The profundal zone is a deep zone of an inland body of freestanding water, such as a

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much lar ...

or pond, located below the range of effective light penetration. This is typically below the thermocline, the vertical zone in the water through which

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

drops rapidly. The temperature difference may be large enough to hamper mixing with the littoral zone in some seasons which causes a decrease in oxygen concentrations. The profundal is often defined, as the deepest, vegetation-free, and muddy zone of the lacustrine benthal. The profundal zone is often part of the aphotic zone. Sediment in the profundal zone primarily comprises silt and mud.

Organisms

The lack of light and oxygen in the profundal zone determines the type of biological community that can live in this region, which is distinctly different from the community in the overlying waters. The profundal macrofauna is therefore characterized by physiological and behavioural adaptations to low oxygen concentration. While benthic fauna differs between lakes, Chironomidae and Oligochaetae often dominate the benthic fauna of the profundal zone because they possess

hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...

-like molecules to extract oxygen from poorly oxygenated water. Due to the low productivity of the profundal zone, organisms rely on detritus sinking from the photic zone. Species richness in the profundal zone is often similar to that in the limnetic zone. Microbial levels in the profundal benthos are higher than those in the littoral benthos, potentially due to a smaller average sediment particle size. Benthic macroinvertebrates are believed to be regulated by top-down pressure.

Nutrient cycling

Nutrient fluxes in the profundal zone are primarily driven by release from the benthos. The anoxic nature of the profundal zone drives

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...

release from benthic sediment. This can drive phytoplankton production, to the point of a phytoplankton bloom, and create toxic conditions for many organisms, particularly at a high pH. Hypolimnetic anoxia can also contribute to buildups of iron, manganese, and sulfide in the profundal zone.

References

Aquatic ecology Aquatic biomes {{Ecology-stub

Organisms

Nutrient cycling

See also

References