Chapter 7, Estuaries, in: Introduction to the Biology of Marine Life (9th Edition)

The Chesapeake Bay, San Francisco Bay, Great South Bay, Tampa Bay, Puget Sound, and the Mississippi River Delta, the Hudson River Bay, Columbia River and Willapa Bay in Oregon, are among 100 bodies of water designated as estuaries in the US. Over 1/3 of the US population lives within the drainage basins of these estuaries.

Types of estuaries:
a. Coastal plan estuaries;
b. Bar built estuaries;
c. Coastal lagoons;
d. Deltas;
e. Tectonic estuaries;
f. Fjords.

Because of the patterns of freshwater and seawater mixing, there is an inward flow of nutrient rich seawater along the bottom of th estuary and a net outward flow at the surface, creating an estuarine upwelling. The time necessary for the total volume of water in an estuary to be replaced is called the flushing time.

The body fluids of osmotic conformers fluctuate to remain isotonic with the water. Most estuarine animals are stenohaline, tolerating exposure only to limited salinity ranges. A few species are euryhaline, capable of withstanding a wide range of salinity.
The areas of highest elevation are the salt marshes, essentially wet grasslands or wetlands with mostly halophytes, creating a transition zone between land and estuary communities. Most of the vegetation decays and enters estuarine food webs as detritus. Microbes, decomposing bacteria, protists and fungi break down especially the cellulose cell walls, which are undigestible for most estuarine animals.
Mudflats are intertidal muds or estuarine expanses composes primarily of rich muds that are exposed at low tide. Diatoms, larger algae and seagrasses are found on mudflats. The epifauna of mudflats are dominated by gastropod mollusks, crustaceans, and polychaete worms. The intersticial water a few centimeters below the mud surface is generally devoid of oxygen. Bivalve mollusks extend tubular siphons through the anaerobic mud to the oxygenated water above.
A channel is the part of an estuary that is filled with water under all tidal conditions. Channel areas are also used as spawning and nursery areas by among others herring and sole.

The Chesapeake Bay system is a cascading series of five estuaries, linked together when the ancestral Susquehanna River valley flooded after the LGM (Last Glacial Manifestation). Existing salinity gradient creates an upper Bay low salinity zone, a mid Bay brackish zone, and a lower Bay marine zone. The tides have a vertical range of 1 to 2 meter.
Oysters spawn, mature and die within the confines of the Bay. Horseshoe crabs and menhaden are marine species that use the Bay only for early life stages. Juvenile sea turtles like the green sea turtles graze on the slowly shrinking eelgrass beds. The blue crab larvae are released and develop in coastal waters outside the Bay. Young blue crabs return to the Bay and seek eelgrass beds and salt marshes as winter nursery areas for food and protection, until they grow sufficiently to exploit other estuarine habitats.