Salt Marshes and Estuaries
Muddy’bottomed salt marshes are among the most interesting inter-tidal shores. Much of the high primary productivity of a salt marsh comes from sea grasses, mangroves, and other vascular plants that can prosper in a marine (or partly marine) environment.
Salt marshes often form in estuaries, a broad, shallow river mouth where fresh water and salt water mix. A typical characteristic of estuaries is reduction of wave shock. Surf is blocked from estuaries by long-shore bars or by twisting passages connecting to the ocean. The salinity of water within an estuary may vary with tidal fluctuations, from sea water through brackish water (mixed salt and fresh water) to fresh water. Many of the organisms living in estuaries are able to tolerate varying salinities; but in areas near the river entrance the water may be almost fresh, while near the outlet it may be of oceanic salinity. These different salinities often lead to distinct horizontal zonation of organisms. Temperature range is also potentially extreme, especially in the tropics or during the temperate-zone summer, when receding tide abandons residents to the heat of the sun. Strong currents may move in estuaries as the tide rises and falls and the river flows. Flowing water takes the place of waves in mixing nutrients and gases in the inter-tidal estuary community.
Estuarine marshes are richer and exhibit greater species diversity than marfishes exposed only to seawater. Primary productivity in estuaries is often extraordinarily high because of the availability of nutrients, the great variety of organisms present, strong sunlight, and the large number of niches. Decomposition of fast growing, salt tolerant plants provides the raw material for the large and complex food web and rapid nutrient turnover characteristic of these communities. The standing biomass (mass of living matter per unit area or volume) in a typical estuary is among the highest of any marine community.
Estuarine organisms show unique adaptations to their rich and variable environment. Some estuarine plants trap silt particles at their roots, thus countering the erosive action of current w. Small plants are often lamentous, bristling with tiny projections used to anchor themselves to the substratum.
Large plants have extensive root systems to hold themselves in place and to colonize new areas. Most of the resident animals burrow into the muck, scurry rapidly across the surface, or hide in the vegetation. Clamshell and snails work their way through the substratum, obtaining food and shelter at the same time. Some worms dig for targets of opportunity, and crabs dart for any interesting morsels.
Estuaries are sometimes called marine nurseries because so many juvenile organisms are found there. This is especially true for fishes. Many pelagic species spend their larval lives in the protective cones of an estuary, taking advantage of many feeding opportunities available. The human pressure of development and pollution are thus doubly stressful in estuaries, affecting both permanent residents and sensitive larval stages of open water animals.