Some Major Ecosystems

The aquatic or hydrospheric ecosystem is a system in which living and non-living components exchange materials and energy within water. Salt-water oceans and seas are the largest ecosystems on earth, forming about $71$ of its surface, while fresh water ecosystems cover less than $1$. Water lends aquatic systems four common features. It changes temperature more slowly than air, so the temperature range is moderate enough to support life. Although it looks transparent, it absorbs a considerable amount of light, and even in the clearest water the intensity falls so fast with depth that at $600$ feet or more there is barely enough light for photosynthesis. Nutrients tend to be concentrated near the bottom sediments, in the very places where light is often too poor to use them. And water is abundant, at an appropriate temperature for metabolic work.

A lake is read zone by zone, and the dividing line is light. The littoral zone is the shallow near-shore water, where plants find abundant light, anchorage and nutrients from the sediments; it holds the most diverse plants, with water lilies, entirely submerged vascular plants and algae at its deepest edge, and it traps sediment carried by the stream, raising the nutrient content. It also holds the greatest diversity of animals, from small crustaceans, insect larvae, snails, flatworms and Hydra to frogs, aquatic snakes and turtles, together with the plankton. Phytoplankton are drifting photosynthetic protists, bacteria and algae; zooplankton are drifting animals such as protozoa and tiny crustaceans. Out in open water, the upper limnetic zone is well lit, and below it the profundal zone has light insufficient for photosynthesis, so its organisms, including decomposers and detritivores such as snails, insect larvae, bacteria, fungi and fishes, are nourished by detritus falling from above.