Patchy salinity patterns. Even within a very localized system, there can be fine-scale variations in salinity influenced by other abiotic variables, such as substrate type and microtopography, and biotic variables, such as substrate organic content and the presence or activity of local species. Substrates of fine particle size and high organic matter, for example, may maintain lower salinities due to lower evaporation rates than coarser, more mineral soils. The presence of microtopography (depressions, peaks, slopes) may also vary evaporation rates and therefore salinity. The presence of organisms that directly or indirectly alter salinity and, in turn, the associated community (i.e., ecosystem engineers) may contribute to fine-scale salinity variability. The presence of shade-casting species, such as plants with dense canopy like trees, grasses, or mat-forming species would likely maintain lower salinities than areas without such (or any) plants.
Activities of organisms present could also alter salinity -bioturbators might turn surface soils minimizing evaporation and salinity accumulation, and salt-excreting species, such as tamarisk, may increase local salinities.
Vertical gradients. Salinity gradients exist across intertidal zones, with distance from the sea into the upland. Whether tidal flats, marsh, rocky intertidal, or sandy beach, salinities generally increase in arid regions and decrease in nonarid regions as one heads from lower to upper intertidal. Freshwater upland runoff and evaporation rates of tidal water, wave swash, and sea spray determine the extent and severity of the gradient. Organisms are thought to be limited by abiotic factors, such as high salinity and desiccation or freshwater runoff, in the upper intertidal and biotic factors, such as predation or competition, in the lower intertidal where stresses are not limiting.
Salinity gradients can also be found in water columns. In estuaries where wave energy and turbulence is low, salinity can have complex vertical patterns. Fresher river water often flows over the denser seawater in estuaries, forming a salinity wedge that migrates horizontally with the tides. The duration and thickness of the wedge varies with rainfall and runoff, as well as amount of mixing due to wind and water turbulence. Some organisms, such as crab larvae, may use these salinity gradients and boundaries as a cue for migration direction or settlement. While mobile species may be able to avoid undesirable salinities, less mobile species need to be able to tolerate the daily and seasonal fluctuations associated with tidal cycles and weather patterns.
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