Bunnvann

Bunnvann is a term used in hydrology and limnology to describe the bottom water layer of a body of water, typically the hypolimnion of a stratified lake, fjord, or reservoir. It lies beneath the warmer, lighter surface water and is often separated from the upper layers by a thermocline in summer.

During thermal stratification, the surface layer (epilimnion) remains warmer and less dense, while the deeper water remains cooler and more stable. In summer, bunnvann can become depleted of dissolved oxygen due to the respiration of organisms and the decomposition of organic matter, with limited mixing replenishing the oxygen from above. Redox conditions in bunnvann may become reducing, leading to the accumulation of substances such as iron, manganese, and in some cases hydrogen sulfide near sediments.

Seasonal turnover during spring and autumn mixes the water column, re-oxygenating the bottom layer and redistributing nutrients. Ecological implications include impacts on bottom-dwelling life, which may be stressed by low oxygen, and potential release of nutrients from sediments that can influence productivity when mixing resumes.

Measurement and management involve profiling the water column with instruments like CTD sensors to monitor temperature, salinity (where applicable), and dissolved oxygen, as well as collecting water and sediment samples. Management strategies to protect bunnvann quality focus on reducing nutrient inputs to prevent eutrophication and, in some managed systems, artificial aeration to maintain adequate oxygen levels in the bottom waters.