Whole ecosystem. When we think of saline systems, we generally think of marine systems, or those influenced by seawater. Estuaries often form where rivers flow into the sea, resulting in a gradient of salinity that decreases with distance upstream from the ocean. Reverse estuaries form where gently sloped, relatively arid coastal areas are flooded by the sea, resulting in salinity gradients that increase with distance from the ocean due to evaporation and lack of fresh water inputs toward the head of the estuary. The structure of associated communities corresponds to changes in salinity, with shifts from higher proportions of marine taxa, such as polychaete worms, large clams, oceanic fish and crustaceans and seaweeds, to higher proportions of brackish and freshwater taxa, such as insects, aquatic vegetation, small clams and mussels, estuarine fish and crustaceans.
Latitudinal variation. Latitudinal variation in climate may influence the levels of salt stress, which in turn affect the structure and controls on associated communities. For example, coastal marshes of the eastern United States generally have higher salinities in the southern, hotter climate than in the northern, cooler climate. The southern marshes are dominated by succulents, with high salinity restricting the lower tidal limits of non-salt-tolerant plants. In comparison, grasses and herbs dominated northern marshes where flooding determined lower limits.
Global patterns. There are global-scale patterns of salinity, both horizontally (regional high sea-surface salinities; e.g., the Pacific Ocean is fresher than the Atlantic) and vertically (e.g., the North Atlantic Deep Water (NADW) is a mass of dense, salty water, part of which flows at a depth of 2000-4000 m on the Atlantic coast of North America). Both precipitation and evaporation affect salinity making the relatively shallow, semienclosed seas of arid climates hypersaline. These include the Red Sea, the Mediterranean Sea, and the Caspian Sea, among others. In fact, it is the large-scale vertical and horizontal distribution of salinity that helps drive global ocean circulation patterns (the thermohaline circulation of the 'global conveyor belt'; Figure 3). These patterns of circulation have profound effects on the ecology of the world, driving climate patterns, propagule distribution, and numerous other large-scale ecological patterns.
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