Estuary Biomes: Definition and Ecology

Estuaries: The Transition Between Freshwater and Marine Biomes

• Estuaries exhibit distinct, variable, and fascinating environments and ecologies due to the blending of fresh and saltwater.
• They support diverse invertebrate populations and serve as crucial nursery grounds for fish and feeding areas for birds.
• They are sometimes considered the most degraded habitat, and provide an ideal setting for studying ecology, conservation, and human impact interactions.
• Estuarine study requires comprehensive ecosystem knowledge.

Introduction to Estuaries

• Most view an estuary as a place where a river meets the sea
• This interaction yields diverse systems, with an estuary being only one type.

Defining an Estuary

• Defining an estuary has proven difficult, with over 40 definitions proposed, with varied criteria like seawater dilution, tidal regime, and geomorphology.
• Traditional focus on salinity can overshadow other environmental and biological variables.
• The Amazon estuary has a tidal influence reaching 735 km inland, but insignificant seawater penetration due to vast river flow; mixing occurs 1000 km seaward.
• Defining by tidal influence differentiates deltas from estuaries.
• Defining an estuary is an inlet of the sea reaching into a river valley up to the tidal rise limit
• Typically divisible into three sectors: (1) lower (free connection to open sea); (2) middle (strong salt/freshwater mixing); (3) upper (freshwater with daily tidal action).
• Defining boundaries using salinity can be difficult
• In unmanaged estuaries, the upper boundary is that of daily tidal influence, truncated by structures in urban areas.
• Defining outer limits is harder; large rivers create reduced salinity plumes far into the sea.
• Estuary boundaries are usually subjectively defined by coastal morphology or management criteria.
• Estuaries are part of a continuum from river to sea, with both biomes strongly affecting estuarine ecology.

Estuary Evolution

• Estuaries differ from marine systems due to their geological ephemerality in temperate zones influenced by glaciation.
• High sedimentation rates lead to infilling and ecological succession.
• During the last glaciation (17,000 BP), sea levels were 130m lower, with a rapid rise following ice sheet retreat, flooding continental shelves and river valleys to create modern estuaries by 6000 BP.
• Because of this, most estuaries are new systems with implications for colonization, diversity, and ecology.

Estuary Classifications

• Estuaries vary globally in geomorphology, tidal range, and salinity, leading to various classification schemes for comparison.
• Topography is a popular classification method that reflects formation after post-glacial sea-level rise.

Topographic Classification

• Coastal Plain estuaries are the most common, formed by river valley flooding after eustatic sea-level rise.
• Fjords are deep estuaries from glacial U-shaped valleys, with a shallow sill limiting seawater exchange and causing anoxic basins.
• Bar-built estuaries form from offshore deposits that restrict seawater flow, creating diluted water bays.
• Estuaries can also be created via isostatic variation in areas of tectonic activity: the San Francisco Bay formed from land sinking.

Change in Sea-Level

• Change in sea-level can be either eustatic (variations in ocean volume), or isostatic (land level variations).
• While topography is a common classification method, it has little ecological relevance except on a broad scale.
• Fjords are typically boreal, while blind estuaries are more tropical.

Tidal Range Classification

• Tidal range also classifies estuaries, having major dynamic and ecological impact, separating them from brackish seas.
• Estuaries exhibit varied tidal amplitudes, from under a meter in tropical systems to over 16m in the Bay of Fundy, Canada.
• The tidal prism (water volume difference between high and low tide) in the Bay of Fundy is ~100 billion tonnes!
• Estuaries are categorized by tidal range: microtidal (<2 m), mesotidal (2-4 m), macrotidal (4-6m), and hypertidal (>6m).
• Urbanization, especially encroachment and flood defenses, enhance these ranges through channel narrowing.

Tides

• Tidal movement is a key estuarine descriptor
• Also, it affects organism function, survival, and distribution by varying physicochemical parameters
• Estuaries are naturally stressful aquatic systems due to environmental variability.

Important Environmental Variables in Estuaries

• Key environmental conditions like salinity, sediment, dissolved oxygen and temperature affect estuarine life

Salinity

• Historically, salinity was considered the dominant factor that defined estuarine survival and distribution of organisms; salinity remains an important, widely variable factor.
• Salinity has two components: spatial/longitudinal variation (0 at river end to 34 at mouth) and temporal (tidal) variation.
• Salinity in tropical estuaries can exceed seawater salinity due to evaporation.
• The most significant salinity variation: conditions changing over time.
• Salinity ranges in the mid-Ythan, Scotland can range from 3 to 29 over a tidal cycle.
• Salinity is measured as Practical Salinity Units (PSU), represented as a number without units.
• Salinity range decreases toward both ends of an estuary away from the mid-estuary.
• Trapped water salinity is less variable in intertidal areas, where heavier seawater fills in and lighter freshwater flows on top of the sediment.
• Salinity of interstitial water in subtidal sediments mirrors overlying water.
• Seasonal variations in freshwater flow rates can influence estuaries.
• Freshwater flow impacts current speed and saltwater interaction, plus the overall estuarine structure

Sediment

• Intertidal mudflats, of soft grey silt, compose estuaries and are difficult to traverse, while emitting a sulphurous odor.
• This mobile sediment makes it difficult to colonize so it has no points of macroalgae attachment/sessile organisms; organisms avoiding mechanical disturbance cannot manage the substratum.
• Fine particles can clog feeding and respiratory structures, which makes locomotion challenging.
• Thriving organisms need to be adapted to the sediment moreso than salinity.
• Large mudflats form within the middle reaches of the estuary under normal conditions, and are comprised of seafloor material.
• Sediment deposition results from the speed of tidal/river currents, but there is an excess of suspended sediment concentration towards the estuary for various reasons.
• Sediments near estuary mouths and heads are coarser due to high tidal/river flows, preventing fine particle settlement.
• Mudflats found in estuaries are full of material that settled out of the water column
• Estuaries have one of the highest levels of secondary production in aquatic systems.
• Deposit-feeding organisms can tolerate stressful mudflat conditions and have extremely high growth rates due to abundant, high-quality food.

Dissolved Oxygen

• Low oxygen levels are uncommon in aquatic systems, so estuaries have some unique conditions that affect life.
• Estuarine waters and sediments have high organic matter and microorganism levels, which consume dissolved oxygen.
• Aerobic bacteria breakdown matter in estuarine waters which depletes oxygen; head/mouth of estuary have normal oxygen.
• An oxygen sag occurs in middle estuaries.
• Oxygen removal from interstitial water in mudflats is more severe; diffusion is prevented by sediment
• Matter is broken down by anaerobic bacteria below the surface by oxygen removal, causing toxic by-products like methane and hydrogen sulfide.
• The effect is black mud with a rotten egg smell (occurs in unpolluted estuaries); adds stress to organisms.
• Low estuarine oxygen is severely impacted by human life.
• Human settlement on estuary banks stimulates removal of oxygen via sewage disposal, organic matter supplementation, and bacterial action.
• Estuarine waters can become oxygen-depleted, threatening ecology when urbanized.

Temperature

• Limited temperature variables, but there are seasonal temperature patterns changing the major trigger for migration and reproduction (fish + large crustaceans).
• Temperate change can be influential and a major ecological factor; temperature also influences the speed of microbial breakdown.
• Bacteria process organic matter rapidly in the tropics/summer of the temperate systems.
• During urbanized areas, oxygen depletion is more severe during the summer when temperate water occurs
• The discharge of water can affect temperature levels
• Thames Estuary’s water temperature increased by three degrees because of power stations.

Conditions Linking Human Influence

• Conditions surrounding and within estuaries are directly connected to human activity impacting the water quality, fisheries, and/or alteration of the surrounding bankside environment.
• Estuaries are the most anthropogenically degraded habitat, and that must be considered when investigating ecology.

Estuary Zonation

• Estuaries are traditionally divided into zones reflecting overall environmental conditions apparent in the region for additional sub-classification.
• Schemes are based on various salinity levels and are highly subjective
• Zones are hard to spatially define in a system that's changing via tidal and seasonal cycles.
• Pritchard's (1967) scheme is based on the mixing of saline and fresh water

Pritchard's Zonation Scheme

• Freshwater zone is defined as the river above the tidal influences
• Tidal freshwater zone is subject to tidally influences, but is essentially freshwater
• Mixing zone features a steady increase in salinity, as distance from the river is gained
• Seawater zone is defined as marine waters with little to no dilution
• Mixing Zone includes: upper mixing (freshwater dominant), middle mixing (steady salinity increase), and lower mixing (marine influence is dominant).
• Location varies with state and freshwater flow.

Estuarine Organisms

• Most of them have evolved elsewhere
• Few organisms have evolved to exclusively live there
• Estuarine organisms are descendants of marine/freshwater and have developed mechanisms to survive

Evolution of Estuarine Organisms

• There is complex disagreement surrounding estuarine evolution, but here are the leading theories:
  • Estuarine evolved from: marine species that tolerate low salinity
  • Estuarine species evolved from: freshwater that tolerate high salinity
• Likely that both processes occurred

Challenges for Organisms

• Maintain a stable internal environment in the face of salinity change
• Known as osmoregulation
• Osmoconformers allow their internal salinity to vary
• This is a passive strategy
• Osmoregulators maintain a stable internal salinity.
• Active/energy demanding

Osmoregulation

• Organisms are osmoregulators because it's hard for osmoconformers to maintain stable internal
• The sediment makes organisms more stable/osmoconformers

Adaptations of Estuarine Organisms

Adaptations

• Have developed adaptations to survive that can be divided into categories
• Physiological Adaptations: affect organisms internal functioning
• Behavioural Adaptations: Affect organisms behavour
• Life History Adaptations: Affect organisms lifecycle

Physiological Adaptations

• Most important: osmoregulation because changes in salinity makes it hard for osmoconfomers
• Important adaption tolerate levels of disolved O2
• Levels are low in sediment
• This helps them extract oxygen from water and breathe
• Has allowed them to switch to respiration when O2 levels are low

Behavioural Adaptations

• Most important burrow, allows them escape predators and avoid currents
• Helps escape salinity and temp
• Ability to migrate
• Move to new habitat/conditions that are good
• Estuarine fish transfer between Estuaries and sea

Life-history Adaptations

• Ability to be able to reproduce quickly
• Reproduce likely to take advantage

Dispersal

• Important adaptation, ability to dispersed widely
• Organisms that dispersed are mor likely to fine habitats

Diversity of Estuarine Organisms

• Estuaries are home to a range of organisms including algae plants invertebrates fish and birds
• Diversity influenced by: salinity, temp, sediment type, food O2 availability

Bacteria

• Most abundant
• Play role in decomposition and cycling nutrients

Algae

• Algae: abundant and have food
• Are important primary producers use sunlight to produce food
• Plants
• Home to plants (mangroves) plants animals use for food

Invertebrates

• Animals w/out backbones most diverse
• Play role in foodweb and helpful filter wate

Fish

• Very important habitat for fish
• They are food for people

Birds

• Great habitat for birds
• Food from crustaceans/fish

Importance of Estuaries

• Many benefits for animals and humans

Importance

• Water filtration helps to filter H2O
• Flood control helps absorb it
• Fisheries: important for Fish
• Recreation: for boating
• Tourism: major

Global Economy

• Worth trillions

Threats to Estuarie

Threat

• Pollution from sewage/agriculture harm

Habitat Loss

• Harm from development/activities

Climate Change

• Cause sea levels to rise
• Inundating

Invasive Species

• Outcomplete

Estuarine Conservation

Important for benefits they contribute

Conservation

• Recycle
• Conserve
• Restore
• Impact

Estuarine Habitats

• Variety of Habitats including

Habitats

• Mudflaps
• Salt marshes
• Mangroves
• Seagrass
• Oyester Reefs
• Unique community

Mud Flats

• Common
• Worms crab food for bids

Saltmashes

• Birds other animals upper

Mangroves

• Protect from the ocean
• Fish crabs

Seagrass beds

• Found shallow h2o
• Fish crabs

Oyster Reefs

Currents

Estuarine food webs

• Consists of phytoplankton, base made zooplankton they eat each oth

Consumers

• Zooplanktoninvertebrates

Decomposers

• Break down plant
• Estuaries are productive one earth

PromaryProduction

• Rate producers make organic
• Estuaries receive nutrients

Secondary Production

• Secondary production
• Consumers
• Many factors impact

Factors

Nutrient availability

• Important availability

Light availability

• Photosynthesis

Temp

• Metabolic rate affect

Salinity

• Distrubution

Pollution

• HArm reduce productivity

Food Webs

• Base estuarine food webs
• Eats