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Estuaries: Where Rivers Meet the Sea. What is an Estuary. An Estuary is where freshwater from rivers merges with the ocean and is bordered by extensive wetlands (mudflats or saltmarshes ). Characteristics of Estuaries.
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What is an Estuary • An Estuary is where freshwater from rivers merges with the ocean and is bordered by extensive wetlands (mudflats or saltmarshes)
Characteristics of Estuaries • Salinity varies spatially and over the course of a day as the tides move in and out • Nutrients from the rivers enrich estuarine waters making them one of the most biologically productive environments on earth • Salt marsh grasses, algae, and phytoplankton are the major producers • Many organisms are found in the muddy bottom • Estuaries rank among the environments most affected by humans
Functions and Values of Estuaries • Provide open space and recreation • Provide natural flood control • Purify the water • Produce oxygen • Are outdoor laboratories for scientists and educators • Provide sediment traps and erosion control • Serve as nurseries for marine fishes • Provide homes for endangered species • Provide habitat for migrating, wintering and breeding birds • Serve as repositories for native plants and animals
Types of Estuaries – Four basic groups based on their origins • Drowned River Valley or Coastal Plain – Fig. 12.1– formed as a result of the sea invading lowlands and river mouths. This is the most common type of estuary.
Types of Estuaries • Bar-built estuary – Fig. 12.1 – formed when a barrier island or sand bar separates a section of the coast where freshwater enters. (Pictured below is an aerial view of Cape Hatteras, North Carolina.)
Types of Estuaries • Tectonic Estuary – formed as a result from sinking land due to movements of the earth’s crust. San Francisco Bay formed in this way.
Types of Estuaries • Fjords – Fig. 12.2 – formed in a deep valley created by a retreating glacier. (Kenai Fjord National Park, Alaska)
Physical Characteristics of Estuaries – unique because of mixing of salt and freshwater • Salinity fluctuates as the tides move seawater in and out of an estuary. • Organisms are subjected to these changes every day and therefore have adapted to these conditions. • Other factors that influence salinity include; the shape of the estuary, the bottom, wind and evaporation, and freshwater runoff
Physical Characteristic - Salinity • Salt Wedge – a layer of denser, saltier seawater that flows along the bottom Salt Wedge
Physical Characteristic - Substrate • Sand and other coarse material settle out first near the river mouth • Finer particles settle out into “main” body of water, therefore most estuaries are soft mud (silt & clay) which is very dense and rich in organic material • Decomposition by bacteria depletes oxygen in the substrate and produces hydrogen sulfide (rotten egg smell) • Tidal flow keeps water in estuaries oxygen rich
Physical Characteristics • Temperature – fluctuates widely because of tides, organisms that are exposed at low tides also face seasonal temperature changes • Turbidity – estuaries usually have large amounts of suspended sediments which can reduce water clarity
Coping with Salinity • Maintaining the proper salt and water balance is essential for all organisms. • Organisms that live in estuaries must cope with rapid changes in salinity. • Most estuarine organisms are marine species that have evolved to tolerate low salinities.
Stenohaline species can only tolerate a narrow range of salinities (yellow fin tuna)
Osmoconformers (molluscs and worms) – osmotic balance is maintained by changing with the salinity of the water
Osmoregulators keep internal salt balances constant despite salinity levels of the water in which they live
Coping with Salinity Marine fish – have a lower salt concentration than seawater, therefore tend to loose water; adaptations to maintain homeostasis include (Fig. 4.14) • Drinks seawater • Excretes excess salt through gills • Excretes small volumes of concentrated salty urine
Coping with Salinity Adaptations of other marine organisms • Sea turtles/sea gulls/sea lions have glands near the eyes that excrete “salty” tears (Fig. 4-15) • Mangroves/spartina grass – excrete excess salt through leaves Salt crystals on spartina grass
Coping with Salinity – Pickleweed accumulates large amounts of water to dilute the salts they take up.
Adapting to the Mud • Most animals burrow or live in permanent tubes beneath the sediment • Inhabitants tend to be stationary or slow moving • Salinity fluctuations are less drastic than in the water column • Organisms overcome the depletion of oxygen by: • Pumping oxygen rich water into their burrows • Some have blood that contains hemoglobin, an oxygen carrying molecule
Estuarine Communities • Different types of communities are associated with various estuaries • Open Water community includes those organisms that come and go with the tide (plankton, fishes, jellies). • Other types of communities are permanent parts of the ecosystem, these include: • Mudflats • Salt Marshes • Mangrove Forests • Sea-grasses • Oyster Reefs
Open Water Community • Organisms in this type of community include phytoplankton, zooplankton, and marine fish that are flushed in and out by the tides. • Many types of marine fish and shrimp use estuaries as nurseries for their young. • Some fish migrate through estuaries. • Anadromous – marine fish that live their lives in the sea but return to freshwater to breed (salmon) • Catadromous – freshwater fish that migrate to the sea to breed (freshwater eels)
Mudflat Communities • Low tides expose organisms to desiccation, fluctuating temperatures and salinity, and predation. • Primary produces consist of diatoms, bacteria, and some algae • Consumers are typically burrowing deposit and suspension feeders that feed on detritus
Salt Marsh Communities • Salt marshes are grassy areas that extend along the shores of estuaries and sheltered coasts in temperate and subpolar regions. • Salt marshes are subject to the same fluctuations in salinity, temperature, and tidal changes that affect mudflats. • Pronounced zonation of vegetation in salt marsh communities is a result of fluctuations in salinity, height relative to the tide, and rate of evaporation • Cordgrass (Spartiniaalterniflora) is the most common plant found where the marsh meets the mudflat. Helps to stabilize the bottom.
Salt Marsh Communities • Zonation depends on several factors: two important ones are salinity and nutrient availability. • Salinity changes depending on these conditions: • a. frequency of tidal inundation b. rainfall c. tidal creeks and drainaged. soil texture e. vegetation f. depth of water tableg. freshwater inflow • Nutrient availability varies considerably, especially the supply of usable nitrogen and phosphorous. Often there is not enough oxygen present in marsh soils to combine into useful nitrates and phosphates.
Mangrove Forest Communities • Mangrove forests are formed by mangroves, tropical and subtropical trees and shrubs adapted to inundation by seawater. • Grow on protected coasts where muddy sediments accumulate. • The red mangrove is found along the coast of Florida. Can be identified by its prop roots.
