Estuarine Ecology

Estuary - costal, semi-enclosed extended interface between marine and lotic (flowing freshwater) system

Dynamic, noisy and geologically ephemeral

Streams don't produce estuaries - not strong enough

Different definitions based on tidal regime and influence, salinity profiles and geomorphology

Fjord-type (U-shaped) - glacier formation

Ria-type (V-shaped) - river formation

Costal plain type - rivers at the end of their life, creates marshes

Bar-built - sand/silt bars build up due to offshore deposits

Blind - closed off by offshore deposits

Delta-front - riverine deposit formation

Tectonic - earthquake/tectonic driven (San Fracisco Bay)

Caused by balance between volume of river flow and strength of tides

Salt water - denser than freshwater = sinks to bottom

Tides mix water = stronger tides means less variation in salinity

Salt wedge - strong river flow, little mixing

Partially mixed - gradutation of salt to freshwater

Well mixed - vertical mixing leading to horizontal change in salinity

Effects development of salinity profiles - higher tidal range = more mixing

Higher amplitude waves = stronger tidal currents

Tidal resonance - amplification due to tidal waves = high tides (>16m - Bay of Fundy)

• Microtidal - <2m (Poole Harbour)
• Mesotidal - 2-4m (Southampton Water)
• Macrotidal - 4-6m (Thames)
• Hypertidal - >6m Severn

Tides influence by regional/local topography e.g. flow channels

Semi-diurnal (1 a day) or diurnal (2 a day) tides

Both terrestrial and marine sediment inputs in estuaries

Water reaching estuary has reduced sediment carrying capacity as opposing tidal and riverine flows cancel

Heavier sediments deposited first e.g. mud + coarser material e.g. sands deposited at estuary head/mouth = Diverse range of habitats w/ nutrient-rich waters

Micronutrients - interact w/ waters and change oxidation states

• In reduced state - metal ions dissolved and in presence of sea water = precipitation

Effects of sediments and nutrients:

• Increase in deposition rates on flats - decreases water flow and creates salt marshes
• Reduction in erosion rates - biofilms bind sediments
• High nutrient loading and organics - fuels primary/secondary production
• Increase turbidity - reduces light pentraction = reduced photosynthesis
• Organic loading increases bacterial. activity - reduced oxygen and eutrophication

Multiple primary production sources due to diverse habitats

Upper areas - freshwater marshes and phytoplankton

Mid to lower - more benthic algae, marine phytoplankton, sea grass beds and salt marshes

Dominanted by secondary production - terrestrial detritus from rivers and marine allochthonous particulate organic matter 

Bacteria production from sediments in mud flats - large SA for bacteria to grow

Black mud = reduced mud due to bacterial activities

Comination of 1' and 2' production similar to tropical rainforests/coral reefs

Salinity vary depending on time of day - high and low tides

Organisms cannot cope w/ large variation = adaptations to prevent adverse effects (cell crenation or lysis)

Osmoconformers - osmotic pressure matches surrounding areas

Osmoregulators - expend energy to actively excete water

Euryhaline - tolerates large salinity ranges

Stenohaline - tolerates narrow salinity ranges

Fish have lower internal osmotic body pressures than environment

Marine fish actively secrete salts e.g. Cl- (Na+ follows down electrochem gradient)

Freshwater fish swallor lots of water - water contains little ions so actively transported into body

Organisms move in and out of estuary with tide = prevents salt variation affecting them

Organisms bury themselves in sand - reduced salinity fluctuations in interstitial fluids in sediments

Ramane diagram - show steep reduction in freshwater species with salinity

Steep drop diversity - upper to mid estuary

Brackish animals take over afterwards

Diadromous species common in estuaries = fish spending part of life in freshwater and part in salt water

Invertebrates - dominant epifauna and infauna

• Crabs, shrimps, gastropods, polychaetes, bivalves

Fish - use estuaries as nursery groups/move in with tides 

• Flounder, bass, whiting, herring, salmon, eels

Birds - migratory visitors

• Geese, ducks, waders, gulls, cormorants

Plankton 

• Phytoplankton - diatoms and dinoflagellates
• Zooplankton - copepods, scyphozoans and larval stages

Estuaries easy access to sea and sheltered = human settlement hotspot

Harbour construction and dredging - large-scale mods = damage sea beds and remove habitats

Effluent from harbours - nutrient-rich = eutrophication 

Pollutants e.g. antifouling agents and industrial/agricultural runoff - affect taxa and accumulate in food chains

Large, enclosed seas

Limited ocean water exchange and large freshwater inputs

Examples - Baltic Sea, White Sea, Black Sea, Sea of Azov

Sea of Azov - shallowest sea w/ high variable salinity and high evaporation rates 

= salt easily mined from it