Man and his Environment

Aquatic environments are read by light and by depth. In a lake, the littoral zone is the shallow near-shore water where plants find abundant light, anchorage and adequate nutrients from the bottom sediments; water lilies, entirely submerged vascular plants and algae flourish at its deepest region, and the plants trap sediments carried by the stream. It holds the greatest diversity of animals in the lake, from small crustaceans, insect larvae, snails, flatworms and Hydra to frogs, aquatic snakes and turtles. Outward the open water divides into an upper limnetic zone and a lower profundal zone, where light is insufficient for photosynthesis and the organisms, among them snails, certain insect larvae, bacteria, fungi and fishes, are nourished by detritus falling from the littoral and limnetic zones above.

The sea carries the same logic deeper. The bathyal zone extends from the surface to a depth of about $2000$ metres and comprises pelagic and benthic parts; it is aphotic, meaning it lacks light for photosynthesis, so it holds no producers and depends entirely on material drifting down from above. Below $2000$ metres lies the abyssal zone. Above the photic region, productivity falls with light, and the aphotic zone is the least productive area in the sea. A pond is treated as a complete ecosystem, since it holds the biotic and abiotic components and the nutrient cycling needed for life, and its producers are phytoplankton rather than plants rooted in soil. Rate of a biological activity in a nutrient-rich pond is eutrophication.