many or most important fishery stocks are now being exploited at levels considered to be at or over the maximum sustainable level. Controlling fisheries is vital for the health of the oceans, but there are other major ways in which human activity is affecting our oceans. In this chapter, the direct effects of adding materials to the sea (pollution) and indirect effects on the oceans through human-induced climate change and global warming are described. Mitigation of these complex problems requires action at all levels from the individual to governments, including whole ocean management as well as protection in the form of marine protected areas (MPAs). 9.1 Marine pollution The term ‘marine pollution’ is really a convenient term that covers virtually any sub- stance released into the oceans by human activities and that has a deleterious effect on marine organisms and ecosystems, or is a nuisance to mankind. Effects of these materials are wide-ranging but include smothering, entanglement and poisoning of organisms, interference with behaviour and physiology and increase or decrease in biological pro- ductivity, with consequent effects on organisms in other parts of the food web. There is also increasing evidence that underwater noise should be considered as a form of pollu- tion. It is certainly having detrimental effects on a wide variety of marine organisms and not just the obvious ones such as cetaceans, seals and other marine mammals. The extent to which the oceans are being adversely affected by their use as a dumping ground for an ever-increasing quantity and variety of human and industrial Elements of Marine Ecology r 2022 Elsevier Ltd. DOI: https://doi.org/10.1016/B978-0-08-102826-1.00013-2 All rights reserved. 459 460 Elements of Marine Ecology wastes is a major cause for conservation concern. Whilst great strides have been made in some parts of the world, in reducing pollutant inputs such as oil spills and raw sew- age, new problems continually arise, of which plastics and micro-plastics are currently (2020) at the fore (see Section 9.1.6). Floating plastic pollution manifests itself at an ocean-wide scale, whilst other marine pollution problems may affect relatively local areas and coasts, with the worst effects often in confined areas such as estuaries and bays. Non-tidal seas such as the Mediterranean and the northern Baltic obviously pres- ent special problems, as do semi-confined areas such as the North Sea. The problem of plastic, however, is certainly a global, ocean-wide problem affecting offshore and inshore areas and marine life throughout the food chain. Degradation of marine habitats and direct effects on marine life from pollution can add considerable stress to ecosystems such as coral reefs. This can result in a lowered ability to withstand other stressors, such as increased water temperature resulting from global warming. A healthy, unpolluted coral reef has a much better chance of recover- ing from bleaching events (Section 9.2.5) than an already degraded system. In the long run marine pollution can only be effectively tackled through waste minimization and reduction of pollution at source. To this end, recycling of materials, innovative ways of reducing or using waste and the use of new and renewable, non- polluting energy sources are all key. Manufacturing products, with the end fate and disposal of the materials they are made from in mind, is another route to minimizing pollution, whether marine or terrestrial. The amount of literature now available on all aspects of marine pollution is large. This important aspect of marine ecology can only be reviewed briefly here and for more information readers should refer to references given in the text and books listed in Further Reading at the end of this chapter. 9.1.1 Capacity of the ‘ocean dump’ From the beginnings of civilization sewage and domestic rubbish have been disposed of mainly by dumping or spreading them over the ground or discharging them into rivers, lakes or the sea. Whilst human populations were at low levels, these simple methods of waste disposal have usually sufficed, because much of this mainly organic refuse is broken down by natural processes of decay. This was and still can be benefi- cial by returning nutrients and minerals to land and water thus maintaining fertility. However, growing towns and cities meant that the natural capacity of their streams and rivers to disperse, degrade and recycle the excrement and rubbish poured into them became overtaxed. Open sewers in the crowded cities of mediaeval Europe were a serious health hazard, causing massive outbreaks of disease. In 1858 Parliament in London had on occasion to be suspended due to the overpowering stench from the River Thames. In later historical times, the much improved health of urban  Human impacts 2: problems, mitigation and conservation 461 populations in developed countries has been attributable in considerable measure to cleaner towns with much safer methods of sewage and waste disposal. This is still not always the case in less economically developed countries. In contrast to land or freshwater, the enormous volume of the sea has theoretically a huge capacity for dealing with sewage and other organic refuse. To the extent that the ocean can accept wastes without detriment to marine and human life, then using it as a natural sink could be considered sensible. However, this has obviously been found to be far from the case. Assumptions that noxious substances that are not readily inactivated in the sea by natu- ral processes, soon become so diluted as to be quite harmless, have proved wrong, particu- larly in the case of persistent chemicals such as DDT and polychlorinated biphenols (PCBs) (Section 9.1.7). However, many coastal towns worldwide still pour untreated wastes directly into rivers and oceans. Industries which produce large volumes of waste have often developed along the coast because, in addition to the many other advantages of ready access to the sea, they obtained a simple and inexpensive outlet for their effluents. In the past it has sometimes seemed preferable to pollute the ocean rather than the land, for example by discharging poisonous wastes into the sea rather than into rubbish tips. However, over time it has become increasingly obvious that the capacity of the oceans to cope with our various inputs is being exceeded, with detrimental effects on both the ocean ecosystem and humans. Rapid growth of populations, together with industrialization and new technologies has and is producing wastes in much greater quantities and variety than ever before. Public health problems have arisen from raw sewage discharges into the sea (see Section 9.1.5), more land has been needed for rubbish tips and industrial spoil and more effluents are reaching the sea via rivers and estuaries. An additional problem is that some of the new materials produced in large amounts, notably many plastics, accumulate because they are extremely long-lasting, being little if at all subject to biological decay. Despite the great size of the oceans and their thorough intermixing, water circula- tion is mostly slow and dispersion of materials in the sea is sometimes a very gradual process. Consequently, where large amounts of wastes are discharged into shallow water, especially into enclosed areas like the Baltic, the North Sea and Mediterranean, concentrations are soon reached which present a variety of problems. Even if wastes are disposed of by tipping far out at sea, pollution may become obvious if persistent floating substances drift back to the coast, or if solids which sink to the bottom are later carried inshore by shifting of sediments along the sea floor. 9.1.2 Chief routes of entry of marine pollutants Marine pollutants find their way into the sea not only through deliberate routes such as sewage discharges and dumping (legal and illegal) but also by a variety of other, not always obvious, routes outlined below. 462 Elements of Marine Ecology Drainage Many pollutants reach the sea either through direct drainage from coastal towns and industries or indirectly via rivers. Dilute industrial effluents, treated sewage and cooling water are often discharged into rivers and estuaries. Fertilizers, pesticides and animal wastes may drain into rivers from agricultural land. Huge amounts of silt resulting from rainforest clearance are carried down to the sea by tropical rivers. Rainwater run- off from cities and towns carries oil, heavy metals and other material into rivers. Dumping Coastal towns and cities discharge raw or treated sewage into coastal waters. In differ- ent areas, tipping at sea is or was used to dispose of sewage sludge, industrial wastes, dredged and mined materials and general rubbish (the latter particularly in undevel- oped countries). Whilst controls are now in place, ships still dispose of at least some of their day-to-day wastes by dumping, legally and illegally oil tanker washings were once a particular problem and this sometimes still occurs illegally. The appearance in 1995 of dangerous phosphorus bombs on Scottish beaches has been attributed to poorly controlled military dumping after World War II. Airborne pollution Many airborne pollutants are dissolved by rain which may then fall over the sea. Others are carried into the sea as dust particles or by solution of volatile materials from the atmosphere. For example, huge amounts of smoke containing polycyclic aromatic hydrocarbons were created by burning oil wells in the 1991 Gulf War, which drifted for many miles over land and sea. Accidents A great variety of objects and substances finds their way into the sea through ship- wrecks and lost cargoes, from plastic ducks and shoes to potentially lethal (to humans and marine organisms) chemicals. Oil spills are another obvious concern. A large amount of unexploded ordnance remains in the American Liberty ship SS Montgomery which was wrecked in 1944 in the Thames Estuary, near the town of Sheerness in the UK. It remains a significant shipping hazard with the three masts of this ship clearly visible above water, a well-remembered landmark of the author’s childhood holidays. 9.1.3 Persistence of pollutants The long-term effects of waste disposal at sea must obviously depend upon the length of time taken for material to be broken down to a harmless form. The extent to which continuous addition of substances may be cumulative in effect will depend partly on their persistence in the water. Pollutants can be roughly classified as transient,  Human impacts 2: problems, mitigation and conservation 463 moderately persistent, very persistent and virtually permanent. Examples of the length of time that various litter items may persist are given in Table 9.4 in Section 9.1.6. These are maximum times which may be shortened by wave action and abrasion. Transient pollutants Sewage, some domestic rubbish and certain industrial organic wastes are rapidly biode- graded to inorganic form by marine bacteria within a few days, unless processes are retarded by oxygen deficiency in slow-mixing waters. Mineral acids are quickly neu- tralized by the large alkali reserve of seawater. Moderately persistent pollutants Oil and many organic industrial effluents, including solid wastes such as organic fibres and pulps, are only slowly degraded by natural processes, often taking many months for complete decomposition. The time taken for untreated oil pollution on rocky shores to disappear depends on the exposure of the shore to wave action. In most cases the majority of the visible oil will have gone within about 2 years, but thick tarry deposits can remain on the upper areas of sheltered shores for many years (IPIECA, 1995). Evidence of past coal mining activities on some shores in County Durham, UK can still (2020) be seen in the form of black sand patches consisting of tiny grains of coal, although most of the mines closed many decades ago. Very persistent pollutants Many artificially produced chemicals are highly stable in seawater, undergoing only very gradual natural degradation. These include some detergents, PCBs and some widely used pesticides including dichloro-diphenyl-trichloroethane (DDT), hexachlor- obenzene and Dieldrin (see Section 9.1.8). Virtually permanent pollutants Many plastic waste items, such as plastic bottles, will last indefinitely (see Table 9.4) either intact or in pieces. Glass bottles, if undisturbed by waves, could theoretically last a million years! Some radioactive isotopes are very long-lived pollutants. Addition to seawater of certain toxic metals may produce a long-lasting increase of their concen- tration in the sea. Even if the pollution ceases, the concentration of such metals may fall only very gradually, by dilution and by various processes of sedimentation and pre- cipitation. Solid inorganic residues such as mine spoil may become a permanent part of the sea bottom. 9.1.4 Regulations Regulation of deliberate dumping and pollution in the world’s oceans has been slow in coming and although there are now a number of international and many national 464 Elements of Marine Ecology regulations, enforcement and compliance remain major problems. Only a few exam- ples of important contemporary legislation can be given here. International regulations International regulations require agreement and ratification between countries and are rarely applicable on a truly worldwide basis. Nevertheless, considerable progress has been made in recent decades through the United Nations International Maritime Organization (IMO), which is responsible for the prevention of marine and atmo- spheric pollution by ships. The IMO has adopted 21 treaty instruments that are directly related to the marine environment. The important international convention known as MARPOL (International Convention for the Prevention of Pollution from Ships) was initially concerned with oil pollution, but now includes a much wider range of measures concerned with safeguarding the marine environment against pollu- tion from ships. Various annexes cover important pollution sources including chemi- cals, sewage, garbage, air pollution and emissions from ships (Table 9.1). Oil and litter both have very visual impacts and these have particularly aroused public awareness. The deliberate discharge of oily ballast waters and tank washings from ships is illegal under MARPOL, as is dumping any plastic items over the side of ships. European regulations Sewage pollution of beaches has become a major topic in Europe in recent years. There are now two main EC directives relating to sewage pollution: The Bathing Water Directive (BWD) (1975) and Revised Bathing Water Directive (2006): This aims to protect recreational users of bathing waters from health risks asso- ciated with sewage pollution. Bathing waters are designated by each EC member state and are tested annually. Whilst a good concept, there were initially considerable pro- blems with the standard tests used and confusion over various beach awards such as the EC Blue Flag award and the Seaside Awards. These awards can be given to bea- ches with widely differing water quality. In the UK, the Marine Conservation Society (MCS) produces an annual Good Beach Guide, an independent source of information Table 9.1 The six implemented MARPOL annexes. Annex number Date entered into force Ship pollution type I 1983 Oil II 1987 Noxious liquid substances in bulk III 1992 Harmful substances carried by sea in packaged form IV 2003 Sewage V 1998 Garbage VI 2005 Air pollution IMO 2020 2020 Emission standards for fuel oil  Human impacts 2: problems, mitigation and conservation 465 about the state of Britain’s beaches. This was initially produced in book form and can now be found online (http://www.thebeachguide.co.uk). The BWD requires mem- ber states to monitor and assess their bathing waters for a minimum of two parameters of faecal bacteria. The EC Urban Waste Water Directive (1991): The aim of this EU directive is to pro- tect the environment from the effects of urban wastewater. It lays down minimum levels of treatment required for different sizes of settlements and takes into account where the settlements are. Coastal and other settlements over 10,000 people dischar- ging into defined ‘sensitive’ areas have more stringent requirements, such as tertiary treatment (removal of nitrates and phosphates). The UK implementation of this direc- tive is described in DEFRA (2002) which also explains treatment processes and the levels of treatment required for different discharge areas (such as coastal waters). The UK phased out sewage sludge dumping at sea in the 1990s. 9.1.5 Sewage In spite of directives such as those described in Section 9.1.4 beaches in the UK and the EU are still periodically contaminated with sewage-related debris (SRD), including wet wipes and vaginal tampons. Much of this material reaches beaches when treatment plants are bypassed (both with and without permits) when material is discharged via combined sewer overflows into rivers or directly into the sea. Some sewerage systems collect both sewage and storm water and if the volume of water collected is too high due to heavy rain, then discharges of untreated and unfiltered waste may be permitted (Box 9.1). Sewage-borne infections such as typhoid, viral hepatitis, enteric infections and ear, nose and throat infections have been associated with exposure to sewage-polluted sea- water. Infection may be transmitted to humans directly through contact with water or spray, or indirectly by consumption of marine foods. In the UK there have been sev- eral cases where surfers have suffered severe illness and paralysis thought (but not con- clusively proved) to be due to infection from faecal viruses. This was one of the BOX 9.1 Wet wipes and fatbergs. Beaches polluted with SRD often result from plastic-containing personal products incorrectly disposed of down the lavatory. Wet wipes are a prime example that have become a problem on many beaches in the past decade or so. According to the UK Marine Conservation Society, the number of wet wipes found on UK beaches increased fourfold between about 2006 and 2016. Although described as ‘flushable’ and looking like paper, the majority are not biodegradable and are made of polyester and polypropylene. When mixed with fatty material in sewers, so-called ‘fatbergs’ can result which cause numerous blockages in the sewerage system. Wet wipes on the beach mostly get there via combined sewer overflows (see Section 9.1.5). 466 Elements of Marine Ecology drivers behind past UK public campaigns such as ‘Surfers Against Sewage’. It also spurred the development of the MSC Good Beach Guide (http://www.mcsuk.org/ nearyou) listing and rating swimming beaches for their cleanliness. The increased bacterial content of sewage-polluted water can favour filter feeders and can result in good growth of bivalves such as cockles and mussels. Shellfish from such areas would require thorough purification before being eaten. Some viruses sur- vive longer than bacteria in seawater and may remain in shellfish even after they are bacteriologically clean. In the UK certain areas are designated as Shellfish Water Protected Areas under the Water Framework Directive, where environmental objec- tives particularly concentrate on maintaining clean water. Sewage treatment Sewage enters the sea via short and long sea outfalls, stormwater drains and rivers. Although sewage is treated before discharge in the UK, raw sewage still gets into coastal waters through emergency storm water drains, as mentioned above. This is per- mitted on a restricted basis, but increasing rainfall and storm incidences mean that releases are increasing in frequency and often exceed permitted levels. Throughout Europe (and in most developed countries), increasing efforts are being made to treat sewage effectively before discharge. This clearly reduces the impact of the sewage both on the marine environment and on coastal amenities. Sewage pollution of coastal and estuarine waters is usually most severe during the summer when temperatures are raised, river outflows are reduced and seaside populations often increased. Untreated domestic sewage consists mainly of waste water and solids from toilets, sinks and drains, which includes detergents (often containing phosphorus), other che- micals and sanitary products in fact, anything flushed down lavatories, correctly or incorrectly. Untreated industrial waste can also enter sewers through illegal discharges and accidents, along with oil and run-off from road systems. Raw sewage discharged into the sea may therefore contain large quantities of plastics, metals such as arsenic, cadmium, copper, mercury and lead, as well as organic matter, petroleum products, fats, solvents and dyes. Thus there is considerable potential for human health risk and for ecological damage when untreated sewage is discharged into the sea. Prior to about 1990 a large proportion of discharges from coastal towns and cities in the UK received no treatment or were just screened. The legacy of this in terms of persistent contami- nants remains. Sewage treatments vary between countries, but in the UK there are four main levels of sewage treatment (DEFRA, 2002), the levels required depending on where the waste will be discharged: Preliminary large solids are screened out and grit removed; Primary settlement of suspended solids;  Human impacts 2: problems, mitigation and conservation 467 Secondary biological treatment through bacterial breakdown in filter beds and activated sludge processes; this can reduce the effluent’s biological oxygen demand (BOD) by as much as 99%; Tertiary treatment various additional treatments such as nitrate and phosphate reduction, and disinfection by UV light. Eutrophication Nutrients are essential to fuel growth of phytoplankton and algae in the ocean. However, too many nutrients, particularly nitrates, added to the water through direct sewage discharges, river discharges (inland treated sewage water is discharged into rivers) and agricultural run-off can cause problems of eutrophication (Box 9.2). This is where excessive nutrients cause algal blooms and often anoxic conditions and can occur in both fresh and marine waters. Enclosed areas with restricted tidal circulation such as bays, river estuaries and sea lochs are often worst affected. When algal blooms die off and the material sinks, there is an accumulation of organic debris on the seabed. This allows bacteria to multiply and in doing so they deplete the available oxygen supplies through respiration. This results in anoxic conditions at the sedi- ment water interface. It is often indicated by the presence of species such as the poly- chaete worm Capitella capitata, which can survive and do well in polluted conditions. Rising coastal nutrient input can contribute to ‘dead zones’ or ‘dead areas’, particularly in pockets of deepwater in enclosed seas, where nothing can live except anaerobic microor- ganisms (see Oxygen in Section 2.2.3). ‘Red tides’, (see Section 4.2.6) are blooms of toxic phytoplankton that are also associated with excess nutrients, often supplied by sewage. These can poison fish and shellfish and may indirectly cause disease in humans. BOX 9.2 Starfish plagues and nutrient input. Crown-of-thorns starfish (Acanthaster planci) or COTs as they are popularly known are a major predator of corals, which under favourable conditions (for them), can reach plague proportions. On the Australian Great Barrier Reef (GBR) such plagues cause significant coral loss. There are many underlying reasons that may cause starfish numbers to increase dra- matically, including overfishing (removes their predators). However, research by the Australian Institute of Marine Science indicates that land runoff may be another underlying reason. Levels of phytoplankton on coral reefs are naturally low, but inputs of nutrients can cause rapid increases in their levels. The starfish larvae feed on phytoplankton and their sur- vival is greatly enhanced when their food supply is increased. Storm and monsoonal events cause land runoff and can carry fertilisers and grazing animal faecal material into the GBR lagoon and onto reefs. Dense breeding populations of COTs can then develop and once established, larval numbers can rise sufficiently that ocean currents may carry enough larvae to establish outbreaks on reefs not directly affected by runoff. 468 Elements of Marine Ecology 9.1.6 Plastic and other litter During the British Steel Round the World Yacht Race in 1992 93, the crews took part in a project called ‘Ocean Vigil’. Part of this project involved recording floating rubbish. The crews reported oil slicks, plastic containers and bags, fishing nets, wood, oil drums, shoes and a freezer! Rubbish was present even in the remotest parts of the Southern Ocean. Nearly three decades later the ocean litter situation remains severe, in spite of international legislation such as MARPOL (see Section 9.1.4) aimed at control- ling inputs from shipping. Ocean litter has many origins and obviously not all of it comes from the world’s shipping fleet. However, a significant amount of it does, mainly through illegal dumping and accidental losses. Waste reception facilities at ports around the world vary considerably and inadequate or expensive facilities are an incentive for ships not to follow the rules. There are currently (2020) around 53,000 ships in the global merchant fleet and the global fishing fleet in 2015 was estimated at 3.7 million (Rousseau et al., 2019). Other shipping includes recreational boating and cruising. Looked at in total, the majority of marine litter actually comes from the land, via sewage storm water overflows, the public (litter left on beaches, washed into water- ways and blown in from streets), lost or discarded fishing gear and other unidentified sources (Table 9.2). In some coastal areas, badly sited and now disused landfills dis- gorge a constant stream of rubbish as the sea erodes the coastline. In developing coun- tries rubbish disposal facilities may be poor or non-existent, leading to huge amounts entering rivers and estuaries, in the case of plastic sometimes to the extent that local fishing is impeded. Obtaining accurate estimates of how much litter waste enters the ocean each year is difficult, but for plastic items alone the figure is thought to be around 8 million tonnes. Much of this eventually gets washed up on the seashore and plastic makes up by far the largest percentage. Even in the mid-1990s itemized counts of beach litter made in the UK during the MCS annual ‘Beachwatch’ indicated around 60% was Table 9.2 Sources and percentages of beach litter recorded from UK beaches (average number of items counted per 100 m) during the Marine Conservation Society (MCS) annual beach cleans. Sources 1996 (%) 2018 (%) 2019 (%) Non-sourced (eg, too small to identify) 33.5 48.7 44.6 Litter from public (eg, drink bottles) 22.1 28.6 30.4 Fishing (eg, nets) 12.5 12.1 14.7 Sewage-related debris (eg, sanitary) 13.5 6.2 5.9 Shipping (dumped or lost overboard) 17.4 3.2 3.3 Fly-tipped, eg, TVs 1 1 1 Medical, eg, syringes 0.1 0.2 0.2 Litter from public is that left on beach, blown in or carried in by waterways. Data for 2018 and 2019 taken from Great British Beach Clean online reports and for 1996 from the equivalent print report, Beachwatch 1996.  Human impacts 2: problems, mitigation and conservation 469 plastic and polystyrene. Today that figure is thought to be far higher, some estimates suggest 80%. Plastic litter includes everything from toys through to drinks bottles, plas- tic bags and fishing nets and ropes. Most of the litter categories listed in Table 9.3 con- tain or are predominantly plastic. Most plastic degrades only very slowly and even if all plastic was prevented from entering the ocean now, it would take many, many years for the situation to improve (see Table 9.4). A beach covered in plastic is very unpleasant for us, but is often fatal to marine life. Animals can be poisoned or starve after eating plastic, or they can become entangled and trapped. Classic examples include air-breathing turtles, seals and Table 9.3 Average number of items recorded per 100 m stretch of shore during the 2019 Marine Conservation Society Great British Beach Clean. Litter category Average no. items/100 m Plastic/polystyrene (0 50 cm) 143.0 Cigarette butts 42.6 Glass (other) 33.4 String/cord (thickness 0 1 cm) 32.6 Packets (crisps, sweet, lolly, sandwich) 30.9 Fishing net (small) 21.3 Caps/lids 20.4 Wet wipes 19.2 Fishing line 18.8 Plastic/polystyrene (other) 16.0 Source: Data from https://www.mcsuk.org/media/mcs-gbbc-2019-report-digital.pdf. Table 9.4 Approximate probable timescales (maximums) for the breakdown of litter in the marine environment to an unrecognisable form. Litter item Persistence (years) Glass bottle Indefinitely (undisturbed) Plastic bottle Indefinitely Aluminium cans and ring pulls 80 100 Tin cans 50 Leather, eg, shoes 50 Nylon material 30 40 Plastic bags 10 20 Plasticized paper 5 Wool garments 1 5 Cigarette butts 1 5 Orange peel and banana skins 2 It must be remembered that plastic may break down into ultimately microscopic pieces that remain in the environment (see Microplastics in Section 9.1.6). 470 Elements of Marine Ecology BOX 9.3 The Great Global Nurdle Hunt. Beaches all over the world are contaminated by plastic nurdles to a greater or lesser extent. The Great Global Nurdle Hunt uses volunteers to collect and count nurdles using a standard methodology. It was established in 2012 by Fidra, a Scottish environmental charity, to docu- ment nurdle pollution in the UK and provide scientific data on the extent of the problem. In 2017 one beach in Cornwall, UK was found to have 127,500 pellets on a single 100 m stretch of beach. In 2019 volunteers in 50 countries took part. The data are intended to inform and guide governments and decision-makers towards ways to eliminate this problem. cetaceans which are drowned by floating fishing net and plastic sheeting. Turtles, espe- cially the Leatherback (Dermochelys coriacea), eat plastic bags mistaking them for the jel- lyfish that they normally devour. The bags make them feel full so that they stop feeding properly. In addition, chemical plasticizers may poison them. Even birds and fish may be poisoned in this way. In the English Channel, dead cod have been found killed by ingesting plastic cups thrown overboard from cross-Channel ferries. Tiny floating plastic pellets known as nurdles are mistaken for fish eggs or other plankton and eaten by seabirds and fish. The nurdles are the raw material for the manufacture of a wide range of plastic items and are produced and transported all over the world. They enter the marine environment through spillage into rivers or directly into the ocean through lost and damaged cargoes (Box 9.3). Microplastics Whilst nurdles are visible to the naked eye, a microscopic but an even more insidious threat comes from much smaller plastic particles and fibres that have now been found floating almost everywhere in the ocean, including sea ice in the Arctic. Continuous Plankton Recorder (see Section 4.6.2) survey work in the northeast Atlantic has shown that microplastic abundance has increased significantly since the 1960s. These microscopic plastics are partly derived from the breakdown of larger plastic items, including nurdles, into smaller and smaller pieces. However, a major input comes via rivers and estuaries into which treated (and untreated) sewage is discharged. This water contains immense numbers of plastic microfibers and microbeads, which, at sizes mea- sured in micrometres (µm), are invisible to the naked eye. The main source of micro- plastic fibres in these discharges comes from water used for washing clothes. Most clothing sheds fibres, but whilst those from natural materials such as wool and cotton are ultimately broken down completely, fibres from polyester and other synthetic materials (basically plastic) persist on a much longer timescale. Some research also sug- gests that another major input is of fibres shed whilst simply wearing clothes and ulti- mately carried to the ocean via wind. Microbeads are solid microplastic ingredients used in the manufacture of innumerable household products, particularly cosmetics.  Human impacts 2: problems, mitigation and conservation 471 Since cosmetics are often washed off after use, the insoluble microbeads end up in the sewage system. Microplastic particles are also thought to enter the ocean indirectly from other sources such as wear particles from car tyres (Knight et al., 2020). There is currently (2020) considerable ongoing research into the occurrence, sources and effects of microplastics on the marine environment. They have now been found throughout the ocean food web, from plankton right through to humans. The persistence up the food chain and effects at various levels are still being researched. What is known is that microfibers (whether from natural or plastic-based materials) are ingested by key planktonic organisms such as copepods when feeding. This can directly affect their feeding and reproduction. Experimental work also suggests that marine systems may be indirectly and widely affected through mechanisms such as the slower than normal descent to the seabed, of faecal pellets containing microplastics (Coppock et al., 2019). The effect of microplastics is an emerging field which will undoubtedly be the subject of continuing research and reviews. A small step towards the containment of the problem was made by the UK government in 2018 when it banned the sale of products containing microbeads, such as rinse-off cosmetics and personal care products. Artificial reefs Some types of rubbish such as tin cans and glass jars can be beneficial to marine life in some ways. Divers often report blennies and other small fish living in such items and sessile marine life soon settles and grows on solid items. Parts of the sea floor in the vicinity of large ports are littered with clinker and ash dating from the time when steamships were powered by coal-burning furnaces. Clinker and many other objects on the seabed can encourage the development of a larger and more diverse population by providing a range of cavities and holes offering protection and concealment. Artificial hard substrata such as offshore wind farms, lighthouses, pontoons and oil plat- form legs, can increase biodiversity by providing a foothold for algae and sessile animal species, particularly in otherwise purely sediment areas. Purpose-built artificial reefs have been constructed in many parts of the world using a variety of materials, including natural rock, cinder and clinker blocks and even old car tyres (Collins and Jensen, 1996) (Fig. 9.1). Whilst some of the early designs, such as car tyres, were not very successful, artificial reefs constructed of long-lasting and non-polluting materials, such as steel and limestone can enhance depleted envir- onments. The aim is often to encourage a greater variety and abundance of fish life for conservation purposes or the benefit of local fishermen. Shipwrecks are well known for their ability to act as a focus for marine life and new wrecks soon become colo- nized. Some fish seem to be directly attracted to large objects such as wrecks and will move in well before the wreck can provide anything other than shelter. Groupers have been observed to move into a wreck in the Arabian Gulf only days after it sank 472 Elements of Marine Ecology (A) (B) (C) Figure 9.1 Examples of purpose-made artificial reef modules. and many miles from the nearest rocky area or reef (Dipper, 1991). De-commissioned ships and even aircraft can be cleaned of contaminants and deliberately sunk, often with the dual aims of studying their colonization by marine life and as attractions for tourists, scuba divers and recreational fishermen (Hiscock et al., 2010) (see also Box 9.4). The question of dumping de-commissioned oil platforms and installations at sea became topical in the UK in the late 1990s, as many platforms reached the end of their useful life. This came to the fore in 1996 with the de-commissioning of the giant Brent Spa platform. Whilst some scientists argued that deep ocean dumping was an ecologically sound option, others disagreed and public opinion was so strongly against it that disposal on land was finally agreed. The deep ocean is vast and, in relative terms, sparsely populated by marine organisms. However, water exchange in the depths is extremely slow as are rates of breakdown and decomposition. Deep-sea com- munities may be easily disrupted since reproductive rates are often very slow. Even if the top structures are removed, the bases, containing crude oil and other toxins, remain as do arguments over the environmental benefits of their removal or non- removal. Waste disposal in the deep ocean is discussed by Angel (1996). 9.1.7 Pesticides Pesticides and herbicides enter the sea from agricultural runoff, rivers and by airborne transfer (particularly DDT). In estuaries the quantities of pesticide in the water tend to  Human impacts 2: problems, mitigation and conservation 473 BOX 9.4 Ecoengineering. Biodiversity loss (see Section 9.5) does not just refer to species but also to habitats and habi- tat loss itself leads to species loss. When artificial coastlines such as harbours, flood defences, coastal roads and tourist developments are built, there is inevitably a loss in the diversity of intertidal habitats and micro-habitats. In the past such structures were very often built with little regard for the environment. However, such artificial coastline structures can be designed to mitigate this loss through effective ecoengineering. A small-scale, simple exam- ple is the addition of artificial rock pools to vertical walls, to mitigate the effect of losing the complex three-dimensional habitat provided by natural rock pools and crevices. Whilst bar- nacles and seaweeds may be able to settle and grow on vertical walls, little else can. Artificial rockpools installed at different levels on a vertical wall will be exposed to air one by one as the tide goes out, mimicking the situation on a sloping rocky shore. In the UK various recent designs have proved successful in increasing biodiversity, either when incorporated at the design stage or retro-fitted (Hall et al., 2019). The design and success of ecologically engineered shorelines are discussed by Morris et al. (2019), who describe three approaches: hard, soft and hybrid. Hard ecoengineering attempts to compensate for ecological losses resulting from structures such as sea walls, as with the artificial rock pools just described. Soft ecoengineering aims to restore or create natural sea defences such as saltmarshes and mudflats (see Section 6.8, Figure 6.21) whilst hybrid solutions attempt to combine the two approaches. vary seasonally according to local agricultural practice. Around the British Isles there are often two peaks, one in early summer following springtime dressings on crops and orchards and a second peak in late autumn following the use of pesticides on autumn- sown wheat. Dichloro-diphenyl-trichloroethane When it was first developed in the 1940s, DDT was heralded as a wonder chemical capable of killing malarial mosquitoes and many other pests and its use certainly saved many human lives. It was only much later that the environmental consequences, both on land and in the ocean, were realized. Chlorinated hydrocarbons such as DDT and PCBs have become widely dispersed throughout the oceans, even to the Arctic and Antarctic. The main problem is that these chemicals only degrade very slowly and remain in the environment, which makes them readily available for biomagnification in food chains. Phytoplankton take up DDT because the large hydrocarbon molecules are not very soluble in water, but are very soluble in fats such as the oils in diatom cells. Zooplankton such as copepods eat the phytoplankton with its burden of DDT and do not break down or excrete it. Small fish eating the copepods gain yet more DDT and so on right up to mammals, birds and the human population. The effects of DDT 474 Elements of Marine Ecology pollution are varied. Some marine organisms, especially crustaceans, are extremely sen- sitive to both organochlorine and organophosphorus compounds and are killed by very small concentrations. In others, reproductive success is affected. Dramatic repro- ductive failures in fish-eating seabirds such as pelicans, cormorants, terns and fish eagles and in land raptors such as ospreys in the early 1970s, were finally attributed to a thin- ning of their egg shells and consequent breakage, caused by DDT. Some animals, including seagulls, have a high resistance to DDT and were not affected. The use of DDT was banned outright in many countries in 1972 and restrictions were placed on its use in others. However, DDT (and other persistent pesticides) are still (2020) man- ufactured and used in some countries, particularly India. Tributyltin Tributyltin (TBT) is a very effective organotin biocide that was developed and widely used as an anti-fouling paint on boats and ships from the 1960s to the 1990s. By then it had become apparent that the use of such chemicals was causing significant problems in the marine environment, particularly for molluscs. TBT causes various deformities and had such a bad effect on oyster farming in the UK that the industry practically collapsed. In dog whelks (Nucella lapillus), it causes a condition known as imposex, where females develop male sexual characteristics and breeding is impaired. Dog whelks live for 5 6 years and spend all their lives on the same stretch of shore. They are therefore very good indicators of TBT pollution and were used as such when the case for banning TBT was being investigated. The chemical enters the ocean as the paint ages and can also enter the marine environment when ships’ hulls are stripped and re-painted. Other effective anti-fouling chemicals have since been developed, but any such biocides have the potential to cause further problems and require extensive testing. 9.1.8 Toxic (heavy) metals Many heavy metals such as arsenic, lead, cadmium, mercury and copper are naturally present in seawater at very low concentrations reflecting their low solubility. Various organisms need some of these in very small amounts, for normal metabolism (see Iron and trace metals in Section 2.2.2). However, increased concentrations resulting from pol- lution may be harmful both to marine organisms and to humans. The concentration of heavy metals in the water may be raised locally by discharges from many industrial pro- cesses and in sediments they may become very high. Where it still occurs, sewage sludge dumping provides a significant input. Metals may also be released into the water from sediments disturbed by dredging, or by changes in pH or redox potential. Shellfish such as oysters, mussels and clams bioaccumulate metals, but do not seem themselves to be greatly affected. In contrast, most fish and crustaceans excrete any metals they take in with their food with the exception of mercury and cadmium. Levels of these latter two metals in top predatory fish, such as tuna, may exceed levels considered safe for  Human impacts 2: problems, mitigation and conservation 475 human consumption if the fish are eaten in large quantities. High levels have also been found in dead killer whales washed ashore and may have contributed to their deaths. Mercury is present in the effluents from several industries, for example those involved in the manufacture of chlorine, acetaldehyde, caustic soda and paper. It is contained in many agricultural fungicides and some are released by the burning of fos- sil fuels. Mercury poisoning was the cause of the most serious human catastrophe yet arising from marine pollution, an outbreak in 1953 at Minamata in Japan, of what was at first thought to be a mysterious disease. Later the problem was traced to the con- sumption of local-caught fish and shellfish contaminated by mercuric wastes dumped into the bay by a nearby chemical factory. Two hundred and thirty people subse- quently died and a thousand others suffered permanent cerebral and nerve damage from this source. A second outbreak killing five people and affecting 30 others occurred in Japan in 1965 near the mouth of the river Agano from a similar cause. These tragic events illustrate the folly of assuming that discharges into coastal waters are safely diluted and dispersed. Metallic mercury is virtually insoluble in water and very little is absorbed into tissues except in those potentially exposed to long and con- tinuous contact, such as dentists. However, in water, metallic mercury is acted upon by micro-organisms which gradually convert it into a variety of organic compounds, notably to methyl-mercury compounds, which are soluble and highly toxic and read- ily absorbed and concentrated by organisms. Arsenic and other heavy metals were present in some detergents and arsenic was widely used in pesticides and herbicides until at least the 1960s. In seawater it exists mainly as arsenate but a proportion becomes converted to the more highly toxic arse- nite. Arsenic compounds are readily concentrated in the tissues of certain marine fish. Lead is often present in the effluents from mine workings in metalliferous areas and becomes concentrated in the tissues of some marine species. Other metals which may also be present in marine sediments near river mouths carrying mine-washings include cadmium, chromium, nickel, copper and zinc, all of which have been found in high concentrations in various worms and molluscs from these areas (Clark, 2001). Lead also enters the sea from the air due to atmospheric lead pollution from the exhaust fumes of vehicles. The use of lead additives (tetraethyl lead) in fuel was banned in Europe and North America in 2000 due to the realization that it was causing great damage to people’s health but is still used to some extent in the aviation industry. Many marine organisms concentrate heavy metals and it appears that this increases their tolerance to even greater concentrations. Certain metals, such as copper, are essential for normal enzyme activity but may become enzyme inhibitors at high con- centrations. Except for the Minamata tragedy there is not much evidence of human detriment from metal contamination of marine foods. Some fatalities have been reported where chromium and cadmium contamination of shellfish were implicated. There is also the danger that several metals may act additively or synergistically. 476 Elements of Marine Ecology 9.1.9 Oil The topic of oil pollution in the ocean is a huge one with a long and continuing his- tory. In 2010 the unthinkable happened and the offshore drilling rig Deepwater Horizon exploded. As well as the tragic loss of life, the loss of oil from the well into the Gulf of Mexico was estimated at nearly 5 million barrels. Of all the many types of pollution to which the oceans are subject, oil is perhaps the one to have caught the public’s attention the most. It first became apparent to the public as beaches became oiled when ships sank during World War II. Large-scale spills result in oiled beaches and kill huge numbers of marine animals especially seabirds. Sources of oil pollution In most decades since the war, there have been dramatic instances of gross oil fouling of long stretches of coastline from tanker accidents notably the wreck of the Torrey Canyon in 1967 affecting beaches in Cornwall and Brittany; the Amoco Cadiz in 1978 fouling the north Brittany coast; the Exxon Valdez in 1989 affecting beaches in Prince William Sound, Alaska; and the Sea Empress in 1996 which oiled shores in Pembrokeshire, UK including Skomer Marine Nature Reserve (MNR). These events have been widely publicised and attention is drawn not only to the damage done to amenities but also to the destruction of tens of thousands of seabirds and other marine life and possible devastation of local fisheries, fish nursery areas and shellfish beds. However, although such accidents are dramatic, considerable amounts of oil enter the marine environment from other less publicised sources (Table 9.5). The figures in such estimates must of necessity be very approximate, but it can clearly be seen that pollution from domestic and industrial discharges easily exceeds that from tanker acci- dents. Although these figures were compiled several decades ago, this is likely to still Table 9.5 Sources of inputs of petroleum hydrocarbons into the world’s oceans and estimates of yearly inputs. Source Range of estimates (thousands of tonnes) Urban run-off and discharges 2500 1080 Operational discharges from tankers 1080 600 Tanker accidents 400 300 Non-tanker accidents 750 200 Atmospheric deposition 600 300 Natural seeps 600 200 Coastal refineries 200 60 Other coastal effluents 150 50 Offshore production losses 80 50 Source: Data taken from various sources between 1973 and 1981 based on GESAMP, 1993. Impact of oil and related chemicals and wastes on the marine environment (GESAMP, 1993).  Human impacts 2: problems, mitigation and conservation 477 be the case since increased safety measures, such as double-skinned tankers, have sub- sequently been introduced. However, large-scale accidents involving oil rigs and struc- tures other than tankers can skew such figures year to year. Impacts of oil spills A great deal of research has been done on the effects of oil pollution on the marine environment and this is a major topic in most marine pollution books. After every major oil spill, research papers are published on the effects. Only a few major aspects can be dealt with here. Apart from contamination of shores, which if severe can lead to the destruction of much of the intertidal population, the major threat from floating oil pollution is to seabirds and mammals. Oil readily penetrates and mats the plumage of seabirds, mak- ing flight impossible and leading to loss of buoyancy and heat insulation. Attempts to preen off the oil lead to ingestion and gut irritation. Even with modern prevention and cleanup measures, many hundreds of thousands of seabirds are destroyed annually by oil fouling. The species at greatest risk are those which live mainly on and in the water, for example puffins, guillemots, razorbills, shags, cormorants, ducks and divers. Seals and sealions may also suffer as oil fouling of their fur reduces heat insulation, though a thick layer of blubber mitigates this. Oil in their eyes can cause irritation or blindness. The impact of oil on marine organisms depends on characteristics of the oil spill, such as its toxicity and viscosity, the amount of oil and the time for which it is in con- tact with the organism. Marine organisms and different life stages of organisms also vary in their sensitivity to oil (see Table 9.6). For example, on seashores, brown seaweeds are protected by their slimy covering of mucilage, such that the oil washes off relatively eas- ily. Barnacles and sea anemones can also survive covered in oil for several days. Grazers such as sea snails and limpets are much more susceptible. Eggs and larval stages of fish, crustaceans and some other groups tend to be more susceptible than adults. Toxicity: Crude oils and oil products differ widely in their chemical composition and therefore in their toxicity. The direct toxicity of oil to organisms is attributable mainly to the lighter aromatic components. These fractions usually evapo- rate fairly quickly, so the longer oil stays at sea before reaching the shore, the less toxic it becomes to intertidal organisms. The heavier fraction which remains after weather- ing appears to be less directly toxic and in general, well-weathered crude oil has fewer effects when washed ashore. The greatest toxic effects in the field have been caused by spills of light oil, especially when these have been in a confined area. However, if weathered oil comes ashore in great quantity, intertidal populations may be killed by smothering with clinging, tarry material. In addition to its direct lethal effects, oil may cause death by inducing a state of narcosis in which animals become dislodged from their substrata. Though some may recover and re-establish themselves, others succumb 478 Elements of Marine Ecology Table 9.6 Sensitivity of marine animals, plants and algae to oil pollution. Group Comments Mammals Whales, dolphins, seals and sealions have rarely been significantly affected in large numbers. Sea otters are more vulnerable because of their way of life and fur structure. Birds Birds using the air water interface are at risk, particularly auks and divers. Badly oiled birds usually die. Fish Eggs and larvae in shallow bays may suffer heavy mortalities under slicks, particularly if dispersants are used. Large kills of adult fish are rare. Adult fish in farm pens may be killed or stressed such that they succumb to disease. Invertebrates Invertebrates including molluscs, crustaceans, worms of various kinds, sea urchins and corals may suffer heavy casualties if coated with fresh crude oil. On the shore, barnacles are more resistant than limpets and snails such as winkles. Planktonic Serious effects on plankton in the open sea are unusual. This is probably organisms because high reproductive rates and immigration from outside the affected area counteract short-term reductions in numbers caused by the oil. Larger algae Large algae such as kelps and brown wracks have a mucilaginous coating which often prevents oil from sticking to them. Oil sticks better to dry algae such as those high on the shore and these may be broken by waves due to the weight of oil. Marsh plants Some species of plants are more susceptible than others. Perennials with robust underground stems and rootstocks tend to be more resistant than annuals and shallow-rooted plants. If, however, perennials such as the grass Spartina are killed, the first plants to recolonize the area are likely to be annuals such as glasswort (Salicornia). This is because such annuals produce large numbers of tidally dispersed seeds. Mangroves The term ‘mangrove’ applies to several species of tree and bush. They have a variety of forms of aerial ‘breathing’ root which adapts them for living in fine, poorly oxygenated mud. They are very sensitive to oil, partly because oil films on the breathing roots inhibit the supply of oxygen to the underground root systems. Source: Modified from IPIECA (1991). through being washed into the strandline where they cannot survive. In the past, many shore animals were killed as a result of cleaning operations that used detergents sprayed onto the oil to emulsify and disperse it. The present generation of detergents are much less harmful when used properly and in appropriate circumstances. Tainting of seafoods such as fish, shellfish and crustaceans may be caused by absorp- tion of hydrocarbons into the tissues. This imparts an unpleasant oily taste and there is concern over the dangers to humans from eating polluted seafood since some oil- derived hydrocarbons may be carcinogenic (IPIECA, 1997).  Human impacts 2: problems, mitigation and conservation 479 Fate of spilt oil: When oil is first spilt, there is an initial evaporation of the lighter fractions. In the Amoco Cadiz incident, the spilt oil was mainly light crude and approx- imately 30% of it evaporated. Much of the remaining oil formed a stable water-in-oil emulsion known as ‘chocolate mousse’. This tends to float and is often the form in which oil ends up on the shore. On some species these emulsions are more adherent and harmful than non-emulsified oil. After evaporation of the lighter grades, oil remaining floating on the surface or deposited on the shore gradually disappears, mainly as a result of biodegradation by microorganisms. This is assisted by mechanical breakup of lumps and patches by wave action. For example waves may re-float oil from the shore and break it up into small droplets. This increases the surface area and aeration of the oil and thus assists biodeg- radation. In the case of the Braer spill off Shetland in 1993, the wave action was so severe that most of the oil was dispersed through the water column and very little came ashore. Although most biodegradation is via microbial action, some are ingested by larger animals and at least partially broken down. Some oil may be partially broken down by chemical degradation catalysed by exposure to sunlight. Remaining products may then be more easily biodegraded. The rate and extent of chemical degradation are affected by light intensity and duration, aeration and oil thickness. Well-weathered oil at sea may form ‘tar balls’ which may continue to be washed ashore for many months after the spill, causing a great nuisance to beach users (Fig. 9.2). SHORELINE OPEN OCEAN EVAPORATION FROM SLICK PHOTOLYSIS AND FROM SOLUTION FORMATION OF WATER IN OIL AEROSOL AND WEATHERING EMULSION (MOUSSE) SPRAY FORMATION STRANDING ONSHORE DR IF T I N G DISPERSON DISSOLUTION (OIL IN WATER FROM SLICK TRANSPROT EMULSIFICATION) IN LANGMUIR CIRCULATIONS ADSORPTION ONTO AND PHOTOLYSIS LARGER DROPLETS SORPTION ON PENETRATION INTO BEACH, RISE AND COALESCE SUSPENDED MIGRATION AND RELEASE SOLIDS DISSOLUTION VERTICAL OF DISPERSED HORIZONTAL DIFFUSION DEGRADATION AND OIL INGESTION AND DIFFUSION UPTAKE BY BIOTA DEPURATION BY BIOTA SINKING (NATURAL AND IN FAECAL PELLETS BIODEGRADATION UPTAKE AND RELEASE FROM SEDIMENT Figure 9.2 Schematic diagram of oil spill processes at sea and shorelines. Reproduced with permission from Mackay, D., 1985. Chapter 2: The physical and chemical fate of spilled oil, In: Engelhardt F.R. (Ed.), Petroleum Effects in the Arctic Environment. Elsevier Applied Science Publishers, London. 480 Elements of Marine Ecology Oil pollution of beaches The fate and effects of oil washed up on beaches depend not only on the variables such as oil type mentioned above but also on the energy level of the shore (degree of exposure to wave energy) and on substratum type. In general, the more exposed the shore, the quicker the biological recovery time of the intertidal benthos. This applies both to rocky and sedimentary beaches. Oil does not remain long on wave-battered rocky shores and where vertical cliffs are present the oil may never reach them due to the action of reflected waves. In contrast, a gradually sloping boulder shore in a shel- tered sea loch is likely to trap the oil which will get under the boulders and sink into any sediment beneath. Once oil gets down into the substratum, it is protected from wave action and will take much longer to disappear. Freely draining sediment shores of coarse sand, gravel or stones allow easy penetration of oil. With time, the oil may become more viscous due to evaporation and weathering and cannot escape. Firm beaches of waterlogged fine sand or mud resist penetration and most of the oil will be washed away on subse- quent tides. However, where there are fine, productive sediments supporting rich populations of burrowing animals and rooted plants, the oil will penetrate deeply through burrows and down root systems. These are the types of conditions found in salt marsh and mangrove systems. Once the oil penetrates such shores, it may remain for many years in an un-weathered state. Regeneration of mangrove systems has often been prevented by chronic oiling leaking from such sediments for many years after a spill. Oil that has penetrated shallow sublittoral sediments will similarly be protected to some extent from natural degradation and may cause persistent fouling on subsequent release from wave-churned beaches long after the original pollution source has ceased. The general topography of the area and the weather can also modify the effects of spilt oil. If oil drifts onto an irregular coast where there are headlands, inlets, rocks and islands, it will generally form a thinner covering than on a straight shoreline because of the greater distance over which it is spread. Oil on rocky shores tends to be carried up the shore towards high water levels, whereas on sandy beaches much more of it is spread over a range of shore levels. Strong onshore winds may carry some of the pollutants inland in windborne spray. This was of particular concern following the Braer oil spill in 1993 in Shetland. Here oil was carried inland by gale-force winds, affecting sheep, cattle and humans. The persistence of oil on a shore can therefore vary from a few months to many years (IPIECA report series, 1991 1997). Recovery of shore populations: The restoration of an intertidal population after destruction by oil pollution generally involves a sequence of stages in which different species are successively dominant. This sequence is now well recognized on British rocky shores. Oil spills that end up on rocky shores usually kill large numbers of gra- zers such as limpets. This means that, provided no further oil comes ashore, there is  Human impacts 2: problems, mitigation and conservation 481 usually a rapid settlement and growth of microalgae which turn the rocks green. Fast- growing green macroalgae, especially Enteromorpha and Ulva, follow. This is often known as the greening phase. In the continued absence of grazers, sporelings of the larger, slower-growing normal intertidal brown algae settle and grow, resulting in an abnormally dense cover of seaweeds. This is the fucoid phase. Meanwhile, juvenile limpets and gastropod snails settle out of the plankton in shel- tered crevices and start to grow rapidly on the abundance of food. Limpets (Patella spp.) often become abnormally numerous and fast-growing. The effect of this increas- ing number of herbivores is the gradual reduction of the algae populations to normal levels, followed in turn by a decline in number of grazers. Space becomes available for resettlement by barnacles. This is the limpet phase. The shore may look comparatively normal within 2 3 years. However, the fine bal- ance between grazers and algae may take much longer to stabilize. In the case of the Torrey Canyon disaster, the shores around Cornwall took 10 years to return fully to nor- mal, mainly because large amounts of toxic detergent were used to clean the beaches. Beach protection and cleaning There is no doubt that in the case of oil spills, the old adage ‘prevention is better than cure’ has never been truer. However, with oil exploration and distribution still playing a vital part in energy budgets, the opportunities for spills remain in spite of consider- able advances in tanker design and in legislation. Many spills result from human error and from severe weather conditions, but economic factors also come into play as building new tankers is an expensive business. The likelihood of oil tanker disasters has been greatly lessened by the requirement (via the MARPOL convention) from 1996 for all new tankers to be built with double hulls and the gradual phasing out of existing single-hulled tankers. Protection: Various measures are used for the protection of beaches from oil pollu- tion, when it occurs. Permanent and temporary floating booms and barriers may be used to prevent entry of oil into inlets and estuaries. In Sullom Voe in Shetland, UK, where there is a massive oil terminal, such barriers are kept permanently on station at the various arms of the loch, ready for deployment. Temporary booms were used in the Exxon Valdez disaster to prevent the entry of oil to salmon hatcheries. Spilt oil may also be impounded with booms and skimmed up. Such booms are often rendered ineffective by strong waves or currents. Clean-up methods: It is now generally accepted that there is no single clean-up method appropriate for all spills. Each spill is different and careful assessment is needed before deciding on a course of action (IPIECA, 1991; IMO/IPIECA, 1996). The action taken must be capable of significantly reducing the recovery time of the shore to below that which natural weathering will achieve. Sometimes cleaning may be undertaken for economic and amenity reasons rather than for the wildlife alone. 482 Elements of Marine Ecology Clean-up operations have sometimes increased the impacts on marine life and extended the recovery time for marine populations. These have usually been where aggressive techniques such as the use of high-pressure hot water (e.g. Exxon Valdez spill) and excessive use of dispersants (e.g. Torrey Canyon spill) have been used. These methods often kill off key species that have survived the initial oiling. However, even attempts to use non-aggressive mechanical clean-up methods can be damaging when used inappropriately. When attempting to clean mangrove areas, long-term damage has sometimes been done by the heavy vehicles needed to get into the area. Mechanical clean-up methods can sometimes be used to remove bulk oil and these generally cause little damage provided they can be used with minimal trampling and may also prevent further pollution from mobile oil. Suction pumps collect oil from gullies and rockpools, or from trenches dug to collect it. Straw bales and a variety of absorbent materials can be used to mop up small areas of surface oil. Mats of absorbent material can be unrolled onto the surface of the water and have been used very suc- cessfully for small spills, especially in fresh water. Low-pressure flushing with saltwater at ambient temperature washes shores, with little physical damage to marine life but requires booms, skimmers and a large team to collect the oil. Oiled weed and other debris can be collected by hand and oil-saturated sand dug up and disposed of elsewhere. As mentioned above, chemical dispersants have been much used for beach clean- ing, especially on British shores. They soon make the beach more pleasant for human enjoyment but have often proved much more damaging to intertidal organisms than the oil itself. Modern dispersants have low toxicity and may sometimes be appropriate for shore clean-ups. Nowadays they are more usually used on offshore slicks to pre- vent their coming ashore. They may, however, cause the oil to disperse into the water column, sink and affect the benthos. Even so, dispersants might be used, for example to prevent oil from entering a sensitive mangrove area whilst accepting there might be some detrimental effects to nearby coral reefs (IPIECA, 1992, 1993). Response capacity for oil and other pollution incidents varies greatly between countries around the world. The International Maritime Organisation (IMO) is responsible for providing practical technical guidance and has developed best practices and new technologies to help countries develop their ability to be prepared for and respond appropriately to such incidents. IPIECA, a not for profit association estab- lished in 1974, works jointly with the IMO in this respect (http://www.ipieca.org). 9.1.10 Radioactive pollution Artificially produced radioactive material enters or has entered the ocean from two main sources: from weapon testing via atmospheric fallout and from discharges, leaks and accidents within the atomic power industry. In 2011 the second-worst nuclear  Human impacts 2: problems, mitigation and conservation 483 accident in the world happened at the Fukushima Daiichi plant in Japan. Situated near the coast, the plant was damaged by tsunami waves generated by the Japan earthquake of March 11th, 2011. The damage resulted in massive explosions and release of radia- tion that spread over both land and sea but with the majority of fallout over the ocean. Of the 440 operational nuclear reactors in the world, about half are, like the Fukushima plant, situated on the coast. This includes the Chernobyl nuclear reactor in the Soviet Union, which exploded in 1986 greatly increasing the radioactive load of the world’s oceans and this is considered the world’s worst nuclear accident. There are continuing concerns that some of the atolls in the South Pacific used in the past for underground testing of atomic devices may eventually collapse and release huge quan- tities of radioactivity. The main radioactive contaminants are strontium-90, caesium-137 and plutonium- 239. Low-level radioactive waste materials are released into the ocean from coastal nuclear plants throughout the world. In the UK, a major source of past radiation pol- lution in the Irish Sea has been via the discharge of cooling water from Sellafield Nuclear Power Station, which was at its highest in the 1970s. The waste is discharged through a pipeline several kilometres offshore. The level of discharge has been consid- erably reduced in the decades since and levels in seawater are carefully monitored. Caesium-137, which does not occur naturally, has been used as a tracer for ocean cur- rents around Scotland and into the North Sea. Some highly dangerous radioactive wastes have also been disposed of by dumping into the deep oceans in sealed containers. Ocean disposal was banned by international treaties in the early 1990s and was restricted before that. However, the legacy remains and it is likely the containers will eventually corrode and the fate of the residues is uncertain. Although there are few direct links between the food chains of the deep ocean and the shallow waters used for commercial fishing, it is known that some deep-water currents surface in the Antarctic. The effects of low-level radioactivity on marine organisms are complex, difficult to document in the wild, and beyond the scope of this book, but may include genetic disturbances and increased mortality both in young stages and in adults. In the Irish Sea, regular monitoring shows that low levels can be detected in sediments, as well as in seawater and also in fish, shellfish and seaweeds. Any increased radiation load from eating seafood from this and similar areas is considered negligible. This would not be the case in the immediate aftermath of nuclear accidents such as those just described. 9.1.11 Thermal pollution Warm effluents discharged from coastal power plants (including nuclear plants) can raise water temperatures in bays and estuaries and may also cause stratification of the water up to several kilometres offshore. Thermal stratification can lead to reduced 484 Elements of Marine Ecology vertical water mixing and therefore reduced oxygen levels in the lower layers and in any case, oxygen is less soluble in warm water than in cold. Warm water may increase rates of bacterial decomposition of organic matter, leading to increased BOD and a consequent decrease in oxygen levels. If the warming is severe then, overall, the effect may be that the lower layers of water become deoxygenated and foul. Migratory fish with high oxygen requirements such as salmon may be discouraged from passing through the area. Warm water may also favour pests such as shipworms (Nototeredo) and gribble (Limnoria), accelerating their growth and extending their breeding seasons and it may also encourage or allow the establishment of invasive non-native species from warmer areas. There are also some potential benefits. Growth rates of useful species such as edible bivalves may be improved and growing these in aquaculture units in the vicinity of warm-water outlets may be beneficial. Some coastal manatees in Florida have been seen to overwinter near the warm-water outflows from power plants as well as in nat- ural warm spring areas. 9.2 Climate change and global warming Whilst the various types of marine pollution described in Section 9.1 tend to affect limited areas, global climate change has the potential to have significant consequences for ecosystems everywhere, whether marine, freshwater or terrestrial. Effects are and will be felt at all scales from individual species to global ecosystems and processes. The speed of change in the global climate has increased since the 1950s and this has become particularly evident since the beginning of the 21st century. The most clearly visible effects of global warming are seen in significant and sometimes dramatic changes in the amount, distribution and seasonality of polar ice and in sea level rises. Global warming is happening as a consequence of increasing levels of ‘greenhouse gases’, particularly carbon dioxide, methane and nitrous dioxide in the atmosphere (see The greenhouse effect in Section 9.2.1). These act to slow the escape of heat radiated back from the Earth’s surface into space. Most of that extra retained heat is absorbed by the ocean, which is effectively a heat storage system. Water can absorb heat with- out a large increase in temperature (see Box 2.1 Specific heat capacity) but also releases it again through direct heating of the atmosphere, melting ice caps and evaporation. Thus heat retained by atmospheric greenhouse gases and absorbed by the ocean today will still be having an effect on the planet decades later. Water temperature has pro- found effects on the distribution, abundance, functioning and physiology of marine organisms (see Section 2.1) and ocean warming has far-reaching consequences for the marine ecosystem. Climate and weather are integrally connected to the ocean and there is already evi- dence that global warming is increasing the frequency and intensity of storms and  Human impacts 2: problems, mitigation and conservation 485 hurricanes. The direction and speed of ocean currents may also be affected with the potential to cause significant changes in the climate of maritime countries in particular. Extremes of weather, sea level rise and melting polar ice are easily perceived manifes- tations of climate change, but major effects such as ocean acidification, coral bleaching and changing distributions of marine species are hidden from public view. Climate change is a hugely important topic, but only a few such examples of the effects of changing environmental conditions on the marine ecosystem can be described here. The Intergovernmental Panel on Climate Change (IPCC) is the United Nations body for assessing the science related to climate change. The most current assessment at the time of writing (2020) is summarised in a synthesis report (IPCC, 2014) and reports from the next assessment are due in 2022. The synthesis reports include useful summary graphs from 1850 to present, depicting sea level change, greenhouse gas con- centrations, anthropogenic CO2 emissions and land and ocean surface temperatures. 9.2.1 Carbon dioxide in the ocean and atmosphere Carbon dioxide is stored in the atmosphere, the land and in the oceans. The latter have an immense capacity to contain CO2, storing by far the largest amount and thus having a major controlling influence on atmospheric CO2 levels. Physical, biological and chemical factors are all involved in the uptake of CO2 by the oceans. Dissolved CO2 in its several forms (see Carbon dioxide in Section 2.2.3) is conveyed between sur- face and deeper levels by currents and water-mixing processes. However, phytoplank- ton also plays a leading role in CO2 transport. Phytoplankton take up CO2 dissolved in the surface waters and use it for photosynthesis. Herbivores eat the phytoplankton, thus taking up the carbon and transporting it to deeper levels during diurnal migra- tions. Here it is released during respiration and defecation or further distributed through the food web. For example, krill living in the surface layers over deep water in the Southern Ocean produce relatively large faecal pellets that have a good chance of reaching the deepsea bed, where the carbon remains locked away. This particularly applies to juvenile stages that live slightly deeper at the base of sea ice where they are more sheltered from currents that might carry the pellets back to the surface (Cavan et al., 2019). The effects of fishing for krill therefore need to consider the potentially large role that it may play in the carbon sink (see Section 8.3.6). The amount of CO2 taken up by the oceans is partly dependent on productivity and is therefore lowest in the tropics and highest in mid- and high latitudes. In the northern hemisphere, uptake of CO2 by the oceans increases during the spring and summer plankton blooms. In fact, in the tropics there is a net discharge of CO2 to the atmosphere throughout the year, but this is more than made up for in higher latitudes. Anything that lowers the productivity of the open oceans will also lower the oceans’ ability to absorb CO2. For example, increased water stratification resulting from 486 Elements of Marine Ecology warming of surface waters could prevent upward flow of nutrient-rich water necessary for phytoplankton production. With the advent of our modern industrial society, the amount of CO2 directly entering the atmosphere has risen dramatically and now stands at about 412 ppm com- pared with 280 ppm in the 1850s. This is a 47% increase since the beginning of the industrial age. The level has increased by about 11% in the 20 years or so since the 4th edition of this book was published. Most of this extra CO2 comes from the burning of fossil fuels and the effects of de-forestation. In the latter, the trees are not only no longer there to photosynthesize and use up CO2 but the forests are often burnt, thus releasing more CO2. The oceans are helping to counteract and moderate this build-up of atmospheric CO2 because as atmospheric levels increase, more CO2 dissolves at the ocean atmosphere interface. This itself is causing ocean acidification (Section 9.2.2), which has the potential to cause major changes in the marine ecosystem. The greenhouse effect This rise in atmospheric CO2 levels is of increasing concern because of a phenomenon that has come to be known as the ‘greenhouse effect’. Energy reaches the earth’s sur- face as short-wavelength radiation in sunlight and leaves again as long-wavelength IR (infra-red) radiation. The escape of IR radiation is slowed by the presence in the atmosphere of greenhouse gases, principally CO2, whilst incoming radiation is unaf- fected. Without these gases, the earth would have an average temperature around about freezing point. However, too great a concentration of these gases delays the escape of IR radiation and allows the earth to warm up. The increase in concentration of CO2 over the past century and a half appears to be having just that effect. The globally averaged combined land and ocean surface temperature has risen by about 1 C since the beginning of the 20th century and the rate per decade has almost doubled since 1980. Considering the immense heat capacity of the global oceans, it has taken a massive amount of heat energy to achieve this. What climate researchers are certain of is that if we continue to add CO2 to the atmosphere at the present rate, then global temperatures will rise, perhaps as much as 1.5 C 4.5 C in the next 50 100 years. Climate scientists generally agree that the rise needs to be kept below 1.5 C 2 C to avert potentially catastrophic environmental changes. Nearly 200 coun- tries have signed up to the Paris Climate agreement of 2015 and have committed to reducing greenhouse gas emissions and some governments have passed legislation committing to specific targets. Mitigation measures include reducing and eliminating the use of fossil fuels through development of renewable energy sources and protec- tion and enhancement of natural carbon sink ecosystems such as peat bogs, forests and seagrass beds. As we have seen above, the absorption of CO2 by the oceans plays a vital role in mitigating the greenhouse effect. However, if the temperature of the oceans increases  Human impacts 2: problems, mitigation and conservation 487 due to global warming, this will reduce the solubility of CO2 and might consequently lead to a net release of CO2 from ocean to atmosphere thus compounding the prob- lem. Obviously, the relationships of these processes with respect to climatic effects are complex and not yet well understood. They are now under intensive study by meteorologists and marine scientists. An early review by Smith and Hollibaugh (1993) showed that the role played by coastal ecosystems in the global carbon budget is even more important than had previously been thought. 9.2.2 Ocean acidification The absorption of large quantities of additional CO2 from anthropogenic sources is increasing the acidity of ocean water. The International Panel on Climate Change (IPCC, 2014) states that the pH of ocean surface water has decreased by 0.1 since the beginning of the industrial era, which is a 26% increase in acidity measured as hydro- gen ion concentration. If levels of carbon dioxide in the atmosphere continue to increase then so too will the acidity of ocean water. The chemistry behind this is described in the section on dissolved gases (Carbon dioxide in Section 2.2.3) and should be read in conjunction with this section in order to understand the process of ocean acidification more fully. Marine organisms have evolved under a stable pH regime, in seawater buffered by natural concentrations of CO2 that allow an abundance of carbonate (CO322) and calcium (Ca2+) ions to exist. Molluscs, crustaceans, echinoderms, corals, calcareous algae, some sponges and foraminifera all use these ions to form the calcium carbonate from which their shells and skeletal structures are made. It is these groups that are most at risk from ocean acidification, with potential effects ranging from the ability to build their shells in the first place and maintain them, to erosion and thinning of shells, particularly of long- lived species, leading to premature predation. Survival rates of larvae may also be reduced (Comeau et al., 2009; Orr et al., 2005); for example sea urchins and brittle stars have skeletal projections that may not develop properly and that are needed to prevent them sinking before metamorphosis. Since CO2 is more soluble in cold water than in warm, it is in the Arctic and Southern Ocean ecosystems that the effects of ocean acidification are being felt first. Evidence for the effects of ocean acidification on marine organisms comes both from direct observations and measurements from experimental laboratory-based work. With the latter, predicted scenarios of rising CO2 levels can be simulated and the effects on various organisms tested. Such experiments have demonstrated changes in calcification rates and serious erosion of shells in planktonic foraminifera (Moy et al., 2009) in corals (De’Ath et al., 2009) and in a variety of other organisms, as well as changes in physiol- ogy and metabolism (Ries et al., 2009). Henehan et al. (2019) used boron isotopes in foraminifera as evidence for the widely supported hypothesis that the last mass extinction 488 Elements of Marine Ecology 66 MYA was caused by an extra-terrestrial meteorite impact. Their data suggests that the impact caused rapid ocean acidification leading to ecological collapse. In the field, comparisons of shallow-water sites where CO2 is vented naturally from the seabed, with nearby sites where it is not, have demonstrated a lack of calcify- ing species in the former, (where the pH was lower) and dissolution of those shelled animals that were present (Hall-Spencer et al., 2008; Martin et al., 2008). In addition, it is already known that, as described in Section 2.2.3, shelled organisms cannot survive below the carbonate compensation depth, the depth below which calcite (calcium car- bonate) dissolves faster than the supply. Research into ocean acidification is relatively new but is vital given the potential ecological consequences of continued rises. On the positive side, Cornwall et al. (2020) have demonstrated in the laboratory that some ecologically important crustose reef coralline algae can gain tolerance to low pH over multiple generations of expo- sure. However, they also demonstrated that the calcification process during the early growth of the algae was very sensitive to ocean acidification. 9.2.3 Sea level rise and melting ice caps Sea level rise as a result of global warming is already apparent, particularly in low-lying countries such as the Maldives in the Indian Ocean. The underlying mechanism is through melting of ice that covers land areas and the subsequent drainage of water into the ocean. Of particular concern is the melting of the immense Antarctic and Greenland ice caps and of glaciers. Additionally, seawater expands as it warms up and as the ocean continues to warm, it will continue to increase in volume. Different geo- graphical areas will and are experiencing different rates of sea level rise. Some areas have a legacy of rising or sinking as a result of rebound from the weight of ice sheets in the last ice age. Other geological processes and differences in prevailing winds and currents can also cause changes in levels. Sea levels are measured using a global array of tide gauges and since the early 1990s, altimetry data from satellites. Tide gauge measurements have been available since the late 19th century, but initially global averages were not that accurate as there were only a few gauges in operation around the world. The available data show that the global mean sea level has risen by about 19 cm between 1900 and 2010 (IPCC, 2014) and that the rate of rise has increased in the last few decades. The best estimate of the current global average rise (as in the second decade of the 21st century) is 3.6 mm per year. Using geological and geophysical data and other observations it is estimated that the rate of rise since the mid-19th century has been greater than the mean during the previous two millennia (IPCC, 2014). Predictions for future rates of sea level rise are difficult to estimate, given the large number of variables involved. However, without a considerable world reduction in CO2 and other greenhouse gas emissions, a rise of at least  Human impacts 2: problems, mitigation and conservation 489 400 800 cm is predicted by the year 2100 and perhaps considerably more depending on rates of ice sheet melt. A great deal of uncertainty remains. Coastal communities such as saltmarsh and mangrove systems can keep pace with a slowly rising sea level by trapping sediment and growing upwards and have successfully done so over the ages. They may have to retreat landwards but this is not a problem unless prevented from doing so by sea walls and barriers (see Figure 6.21). Building of extensive walls to keep back the sea as levels rise could result in the loss of extensive areas of coastal wetlands. Apart from the loss of wildlife, this could also result in the loss of commercially valuable marine fish and other species, when nursery areas are lost. There has been some success on the east coast of England and in other countries, in removing or lowering sea walls to allow and encourage the growth of saltmarsh or man- grove which can act as a natural and adaptable (and relatively cheap) barrier to the sea. 9.2.4 Ozone and ultraviolet A further concern connected to anthropogenic changes in atmospheric gases is the depletion of the ozone layer in the stratosphere. This layer absorbs much of the ultra- violet (UV) radiation from the sun and this is important because ultraviolet-B (UV-B) is harmful to most life forms. It is mainly the UV-B rays from the sun that cause sun- burn and skin cancer in humans and UV light is even used to kill bacteria in some sewage treatment plants. Ozone forms in the upper atmosphere when oxygen mole- cules (O2) are broken apart by sunlight and the free atoms recombine to form ozone (O3). Ozone is unstable and its molecular bonds are easily broken, particularly when chlorine atoms are present. Since the 1970s the ozone layer has been thinning, a phe- nomenon that first came to notice in the Antarctic in 1985, when a huge ‘hole’ was discovered by atmospheric scientists. The destruction of the ozone layer is mainly due to the release of chlorine into the atmosphere, which destroys ozone through a series of chain reactions. Whilst some chlorine enters the atmosphere from natural sources, the majority results from human activity. A major source is from persistent chemicals called chlorofluorocarbons (CFCs) and other halocarbons, used extensively prior to 1987 in air-conditioning plants, refrigerators, aerosol propellants and various manufacturing processes. During the winter, CFCs and chlorine are concentrated over the Antarctic due to complex and unique climatic conditions there. This is why ozone depletion is most marked over the Antarctic. In 1987 the Montreal Protocol came into force phasing out the production and use of these and any ozone-depleting che- micals. The annual Antarctic ozone hole still occurs and was particularly large in 2020. However, the trend in size of the hole is downwards and a potential return to the pre-1980s condition of the ozone layer is predicted by about 2060. The damaging effects of UV-B light to terrestrial plants and crops is well known. However, it is also damaging to phytoplankton and inhibits bacterioplankton activity 490 Elements of Marine Ecology