Chapter 12
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Chapter 12. Estuaries: Where Rivers Meet the Sea. Estuary. Semi-enclosed areas where fresh water and seawater meet and mix Close interaction between land and sea Among the most productive environments on earth Among the environments most effected by humans
Chapter 12
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Chapter 12 Estuaries: Where Rivers Meet the Sea
Estuary • Semi-enclosed areas where fresh water and seawater meet and mix • Close interaction between land and sea • Among the most productive environments on earth • Among the environments most effected by humans • Many cites are located along them (NY, London, Tokyo)
Environmental Impacts • Dredged or filled and transformed into marinas, seaports, industrial parks, cities and garbage dumps
Origins and Types of Estuaries • Scattered along the shores of all the oceans and vary widely in origin, type and size • May be called lagoons, bays • Drowned river valleys or coastal plain estuaries (formed when the sea invaded lowlands and river mouths)
Bar built estuary – formed from the accumulation of sediments that build up and create sand bars and barrier islands dividing fresh and salt water • Tectonic estuaries – formed when the land sank – San Fransicso Bay • Fjords – created from deep cuts by glaciers that later filled in by rivers
Have a unique combination of physical and chemical characteristics from the mixing of salt and freshwater
Salinity • Fluctuates dramatically from place to place and time to time • Seawater 35 o/oo and freshwater 0 o/oo so salinity is somewhere in between • Decreases as you move upstream • Varies with depth – salinity increases as you go down because of density
Salt wedge – seawater is more dense and flows along the bottom with fresher water on top – moves back and forth with the tides
Substrate • Sand to soft mud • Particles are carried in by the river and settle out as the river slows • Mud (mix of silt and clay) is rich in organic material • Sediments are often anoxic – decay bacteria use up the oxygen in the interstitial water
Other Physical Factors • Water temperature varies because of shallow depths and surface area • Large amounts of suspended sediments are typical – reduces water clarity – reduces light
May look like a wasteland • Really are tremendously productive • Home to large numbers of organisms that are of commercial importance • Provide vital breeding and feeding grounds
Live revolves around the need to adapt to extremes in salinity, temperature and other physical factors • Change rapidly and in many ways • Life is not easy so few species have successfully adapted to estuarine conditions
Coping with Salinity • Maintaining the proper salt and water balance of cells and body fluids is one of the greatest challenges facing estuarine organisms
In General • Marine fish – have a lower concentration of salts in their blood as compared to the seawater – constantly drink water – secrete small volume of very concentrated urine – salt also excreted by gills
Freshwater fish – concentration of salt in their blood in greater than the surrounding water so they constantly gain water through osmosis over their skin and gills • Do not drink, secrete large volume of dilute urine, salts are absorbed by gills
Osmosis in an estuary • Most estuarine organisms are marine species that have developed the ability to tolerate low salinities • Euryhaline – organisms that can tolerate a wide range of salinities – most estuarine organisms are • Stenohaline – tolerate a narrow range in salinity
Brackish • Water is intermediate salinity • Some organisms are adapted to this type of environment
Dealing with Osmosis • Since most estuarine organisms are from a marine background they tend to take on water through osmosis • Move • Osmoconformers – body fluids change with the salinity (mollusks and some polychaete worms)
Osmoregulators – keep the salt concentration of their body fluids more or less constant • Salinity lower than blood – get rid of excess water through active transport • Fishes, crabs and some mollusks
Adapting to Mud • Nothing to hold on to • Organisms must burrow or live in permanent tubes beneath the sediments surface • Hard to move in mud – stationary or slow moving • Salinity does not change as much
Oxygen concentrations are low from the decay • Pump oxygen rich water • Blood that contains hemoglobin that holds oxygen
Consist of few species • Species present are in large numbers
Open Water • Type and abundance of plankton varies tremendously with the currents, salinity and temperature • Murky water also limits photosynthesis • Rich supply of fish and shellfish in or near the estuary • Nurseries for young – abundant food
Mudflats • Bottoms of estuaries that become exposed at low tide • Can be extensive where there is a large tidal range and a gently sloping bottom
Low tide in a mudflat • Desiccation • Variations in temperature • Predation • Variations in salinity
Mudflat organisms • Primary producers are not present • Benthic diatoms • Bacteria • Infauna – feed on detritius – deposit and suspension feeders • Bivalves (quahog - Mercinaria mercenaria and soft shelled clam Mya arenaria and razor clams Ensis), fiddler crabs
Mya arenaria Razor Clam
Fishes and birds – important predators • Fishes – high tide • Birds – low tide • Important stopover and wintering areas for many migratory birds
Salt Marshes • Estuaries in temperate and subartic regions usually bordered by extensive grassy areas that extend inland from the mudflats • Also develop along sheltered open coasts • Partially flooded at high tide • Also known as tidal marshes
Wave action is minimal to allows the accumulation of muddy sediments • Tidal creeks, freshwater streams and shallow pools frequently cut through the marsh • Have extremes in salinity, temperature and tides like the mudflats • Muddy bottom held together by the roots
