Q.Productivity of profundal zone will be low. Why ?

The profundal zone is the deep, light-deficient part of a lake where photosynthesis cannot occur, so its productivity is low and its organisms depend on detritus sinking from above.

A lake ecosystem is typically divided, by depth and light penetration, into three main zones: the shallow, well-lit littoral zone near the shore (where rooted aquatic plants grow), the open-water, sunlit limnetic (photic) zone (where phytoplankton photosynthesise), and the deep, poorly-lit profundal zone lying below the depth at which light is still sufficient for net photosynthesis (below the compensation point).

Because photosynthesis strictly requires adequate light, and the profundal zone lies in near-total darkness (or receives only extremely dim light insufficient to exceed the plants'/algae's own respiratory needs), no significant primary production by photosynthetic organisms can take place there. Phytoplankton and rooted plants are essentially absent or non-functional at this depth. Instead, the profundal zone's biological community is dominated by decomposers and detritivores (bacteria, fungi, bottom-dwelling invertebrates) that survive by breaking down dead organic matter (detritus) -- dead plankton, faecal matter, and other debris -- that continually sinks down from the productive littoral and limnetic zones above.

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