Chapter 12

1. Chapter 12 Estuaries: Where Rivers Meet the Sea

4. 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)

5. Environmental Impacts • Dredged or filled and transformed into marinas, seaports, industrial parks, cities and garbage dumps

7. 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)

8. 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

10. Passive margins have broad well developed estuaries

11. Physical Characteristics of Estuaries

12. Have a unique combination of physical and chemical characteristics from the mixing of salt and freshwater

13. 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

14. Salt wedge – seawater is more dense and flows along the bottom with fresher water on top – moves back and forth with the tides

17. 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

18. 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

19. Estuaries as Ecosystems

20. 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

22. Living in an Estuary

23. 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

24. Coping with Salinity • Maintaining the proper salt and water balance of cells and body fluids is one of the greatest challenges facing estuarine organisms

25. 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

26. 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

28. 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

29. Brackish • Water is intermediate salinity • Some organisms are adapted to this type of environment

30. 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)

31. 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

34. 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

35. Oxygen concentrations are low from the decay • Pump oxygen rich water • Blood that contains hemoglobin that holds oxygen

36. Types of Estuarine Communities

37. Consist of few species • Species present are in large numbers

38. 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

39. 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

42. Low tide in a mudflat • Desiccation • Variations in temperature • Predation • Variations in salinity

43. 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

44. Mya arenaria Razor Clam

46. Fishes and birds – important predators • Fishes – high tide • Birds – low tide • Important stopover and wintering areas for many migratory birds

48. 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

50. 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