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Chapter 7
EstuariesEstuaries
Estuaries are semienclosed coastal embayments where freshwater rivers meet and mix with the sea, creating unique and complex ecosystems.
More than one third of Americans live within the drainage basins of our 100 or so estuaries.
Copyright © 2004 Jones and Bartlett Publishers
Types of Estuaries•Estuaries are classified both by their modes of formation and by their patterns of water circulation:– Coastal plain estuaries – Bar-built estuaries – Deltas – Tectonic estuaries
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Types of Estuaries
•Fig. 7.2 Satellite images of three types of estuaries: (a) two coastal-plain estuaries on the U.S. East Coast, (b) several bar-built estuaries, and (c) steep-sided fjords.
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(a) (b) (c)
Estuarine Circulation•Salinity in estuaries typically increases from the surface downward and from the estuary head downstream to its mouth.
•Because of the higher density of seawater, incoming tides move along the floor of the estuary and riverine inputs exit along its surface.
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Estuarine Circulation
•Fig. 7.3 The general pattern of fresh water and sea water mixing in an estuary.
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Estuarine Circulation
•Detritus and other organic nutrients that settle out of the river water are transported back up into the estuary with each tidal cycle, thus creating a unique upwelling process that greatly augments estuarine production.
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Estuarine Circulation
•Fig. 7.4 Cross-sections of a well-mixed (a) and stratified (b) estuaries, with the resulting salinities shown.
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Salinity Adaptations
•Slow-moving invertebrates are osmoconformers. They avoid salinity fluctuations by remaining near the high salinity mouth of an estuary or traveling in and out with the tides such that they always remain within water of a relatively constant concentration.
•All estuarine vertebrates, and most fast swimming invertebrates are osmoregulators. They regulate and stabilize the water and solute concentrations within their body fluids in spite of ambient conditions.
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Salinity Adaptations
•Fig. 7.7 Variations in ion concentrations of body fluids with changing external water salinities for osmotic conformers, partial osmotic regulators, and osmotic regulators.
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Sediment Transport: Creating Habitats
•Sediments are transported into estuaries by rivers that drain coastal watersheds.
•These sediments create some types of estuaries, such as deltas and bar-built estuaries
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Sediment Transport: Creating Habitats
•They also settle out such that large-grained course sediments dominate estuarine heads where current velocity is high and muddy sediments predominate the floor at the mouth of estuaries.
•Sediments not only create the estuary itself, but also promote the development of various microhabitats within the estuary from its head to its mouth.
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Estuarine Habitats and Communities
•Salt Marshes– Salt marshes are intertidal grasslands that
grow along estuarine shores. – Although some grasses are eaten directly by
herbivorous residents and visitors, most grass production enters the estuary as decaying detritus.
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Estuarine Habitats and Communities
•Salt Marshes
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Fig. 7.9 Two types of emergent salt-marsh plants: (a) a stand of salt grass, Spartina, with taller mangroves behind; (b) pickleweed, Salicornia.
Estuarine Habitats and Communities
•Mudflats– Mudflats, or tideflats, are broad expanses of
nutrient-rich fine-grained muds that are exposed during low tide.
– They act as both home and food source for a wide variety of infauna and epifauna.
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Estuarine Habitats and Communities
•Mudflats
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Fig. 7.10 Plant-dominated salt marsh, mud flat, and channel habitats of Chesapeake Bay and other East Coast estuaries, showing their vertical position relative to high tide.
Estuarine Habitats and Communities
•Channels– Channels are home to organisms that cannot
tolerate intertidal life in wetlands or on mudflats.
– Some of these, such as crabs, oysters, and killifishes, live in estuarine channels throughout the year, whereas others simply visit the estuary to spawn (such as herring, soles, and sandbar sharks).
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Environmental Pollutants•Because estuaries are usually supplied with fresh water by rivers and streams that collectively drain vast watershed areas, many pollutants commonly get carried to estuaries where they are trapped and accumulate.
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Environmental Pollutants
•Oxygen-depleting Pollutants Include:– Agricultural fertilizers– Poorly treated sewage
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Environmental Pollutants
•Oxygen-depleting Pollutants
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Fig. 7.15 SeaWIFs satellite view of the U.S. Gulf Coast, with the dead zone at the mouth of the Mississippi River indicated by black.
Environmental Pollutants
•Toxic Pollutants– Heavy metals and chlorinated hydrocarbons
are unquestionably toxic.– The effects of several toxic pollutants are
compounded by their tendency to bioaccumulate.
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Environmental Pollutants
•Toxic Pollutants
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Fig. 7.16 Transfer of DDT to and within marine food webs (arrows). Adapted from Epel and Lee 1970.
The Chesapeake Bay System•The Chesapeake Bay system, a cascading series of five major and numerous minor estuaries, exemplifies the physical, chemical, geologic, and biologic features of a typical estuary.
•It is also a microcosm for the substantial social, economic, and political problems generated by conflicts between natural estuarine processes and the many ways in which we choose to use these unique coastal habitats.
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The Chesapeake Bay System
•Fig. 7.17 Chesapeake Bay and its numerous smaller side estuaries, showing mean surface salinity zones.
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