Bottomwater

Bottomwater is the portion of a body of water that lies at or near the bottom. In stratified lakes, the bottom-water layer corresponds to the hypolimnion, a cold, dark mass beneath the thermocline. During stratification, it becomes isolated from the surface, often experiencing low or zero oxygen (anoxia) later in the season as microbial respiration consumes available oxygen. Its chemical composition tends to be nutrient-rich due to sediment-water exchange, pH can shift, and hydrogen sulfide may accumulate in strongly reducing conditions. These conditions influence the chemistry of sediments and affect benthic organisms, nutrient cycling, and fish habitat.

In the ocean, bottom water refers to the deepest or near-bottom layer, a component of the global thermohaline circulation. Deep water masses such as North Atlantic Deep Water or Antarctic Bottom Water form in polar regions and spread along the sea floor. Bottom water is typically colder and denser than overlying water and can be low-oxygenated in some regions, particularly where respiration exceeds supply, though ventilated basins may maintain substantial oxygen. Its properties—temperature, salinity, oxygen, and nutrient content—vary regionally and seasonally and influence benthic ecosystems and carbon and nutrient cycling through exchange with sediments.

Bottom water is measured and studied with oceanographic and limnological methods, including CTD sensors, bottle sampling, and temperature-salinity profiles. Understanding bottom-water conditions is essential for assessing habitat suitability for bottom-dwelling organisms, interpreting seasonal turnover in lakes, and modeling nutrient fluxes between water and sediments.