Introduction to Estuarine Ecology
John W. Day Jr., Alejandro Yáñ;ez-Arancibia, W. Michael Kemp,
and Byron C. Crump
1.1 Background, Theory, And Issues
We begin this description of estuaries and their functions by defining estuaries very broadly as that portion of the earth's coastal zone where there is interaction of ocean water, fresh water, land, and atmosphere. Large estuarine zones are most common in low relief coastal regions such as the broad coastal plains of Europe and the east coast of North America. They are much less common in uplifted coastlines such as the Pacific edge of North and South America. We begin our assessment as broadly as possible to include all portions of the earth that interact at the edge of the sea because these regions influence the smaller scale ecosystems sometimes more narrowly defined as estuaries proper.
From the vantage point of an orbiting satellite, several of the most basic attributes of estuaries are observable. Plumes of sediment-laden water float seaward on the ocean surface from the largest rivers, such as the Amazon, the Ganges, and the Mississippi. Color differences among various water masses, representing waters of different histories and different biotic richness, are often apparent. Coastal waters in areas with significant riverine input and broad shelf areas generally appear more greenish brown than the deep blue waters adjacent to many other coastlines. There are also atmospheric features of importance to estuaries obvious from space. Clouds commonly form directly over the edges of continents as one manifestation of the atmospheric "thermal engine" that maintains the freshwater cycle on which estuaries depend. At the altitude of a satellite, the dense human populations that proliferate in coastal zones are outlined at night by their lights.
The two most recent geological epochs, collectively named the Holocene, could be called the age of the estuary , for estuaries are abundant today, even though they are geologically tenuous. All present day estuaries are less than about 5000 years old, representing the time since sea level reached near its present level following the last ice age. Human populations flourished during this same period, in no small measure owing to exploitation of the rich estuarine resources of the coastal margin. Most "cradles of civilization" arose in deltaic and lower floodplain areas where natural biota was abundant and where flooding cycles produced the rich bottomland soils and readily available freshwater supplies on which agriculture flourished (Kennett and Kennett, 2006; Day et al., 2007). Early centers of civilization that developed in estuarine or deltaic environments include those of the Tabascan lowlands of Mexico; the valley of the Nile; Tigris-Euphrates, Yellow, and Indus Rivers; and along the Andean coast of western South America where upwelling systems bordered estuarine systems.
Let us now continue our aerial survey of estuaries, but this time at a much lower altitude, about 1000 m, in a light airplane following the course of a coastal plain river in the temperate zone from its headwaters to the ocean. The headwater river is narrow with rapids and falls, but changes near the coast to a larger meandering form with broad marshy areas where the actual edge of the river is not always clearly evident. The color of the water changes from clear blue to yellowish brown as the river picks up silt. As the river water nears the coast tidal currents become apparent and, moving seaward, the influence of tidal currents becomes greater.
Along the banks of the estuary, fresh and brackish water marsh plants grow at the edges of embayments. These marshes are often flanked by rows of houses and yards and spanned by narrow piers to provide access to deeper water. Among these marshes, a variety of wading birds may be observed stalkin