The Seawater Environment and Ecological Adaptations PDF

CHAPTER 2 The seawater environment and ecological adaptations This chapter describes the physical and chemical properties of seawater and how these affect the ecology of marine organisms. The temperature, composition, density and viscosity, pressure, illumination and movements of seawater in the o...

CHAPTER 2 The seawater environment and ecological adaptations This chapter describes the physical and chemical properties of seawater and how these affect the ecology of marine organisms. The temperature, composition, density and viscosity, pressure, illumination and movements of seawater in the ocean all interact to produce the specific environmental conditions which influence the animals, plants and other organisms that live there. In many ways seawater is a much easier environment in which to live than either freshwater or land, and the balance of probabilities suggests that life began in salty water of some sort. As anyone who has swum in the sea knows, seawater provides support for both large and small bodies. Very simple and fragile forms of life such as gelatinous plankton can exist here, but not on land, because the water affords them this support, minimizing the need for structural complications such as skeletons or pro- tective coverings. However, there are also disadvantages. For example in the ocean, whilst photosynthetic organisms such as seaweeds are surrounded by water containing all the chemicals they need in an available form and quantity, light is a limiting factor, except in surface and shallow water. A notable feature of the marine environment is that physical conditions can be remarkably constant over large areas and many marine plants and animals have corre- spondingly wide distributions. Such changes as do occur generally take place slowly, giving time for some organisms to acclimatize. However, stable conditions also allow the evolution of organisms whose environmental requirements are very precise and whose range is limited by quite slight changes in their surroundings. The ocean provides a three-dimensional living space and there are two major gra- dients of physical conditions: geographically between the tropics and the poles and depth-wise between the surface and the seabed. Both are associated with differences of penetration and absorption of solar radiation and therefore with gradients of tempera- ture, illumination and to a lesser extent salinity. Vertical distribution is also influenced by pressure. The distribution of a species is consequently associated with a complex of variables and it is not easy to assess the role of each factor independently. This is made even more difficult because many properties of seawater vary together; for example the temperature of water affects its density. Elements of Marine Ecology r 2022 Elsevier Ltd. DOI: https://doi.org/10.1016/B978-0-08-102826-1.00002-8 All rights reserved. 37 38 Elements of Marine Ecology The effects of variation in single factors can be studied to some extent in controlled conditions in the laboratory, but in this unnatural environment the responses may be abnormal. There is also the complication that several factors often interact in their effects; for example, in some species the tolerance to salinity change is modified by temperature and temperature tolerance may itself vary with salinity. Furthermore, observations on spe- cimens from one locality may not hold for an entire population of wide distribution. Each species may exhibit a range of tolerance for each variable, with populations from one geographical situation differing due to selection or acclimatization. Apart from the effects of the inorganic environment, there are also many ways in which organisms influence each other. Even where physical and chemical conditions seem suitable, a species may not flourish if the presence or absence of other species has a detrimental effect. For example, predation may be too severe, or other competing forms may be more successful in the particular circumstances. The environment may be lacking in some essential resources contributed by other species, such as food, pro- tection, an attachment surface or some other requirement. These biological factors are obviously of great importance, but their evaluation is extremely difficult. Generally, the distribution and success of a species is an equilibrium involving many complex interactions between population and environment which are difficult to unravel. Nevertheless, a start can be made in tracing the complicated web of influ- ences, by first studying the individual environmental variables and the effects these have on the distribution and lives of marine organisms. For more detailed descriptions of the composition, properties and conditions for life in seawater, the reader should refer to one of the many general texts available on oceanography. A small selection of these is listed under Further Reading at the end of Chapter 1. 2.1 Sea temperature Ecological relevance: The high specific heat of water (Box 2.1) and the great volume of the oceans provide a huge thermal capacity and the continual circulation of the oceans and their enormous heat capacity ensure that the extent of temperature variation in the sea is small, despite great geographical and seasonal differences in absorption and radiation of heat. Consequently, the temperature range of the oceans is restricted and temperature changes occur slowly. Except in the shallowest water, the temperature range in the sea is less than that which occurs in most freshwater and terrestrial habitats and the relative stability of sea temperature has a profoundly moderating effect on atmospheric temperature change. Seawater temperature also affects the density of sea- water which is one of the major drivers of ocean circulation. The sinking of surface water due to cooling at high latitudes ultimately carries oxygenated water to the dee- pest depths and thereby makes animal life possible at all depths.  The seawater environment and ecological adaptations 39 BOX 2.1 Specific heat capacity (SHC). The SHC of a substance (such as water) is a measure of the energy (joules) needed to raise the temperature of 1 g of it by 1
C. At 3.93 the SHC of seawater is very high, meaning that it can absorb large amounts of heat energy with only a small change in temperature. When the sun heats water, most of the solar energy is used to break the hydrogen bonds between water molecules. The remaining small part is used to increase the agitation or vibration of the mole- cules, which we detect as a rise in temperature. Such stored heat can be released later. 2.1.1 Geographical and depth variations The highest sea-surface temperatures (SSTs) are found in low latitudes near the equa- tor, where much of the oceanic surface water is between 26
C and 30
C. In shallow or partly enclosed areas like the Arabian Gulf, the surface temperature may rise to as high as 35
C during the summer and conditions are extreme on the shore, where intertidal pools sometimes exceed 50
C. At the other extreme, the freezing point of seawater varies with the salinity and is depressed below 0
C by the dissolved salts. At a salinity of 35‰ (see Section 2.2.1), seawater freezes at approximately 21.91
C. Excluding the shore, shallow water and hydrothermal vents, the extreme tempera- ture range between the hottest and coldest parts of the marine environment is there- fore in the order of 30
C35
C, but in any one place the range of temperature variation is always much less than this. The corresponding land range is nearly 150
C. In high and low latitudes, surface sea temperature remains fairly constant throughout the year. In middle latitudes, surface temperature varies with season in association with climatic changes. The range of seasonal temperature change depends upon locality, but is commonly about 10
C. Off the southwest coast of the British Isles, the temper- ature usually varies between about 7
C in winter and 16
C in summer, while off the north coast of Scotland the range is 4
C in winter to about 13
C in summer. The greatest seasonal variations of surface sea temperature are about 18
C20
C, this range being recorded in the China Sea and Black Sea. Inland seas such as the Caspian also exhibit large ranges. While surface water varies in temperature from place to place and time to time, the deep layers throughout the major ocean basins below about 2000 m remain con- stantly cold at between 21.9
C and 4
C. The coldest (21.9
C) water is Antarctic bottom water, found in the Southern Ocean. However, there are some oases of hot water in the deep ocean in zones of submarine volcanism along tectonic plate bound- aries. Water at temperatures of between 200
C and 300
C spouts out from deep-sea hydrothermal vents (see Section 7.4.6) warming the immediately surrounding water to around 10
C17
C. Along the rift line in the central Red Sea there are areas of 40 Elements of Marine Ecology highly saline water (hot brines) at around 44
C in the Discovery Deep and in the nearby Atlantis II Deep, the water reaches an astonishing 56
C. The normal temperature of Red Sea bottom water is about 22
C. 2.1.2 Thermoclines As you move deeper into the ocean, the temperate drops from a variable surface tem- perature to a constant, cold deepwater temperature. However, the drop in tempera- ture is not a gradual one from surface to bottom. Instead, there is a relatively sharp transition between the warmer, well-mixed upper layer (epipelagic zone) of water and the cold bathypelagic zone. Such a transition is called a thermocline and the zone of change is the discontinuity layer. Although not much used now, the terms thermo- sphere and psychrosphere refer, respectively, to the warm, mixed water layer above the discontinuity layer and the cold, stable water below it, where there is only a slight further decrease of temperature towards the bottom. The thermocline usually occurs between about 200 and 1000 m, that is, within the mesopelagic zone. At low latitudes where the sun warms the water almost constantly, heat absorption at the sea surface produces a warm, light surface layer overlying the cold, denser, deep layers and tropical waters have a permanent thermocline year round. In middle latitudes, the sur- face water becomes warm during the summer months and this leads to the formation of temporary, seasonal thermoclines near the surface, commonly around 1540 m depth. In winter, when the surface water cools, these temporary thermoclines disappear and convec- tional mixing may then extend to a depth of several hundred metres. Below the level to which convectional movements mix the water, there is usually a permanent, but relatively slight thermocline, between about 500 and 1500 m. In high latitudes, heat passes from the sea to the atmosphere. Surface cooling of the water produces convectional mixing and there is therefore little difference in tem- perature between the surface and the deep layers. Through the whole depth of water the temperature range is usually within the limits of 21.8
C to 1.8
C. There is often an irregular temperature gradient within the top 1000 m because the surface is diluted by freshwater from precipitation or melting ice. This forms a low-density layer of colder water above slightly warmer, but denser, water of higher salinity entering from middle latitudes. Below 1000 m the temperature is almost uniform to the bottom, decreasing only slightly with depth. Typical ocean temperature profiles are shown in Fig. 2.1. Thermoclines have a significant effect on the vertical distribution of pelagic spe- cies and can act as a permanent boundary, separating a warm-water population above and a cold-water population below. In temperate areas seasonal thermoclines affect the supply of nutrients and so the occurrence of seasonal plankton blooms (Section 3.4.1).  The seawater environment and ecological adaptations 41 Temperature (°C) Temperature (°C) 5 10 15 20 5 10 15 20 surface mixed layer 1000 permanent thermocline 500 2000 Depth (m) Depth (m) 3000 1000 boom waters 4000 Winter Polar Spring Temperate Summer Tropical 1500 5000 Figure 2.1 Typical generalised ocean temperature profiles: (A) seasonal profiles for temperate lati- tudes, (B) regional profiles in open ocean. From Dipper, F.A., 2016. The Marine World: A Natural History of Ocean Life. Princeton University Press (Wild Nature Press), 544pp. Courtesy Marc Dando and Wild Nature Press. 2.1.3 Temperature tolerances and biogeography The distribution of species throughout the world’s oceans is determined first by which of the major oceans they have evolved in and are effectively confined to and second mainly by water temperature. Many species normally found in one particular ocean or sea could happily survive if moved to another with a similar temperature regime. This happens time and again when non-native species are accidentally introduced (see Section 9.5) and not only survive but thrive. Temperature tolerances differ widely between species, but each is restricted in distribution within its particular temperature range. Classic work on the effects of temperature and salinity on marine organisms was the subject of the first two in the series of Oceanography and Marine Biology Annual Reviews (Kinne, 1963, 1964). Some species can only withstand a very small variation of temperature and are described as stenothermal. Eurythermal species are those of wide temperature tolerance. Strict stenotherms are chiefly oceanic forms and their distribu- tion may alter seasonally with changes of water temperature. Eurytherms are typical of the more fluctuating conditions of shallow water. Sessile organisms have generally a rather wider temperature tolerance than mobile organisms of the same region. 42 Elements of Marine Ecology Whilst other factors play a part in the detailed geographical distribution of a spe- cies, it is temperature that has the greatest effect. Mapping the surface ocean isotherms can give a rough indication of major marine biogeographical regions. Biogeographical regions are geographical areas distinguished by the plants and animals that are found there. An isotherm is a line on a map joining points that have the same temperature at a particular time or on average over a particular period. Classic major world marine biogeographic regions and subregions are listed below and are shown in Fig. 2.2. The boundaries are by no means exact and for the most part, the transition between one fauna/flora and another is gradual, with a broad over- lap of populations. Physical land barriers (some now breached by man-made canals  see Section 9.5) account for some differences between oceanic populations and wide expanses of deepwater can prevent the spread of some littoral and neritic species. In general the populations of the surface, shallow water and shallow seabed fall into three main groups namely, tropical warm-water populations, polar cold-water populations (Arctic and Antarctic) and temperate water populations, which inhabit waters of inter- mediate temperature, with seasonal fluctuations (boreal, antiboreal, warm temperate). These major divisions are subdivided in a variety of ways to take account of local con- ditions and the parameters used to define them. 180° 160° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160° 180° 80° 80° 60° 60° 40° 40° 20° 20° 0° 20° 20° 40° 40° 60° 60° 180° 160° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160° 180° Figure 2.2 Approximate positions of mean annual surface isotherms and corresponding marine biogeographic areas.  The seawater environment and ecological adaptations 43 Warm-water populations are found mainly in the tropical belt where surface water temperatures remain above about 18
C20
C. This warm-water zone corresponds roughly with, but is rather more extensive than, the zone within which coral reefs are found. Coral reefs (Section 7.4.3) are most abundant in clear shallow water where the temperature does not fall below 20
C. Seasonal variations in temperature are relatively small and at the Equator, surface water temperature hardly changes and in most areas lies between 26
C and 27
C. Cold-water populations are found in the Arctic and Southern Oceans where the surface temperature lies between about 5
C and a little below 0
C. In the Southern Ocean the cold water has a well-defined northern boundary at the Antarctic Convergence (see Section 2.7.4) where it sinks below the warmer sub-Antarctic water. The sharp temperature gradient at this convergence effectively forms a barrier and a distinct northern limit to Antarctic species. The southern boundary of the Arctic zone is less distinct, except at the convergences of the Labrador Current and Gulf Stream in the Atlantic and of the Oyo-Shiwo and Kuro-Shiwo currents in the Pacific (see Fig. 2.24). Broadly, the Arctic zone comprises the Arctic Ocean and those parts of the Atlantic and Pacific Oceans into which Arctic surface water spreads, the limiting tem- perature being a summer maximum of about 5
C. The temperate sea areas lie between the 5
C and 18
C mean annual surface iso- therms and here the surface water undergoes seasonal changes of temperature. The colder parts of the temperate regions between the 5
C and 10
C isotherms are termed the Boreal zone in the northern hemisphere and the Antiboreal zone in the southern hemisphere. The rest are warm temperate. The course of the surface isotherms is determined largely by the surface circulation. On the western sides of the oceans, the warmest water reaches higher latitudes and the coldest water lower latitudes, than on the eastern sides. The temperate zones are therefore narrow in the west and much wider in the east, where they extend further to both north and south. The classic biogeographic subdivisions of the intertidal and epipelagic zones are as follows (see also Fig. 2.2): Arctic and Subarctic regions East Asian Boreal region Northwest American Boreal region Atlantic Boreal region North Pacific warm temperate region: East Asian province North Pacific warm temperate region: West American province Atlantic warm temperate region (Lusitanian) Tropical Indo-West-Pacific region Tropical East Pacific region Tropical Atlantic region 44 Elements of Marine Ecology South Pacific warm temperate region South Atlantic warm temperate region Indo-Australian warm temperate region Antiboreal region Kerguelan region Antarctic and sub-Antarctic regions Whilst describing a species as ‘tropical’ or ‘warm temperate’ allows a basic under- standing of that species temperature requirements and geographical distribution, it can- not tell us much more than that. These basic divisions mostly originated from relatively little data, based on particular groups of more easily studied species, such as plankton, seabirds and fish. A recent approach taken by Spalding et al. (2017) uses the huge amount of distributional data now available on world databases such as ORBIS (Ocean Biogeographic Information System). This study analysed the distribution of 65,000 marine species across all taxa and defined 30 marine realms including 12 off- shore deep-sea realms. Not surprisingly their maps bear some similarity to Fig. 2.2 but provide a much more detailed and accurate picture and include deep-sea and benthic as well as pelagic and coastal environments. Biogeographical regions around the British Isles One of the reasons that the British Isles has such a rich and varied marine fauna and flora is that it lies across the region where two major, temperate biogeographical pro- vinces meet: the Boreal, which is centred on the British Isles and the warmer Lusitanian to the south (Fig. 2.3). The Arctic region is also reflected in the presence of some cold-water species in the Shetland Islands and Faroe Islands. The British Isles lie across the 10
C mean annual surface isotherm and in winter the 5
C isotherm moves south along these coasts. It is therefore possible here to distinguish certain species as belonging to a northern group of Arctic and Boreal forms and others as a southern Lusitanian group of Mediterranean and temperate water species. There are seasonal changes in distribution and a broad overlap of populations, but the 10
C isotherm lies approximately between the two groups. Using fishes as an example, the northern group includes Atlantic Cod (Gadus morhua), Haddock (Melanogrammus aeglefinus), Ling (Molva molva), Plaice (Pleuronectes platessa), Halibut (Hippoglossus hippoglossus), and Atlantic Herring (Clupea harengus). Examples of southern forms are Pollack (Pollachius pollachius), European Hake (Merluccius merluccius), Dover Sole (Solea solea), Turbot (Scophthalmus maximus), pilchard (Sardina pilchardus), anchovy (Engraulis encrasicolus), Atlantic Mackerel (Scomber scombrus) and tunas (Thunnus thynnus and T. alalunga). This does not mean that a particular fish, such as a Ling, only occurs in the very north of the region, but it does mean its main distribution is northern. Ling have their southern limit around Morocco and extend north to the Barents Sea and Iceland.  The seawater environment and ecological adaptations 45 Boreal–Arctic region Boreal region Boreal–Lusitanian region Lusitanian–Boreal region Figure 2.3 Major coastal biogeographical regions around the British Isles based on Hiscock (2018). On the seashore almost all boreal species can occur all around Britain and Ireland, but a few that are common in the north and east become scarce or absent towards the southwest (e.g. the barnacle Semibalanus balanoides, the limpet Acmaea tessulata, the blenny Zoarces viviparus and the spindle shell Neptunea antiqua). There are a larger num- ber of species which are abundant in the southwest and often extend up the west coast but are absent in the north and east, the British Isles being the northernmost limit of their range. These southern forms include the barnacles Chthamalus montagui, C. stellatus, and Perforatus (was Balanus) perforatus, the top shells Phorcus (was Monodonta) lineatus and Steromphala (was Gibbula) umbilicalis, the limpet Patella depressa, the Snakelocks Anemone Anemonia sulcata, the prawn Palaemon serratus and the cushionstar Asterina gibbosa. 46 Elements of Marine Ecology Long-term changes in mean annual sea temperatures and short-term weather extremes can alter local patterns of distribution. For example, the distribution of the Toothed Topshell (P. lineatus) eastwards along the coast of the English Channel was affected by the unusually cold winter of 196263 (Hawthorne, 1965). After about 1961, mean annual sea temperatures around the British Isles fell slightly compared with those of the previous 25 years. The distribution of many marine organisms corre- spondingly shifted slightly southwards. In the western part of the English Channel cold-water species such as Atlantic Cod, Norway Pout (Trisopterus esmarkii), Ling and Atlantic Herring became more numerous, whereas warm-water species, notably pil- chards and European Hake, declined in numbers over the same period and tended to spawn later. Effects could also be seen on the relative abundance of some species of barnacles (see Section 6.2). Around southwestern shores, numbers of the southern Chthamalus montagui and C. stellatus steadily increased, along with sea temperature, until around 1960 (although the variation in sea temperature was only about 0.5
C in 50 years). After 1961 or so, mean sea temperatures fell again and the relative propor- tion of the boreal Semibalanus balanoides started to increase. In S. balanoides, high sum- mer temperatures prevent final maturation of gametes and may also affect adult survival. Sea temperatures in the North Atlantic are now rising again and to a greater degree with ocean warming and further shifts in these proportions are extremely likely (see Section 9.2.6). Bipolar distributions Species that are found at high latitudes (polar and subpolar provinces) in both hemi- spheres, but with no evidence for the existence of the species spanning the tropics in between, are said to have a bipolar distribution. This type of distribution could occur as a result of a species (or taxon), originally restricted to one pole, dispersing to the other, or when an event occurs to limit a once cosmopolitan species to the poles. For most polar marine species, especially small, slow-moving and sedentary types, such a dispersal would be very difficult. One possible route is via the deepwater thermohaline currents that convey water slowly right through the ocean (see Section 2.7.4). Changes in temperature and salinity at the poles cause water to sink, carrying marine organisms with it to depths where the temperature remains just above freezing. However, the journey to the opposite pole could take an estimated 400600 years, too long for any one individual. So organisms taking this route would have to repro- duce over many generations as they travelled. Proving that a taxon is truly bipolar is difficult and many species once thought to have a bipolar distribution are now known to be two or more different species (Box 2.2). The amphipod Themisto gaudichaudii (previously known as Parathemisto gau- dichaudi) and cited in previous editions of this book as having a bipolar distribution has now been demonstrated to consist of separate species with T. gaudichaudii in the  The seawater environment and ecological adaptations 47 BOX 2.2 Genetics of bipolar species. Genetic studies are now able to start answering the question of whether there are any truly bipolar marine macro species. During the Census of Marine Life (http://www.coml.org) research expeditions to both poles, 235 species (including ‘worms’, gastropods and crusta- ceans) were documented within both the Arctic and Antarctic zones, which have not been recorded anywhere in between. Some of these may yet be found at locations in between, but genetic analysis should ultimately tell just how closely related each pair is. There is now strong genetic evidence that the shelled pteropod (sea butterfly) Limacina helicina may not be a bipolar species, as considerable genetic differences between Antarctic and Arctic popu- lations have now been discovered (Hunt et al., 2010). This is important in this instance because L. helicina and another pteropod Clione limacina (which may also not be truly bipo- lar) are an abundant component of polar plankton and are vulnerable to ocean acidification. Research using specimens from one pole may not be applicable to the other if they are sep- arate species. Antarctic and T. compressa in the Arctic (Schneppenheim and Weigmann-Hass, 1986). Uriz et al. (2011) have shown that the boreal sponge Stylocordyla borealis does not also occur in the Antarctic, where it had previously been recorded. They determined that the Antarctic S. borealis is actually a new species in the genus. Likewise, the North Atlantic barnacle Semibalanus balanoides and tunicates Botryllus schlosseri and Didemnum albidum were once thought to occur both in Europe and the other end of the world in Australasia, but this has long been disproved. In some cases apparent bipolarity is really a continuous distribution through the colder layers of water underlying the warm surface layers of the tropics. Some pelagic species have adopted a wide depth distribution, which allows them to live in cold sur- face waters in both Arctic and Antarctic waters, but also in cold deepwater between. This is known as tropical submergence. The planktonic chaetognath (arrow worm) Eukrohnia hamata is one such species that lives in surface epipelagic waters at high lati- tudes (Arctic and Antarctic) but at mesopelagic or bathypelagic depths in lower latitudes. Deep-sea distributions Whereas the distribution of many littoral, sublittoral and epipelagic species is fairly fully recorded, knowledge of the distribution of deep-sea species is still very incom- plete. However, over the past few decades, new and improved technologies have allowed much more exploration and sampling of the deep-sea environment. Deep-sea investigations in different oceans have confirmed what had been suspected for some time; that there is greater biodiversity of abyssal species than was thought in the early 48 Elements of Marine Ecology days of such investigations and that some species are relatively restricted in distribution. Many deep-sea species are documented as having a very wide geographical distribu- tion, often through more than one ocean basin and as having a wide depth range (within the deep-sea environment). This makes sense if there are few physical barriers to dispersal and temperature remains fairly constant. There are some recognized boundaries and the most distinct is probably the system of submarine ridges separating the Arctic basin from the North Atlantic which, together with the shallow water of the Bering Strait, form a barrier which some abyssal species cannot cross. Relatively few species appear to be common to the bottom of both Arctic and other deep oceans. However, genetic studies are now challenging the concept of cosmopolitan deep- sea species. For example, the giant deep-sea amphipod Eurythenes gryllus has a recorded distribution in all five major oceans and a depth range covering bathyal, abyssal and hadal zones. Havermans et al. (2013) researched the genetic diversity of this species on a global scale and found nine possible species-level clades, five of which were restricted to a single ocean basin. There was also a clear split between species lineages living above and below 3000 m. Genetic differences were reflected by small morphological differences. It has always been assumed that the deep ocean environment is homoge- neous (especially in temperature) and lacks isolating barriers, thus reducing the oppor- tunities for speciation. It may well be that speciation in the deep ocean is controlled by other subtle environmental differences and barriers. Wider sampling and further genetic studies are needed to determine and re-evaluate species distributions in the deepsea. 2.1.4 Effects of water temperature on physiology Many factors affect the biogeographical distribution of marine organisms, but as already stated, water temperature plays a major role. This is not surprising because, with the exception of mammals, birds and a few exceptional reptiles (Box 2.3) and fish, marine organisms are poikilothermic, that is they cannot control their body tem- perature physiologically and it remains close to the ambient temperature of the sur- rounding water. At the extremes, water temperature in the Arctic and Southern Oceans can be close to 22.0
C. For animals such as fish with a blood circulatory sys- tem, this poses the threat of freezing. Species such as the Blackfin Icefish (Chaenocephalus aceratus) have evolved an antifreeze protein in the blood, done away with red blood cells and can absorb oxygen through the skin. Temperature controls and modifies physiology in a variety of ways, but in simple terms, rates of metabolic processes increase with rising temperature, usually by about 10% per 1
C rise over a range of temperatures up to a maximum, beyond which rates fall off rapidly. A higher metabolic rate allows for more activity such as hunting or  The seawater environment and ecological adaptations 49 BOX 2.3 Marine reptiles and temperature. Very few reptiles have adapted to living in the ocean, with only seven species of turtle, one iguana and a larger number of snakes. They are ectothermic, that is they rely on outside heat to raise their body temperature sufficiently to be active. On land, reptiles thrive in hot environments such as deserts and are rarely found in polar regions. Only one reptile lives in the Arctic and none in Antarctica. Whilst the tropical Marine Iguana (Amblyrhynchus cristatus) must ‘sunbathe’ to warm up after diving for its food in cold Galapagos waters, the Leatherback Turtle (Dermochelys coriacea) has a wide distribution extending as far north as the British Isles and as far south as Tasmania. The other six marine turtles are restricted to the warm water of the tropics. The Leatherback Turtle’s secret is a counter-current heat exchange system of capillaries at the base of its flippers. Heat generated as it swims is car- ried to the body core. Large size and a thick layer of blubber help to retain this generated heat. Temperature even controls the sex of marine turtle hatchlings, with warmer sand nests resulting in larger numbers of males. A pivotal temperature of about 29
C results in an almost equal sex ratio. Global warming is already skewing the sex ratio on some nesting beaches. escaping predators. Heat generated by muscular activity when a fish swims is usually lost rapidly because water has a high heat capacity (Box 2.1). However, some large active fish species have evolved ways of maintaining their swimming muscles, eyes and brain well above ambient temperature, which allows them to effectively extend their geographical range. Elevated core temperature maintains hunting efficiency in cold water and is one of the reasons for the cosmopolitan distribution of the White Shark (Carcharodon carcharias). The mechanism is an effective system of heat recycling, where specialized blood vessel networks recover muscle-generated heat and transfer it to freshly oxygenated, but cold, blood flowing from the gills. These bundles of blood vessels are called retia mirabilia (singular is rete mirabile). Core temperature can be ele- vated 10
C20
C above ambient. Two out of five lamnid sharks the Shortfin Mako (Isurus oxyrinchus) and Porbeagle (Lamna nasus) as well as the singular Thresher Shark (Alopias vulpinus) grace the waters around the British Isles thanks to this adaptation. Several species of tuna (Scombridae) and billfish (Xiphiidae) also have it. Marine organisms will die above and below certain limiting temperatures and marine organisms usually succumb more rapidly to overheating than to overcooling. In freak climatic conditions, extremes of heat or cold may have rapid and devastating effects on marine populations, especially those of the shore. This is evidenced by mass ‘wash ups’ or ‘wrecks’ of mobile animals following temperature extremes. Mid- February 2018 saw thousands of lobsters, crabs and starfish washed up on the east coast of England in early March, many alive but incapacitated by a prolonged cold and stor- my weather event. However, the limits of distribution of a species in the sea do not 50 Elements of Marine Ecology coincide closely with such extremes, but are much more restricted. In normal circum- stances temperature controls distribution in subtle and gradual ways through its influ- ence on major processes including feeding, respiration, osmoregulation, growth and especially reproduction. The effect of water temperature on breeding is one of the key factors influencing the distribution of marine organisms. Temperature regulates reproduction in several ways. It controls the maturation of gonads and the release of sperm and eggs and in many cases the temperature tolerance of embryonic and larval stages is less than that of the adults. Temperature, therefore, has a major influence on the breeding range and period and on mortality rates during early stages of development and larval life. Along the fringes of distribution there are usually nonbreeding zones where the adults can survive but cannot reproduce, the population being maintained by spread from the main area of distribution within which breeding is possible. An example is the Grey Triggerfish (Balistes capriscus) which is a regular summer visitor to the southern coasts of the British Isles. First arrivals are usually in May, swimming, with the help of cur- rents, from areas further south. However, they do not return, but become dazed by cold as winter approaches and are frequently washed ashore. With global warming this species may be set to start breeding in British waters and this will be an interesting story to follow. There is some evidence of this in the form of records of young fish, probably too small to have swum very far (Box 2.4). In temperate seas many species virtually cease feeding during the winter. In some cases reduced feeding is simply the result of shortage of food, but many marine animals definitely stop or slow feeding below a certain temperature. Food requirements are reduced during cold periods because the respiration rate is low and growth slows or ceases. Dead Men’s Fingers (Alcyonium digitatum) a common octocoral found in BOX 2.4 Southern visitors. Exceptionally hot summers such as those of 1995 and 2018 result in the appearance of many southern fish species in UK waters, which are not regular visitors. Some of these such as Big-eyed Thresher Shark (Alopias superciliosus) take advantage of their ability to increase their core body temperature as already described, but others are simply responding to warmer waters. These are often reported by anglers or spotted by divers. Axillary Sea Bream (Pagellus acarne) is normally found south of the Bay of Biscay and is extremely rare in NW European waters. One was caught in 2017 in NW France and in 2018 in Dorset. Sea tempera- tures in the North Atlantic are now rising and such tropical species could become regular visitors if the warming continues. Records of such rarities are not just of passing interest, but are currently being used as tools for monitoring the distribution and abundance of uncom- mon species in response to environmental change.  The seawater environment and ecological adaptations 51 northern Europe shuts down and withdraws its feeding polyps during the winter months (see also Section 3.4.4). These interruptions of growth may produce periodic markings in growing structures, for instance, the annual winter rings on fish scales, which reflect slower growth (Section 8.1.4). Water temperature can also have more subtle effects on feeding, such as affecting the rate of ciliary beat in barnacles. Nishizaki and Carrington (2014) demonstrated that feeding capture rate and efficiency in the North American barnacle Balanus glandula are affected by temperature, but water velocity is also involved. Barnacles also change the rate of ciliary beat for reasons other than feeding (and hence) growth. Despite the depressing effects of cold on growth, it is nevertheless generally observed that where the distribution of a marine species covers a wide range of tem- perature, the individuals living in colder areas tend to attain larger adult sizes than those in the warmer parts of the distribution. However, this is by no means always the case and the temperate water sea urchin Echinus esculentus reaches larger sizes in warmer waters. This trend towards larger size in colder water is associated with a lon- ger growing period, later (energy sapping) sexual maturation and a longer life in cold water. Gigantism is regularly observed in cold environments and many benthic animals in the Antarctic region grow much larger than their counterpart species in temperature and tropical waters. There are also instances of gigantism in the deep ocean and iso- pods in the genus Bathynomus regularly reach 15 cm long, with B. giganteus attaining 36 cm. In temperate waters a 5 cm long isopod is a large one. The key to gigantism has still to be unlocked and the answer may not be straightforward (Box 2.5). Apart from direct physiological effects, changes of temperature have certain indirect effects by altering some of the physical properties of the water, notably density, viscos- ity and the solubility of gases, which in turn influence buoyancy, locomotion and res- piration. The viscosity of water falls considerably with increasing temperature, which may partly account for why many warm-water planktonic organisms have very branched and finely divided appendages compared with their cold-water counterparts. The greater surface area of finely divided appendages increases their floating ability. 2.2 Composition of seawater Ecological relevance: Seawater provides a stable environment and its composition remains almost uniform throughout its extent, despite considerable differences in the rates of evaporation and addition of freshwater in different localities. The composition of present-day seawater may differ in some respects from that of the remote past, but if so, marine organisms have been able to evolve and adjust to changing conditions. Differences in salinity such as are found in estuaries and enclosed seas impose con- straints on which organisms can live there. The levels of specific chemical constituents such as iron and of nutrients greatly affect productivity. 52 Elements of Marine Ecology BOX 2.5 Polar gigantism. Whilst most sea spiders (Pycnogonida) reach only a centimetre or so long, giants lurk in the frigid waters around Antarctica. Colossendeis megalonyx can attain 6 cm long with legs of 50 cm or more. Some Antarctic amphipods, including a species of Paraceradocus, can reach body lengths of more than 5 cm and there are numerous other examples of oversized ani- mals. There may be several explanations for polar gigantism, but cold water seems to be key. The most likely reason is that cold water holds more oxygen per litre than warm water. This ‘oxygen hypothesis’ was first put forward by Chapelle and Peck (1999) and related the maximum body size of amphipods to the usual oxygen content of the water they lived in  the more oxygen the greater the size attained. This hypothesis and others were tested by field and laboratory work on amphipods in the Antarctic (Spicer, 2017). Spicer and Morley (2019) found that body size affected the ability of amphipods to cope with reduced oxygen levels, but similarly sized different species did not always react in the same way. They con- cluded that the larger species would be amongst the first to suffer in a future oxygen-poor ocean. Gigantism is not restricted to polar regions, but is also common amongst amphipods and isopods living in the deep ocean. These animals seldom exceed lengths of 23 cm in shallow and middle depths, but grow to 810 cm or more in the hadal zones. Water temper- ature at these depths is similar to that found in shallow water in polar regions. Seawater is an extremely complex solution, its composition being determined by an equilibrium between rates of addition and loss of solutes, evaporation and the addi- tion of freshwater. The original source of seawater is uncertain, but was probably by condensation of water vapour and solutes released into the atmosphere from hot rocks and volcanic action at an early stage of the Earth’s history. It is uncertain to what extent the composition of seawater may have changed during geological time, but it is not thought to have varied very widely over the period that life has existed. At the present time, many constituents of seawater are still being continually added from vari- ous sources; for instance, in ‘juvenile water’ released from basalts which flow into the seafloor along the separating boundaries of the Earth’s crustal plates (see Section 1.1.6, Plate tectonics), in volcanic gases escaping into both oceans and atmosphere and in processes of weathering and erosion of the Earth’s surface. In some cases solutes are lost from the water by precipitation, for example during deep-sea nodule formation (see Section 1.3.3). Short-term, minor fluctuations of composition occur through bio- logical processes involving absorption and release of solutes by organisms and detritus. Of major importance to marine organisms are interchanges of gases between sea and atmosphere. This is especially relevant today in the light of ocean acidification (Section 9.2.2). The constituents of seawater can be divided into four major categories: major con- stituents, minor constituents, trace elements and gases. The majority of the major  The seawater environment and ecological adaptations 53 constituents of seawater and some of the minor constituents remain virtually constant in proportion and are known as conservative constituents. In contrast, many of the minor constituents fluctuate in amount due to selective absorption by organisms and are termed nonconservative constituents. The latter include nitrate, phosphate, silicate, iron and manganese, dissolved organic matter (DOM) and dimethylsulphide. 2.2.1 Major chemical constituents and salinity The saltiness of seawater comes mainly from sodium and chloride ions, in other words common salt or sodium chloride, which makes up about 85% of the solutes. Magnesium, calcium, potassium and sulphate are the other four most abundant ions and adding these to the mix takes the total to over 99% by weight of the dissolved material in seawater. This forms an approximately 3.5% solution. The saltiness of seawater is usually referred to as its salinity (S), which is the amount of inorganic salts dissolved in seawater expressed as weight in grams per kilogram (g kg21) of seawater. The average in the open ocean is about 35 g kg21, that is S 5 35 parts per thousand (generally written 35‰). The approximate quantities of the major constituents of a typical sample of ocean water are shown in Table 2.1. Exactly where to draw the line between major and minor constituents is open to debate. The concentration of carbon in seawater is variable, but at around 0.4% it could be considered a minor constituent. However, it is obviously of major importance to marine life. The salinity of most ocean water is within the range 34‰37‰ but with extremes both higher and lower than this. The approximate average positions for global surface isohalines are shown in Fig. 2.4. The Atlantic Ocean has a higher aver- age salinity than both the Pacific and Indian Oceans and in general subtropical regions have higher salinities than the rainy belts on and near the Equator. These large-scale variations result from established rainfall patterns, evaporation in hot areas, river input and ocean circulation. Table 2.1 The major chemical constituents of seawater. Element Main ionic state in Concentration in ‰ (g kg21) in % of total seawater average (3.5%) seawater salts Chlorine (Cl) Chloride (Cl2) 19.3 55 Sodium (Na) Sodium (Na+) 10.7 30.6 Sulphur (S) Sulphate (SO422) 2.7 7.7 Magnesium (Mg) Magnesium (Mg2+) 1.3 3.7 Calcium (Ca) Calcium (Ca2+) 0.41 1.2 Potassium (K) Potassium (K+) 0.38 1.0 Total 99.2 54 Elements of Marine Ecology 180° 160° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160° 180° 80° 80° 32 34 35 Arctic Circle 32 5 60° 34 32 60° 32 32 32 34 20 32 33 35 33 40° 36 34 40° 34 38 37 39 35 37.3 40 35 36 35.5 20° Tropic of Cancer 36.5 36 36 20° 35.5 36 35 34 34 35 35 35 35 34 30 36 Equator 0° 35 0° 35 37 36 34 36 37 36.3 37.3 35 20° 20° 35 Tropic of Capricorn 36 36 35 35 36 40° 40° 35 35 34 34 Key >37 60° 36–37 60° 35–36 Antarctic Circle 34–35