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Only 3% of the world's water is fresh. And 99% of this is either frozen in glaciers and pack ice or is buried in aquifers. The remainder is found in lakes, ponds, rivers, and streams.
As one descends deeper in the limnetic zone, the amount of light decreases until a depth is reached where the rate of photosynthesis becomes equal to the rate of respiration. At this level, net primary production no longer occurs.
The limnetic zone is shallower in turbid water than in clear and is a more prominent feature of lakes than of ponds.Life in the limnetic zone is dominated by
Many lakes (but few ponds) are so deep that not enough light reaches here to support net primary productivity. Therefore, this zone depends for its calories on the drifting down of organic matter from the littoral and limnetic zones.
The profundal zone is chiefly inhabited by primary consumers that are either attached to or crawl along the sediments at the bottom of the lake.
Such bottom-dwelling animals are called the benthos.The sediments underlying the profundal zone also support a large population of bacteria and fungi. These decomposers break down the organic matter reaching them, releasing inorganic nutrients for recycling.
The surface water becomes enriched in oxygen
But the water in the profundal zone — being removed from both these sources — becomes stagnant.
In the fall, however, as the surface water cools, it becomes denser and sinks to the bottom — carrying oxygen with it.
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