Practice question

The profundal zone is the deep, open-water region of a lake below effective light penetration. Because irradiance there falls beneath the compensation level, photosynthesis cannot balance the respiration of producers; rooted plants and sustained phytoplankton production are therefore absent. Its food web is dominated by heterotrophic organisms that obtain organic carbon made elsewhere. Bacteria and fungi decompose sinking detritus, while invertebrates and fishes consume particles, microbes, or other animals. In a stratified eutrophic lake, this respiration can deplete oxygen in the profundal water because atmospheric mixing is restricted. Calling the region “only heterotrophic” is a textbook simplification: chemoautotrophic microorganisms can occur under suitable chemical conditions, and organisms may move between zones. Nevertheless, heterotrophy overwhelmingly characterizes this light-limited habitat. Littoral water is shallow enough for rooted vegetation and attached algae, while the limnetic or pelagic surface region supports phytoplankton. The decisive mechanism is the shortage of photosynthetically active radiation, which removes local photosynthetic production and makes the profundal community dependent on imported organic matter.