Estuaries

Estuaries and their associated wetlands differ from other aquatic environments considered in this book in being only partially freshwater. They are, by reason of their topographical location, semi-enclosed bodies of sea water which are diluted with freshwater from terrestrial watersheds. Inflow of fresh water from the river catchment area is periodic, varying mainly with rainfall, and results in estuarine communities which are freshwater or brackish for at least part of the year.

Water movements within the estuary depend on tidal currents, estuarine circulation, river discharge, and inflow from groundwater (Figure 2.18). The extent of these movements depends to a large extent on the morphometry of the estuary basin. Although daily tidal variation in the height of water is normally within 1-2 m, this may rise to over 10 m in funnel-shaped estuaries such as the Severn estuary in the UK and the Bay of Fundy in Newfoundland, Canada.

Figure 2.18 Schematic diagram of the upper reaches of an estuary, emphasizing freshwater input into the system. A number of physical processes contribute to the temporal and spatial variability of the estuarine system. In addition to tidal currents and river discharge, which interact to determine periodic bulk flow, water movement is also caused by vertical estuarine circulation (within the water column), tide-induced residual circulations at the headlands, groundwater inflow, and run-off from surrounding land at times of high rainfall. Anthropogenic input (agricultural, industrial, and domestic) contributes to the river discharge. Physical and climatic processes controlling water movement modulate the freshwater/ saltwater exchange with fringing marshes, mudflats and the adjoining coastal region. Biological transport within the pelagic zones can be passive (e.g., phytoplankton) or active (e.g., fish migration). Mudflats and marshes provide important sites for benthic microorganisms, including bacterial and diatom populations (based on a figure from Geyer et al., 2000)

Figure 2.18 Schematic diagram of the upper reaches of an estuary, emphasizing freshwater input into the system. A number of physical processes contribute to the temporal and spatial variability of the estuarine system. In addition to tidal currents and river discharge, which interact to determine periodic bulk flow, water movement is also caused by vertical estuarine circulation (within the water column), tide-induced residual circulations at the headlands, groundwater inflow, and run-off from surrounding land at times of high rainfall. Anthropogenic input (agricultural, industrial, and domestic) contributes to the river discharge. Physical and climatic processes controlling water movement modulate the freshwater/ saltwater exchange with fringing marshes, mudflats and the adjoining coastal region. Biological transport within the pelagic zones can be passive (e.g., phytoplankton) or active (e.g., fish migration). Mudflats and marshes provide important sites for benthic microorganisms, including bacterial and diatom populations (based on a figure from Geyer et al., 2000)

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