Anoxic monimolimnia: Nutrients devious feeders

Abstract. This study focuses on the role of the meromictic anoxic basins' internal load: (a) during storm events and (b) under the environments' typical stratification conditions. Measurements of physicochemical parameters, nutrients, chlorophyll and hydrogen sulfide, four days after an anoxic crisis in Aitoliko basin as well as data obtained from a biennial basin's monitoring, were used. The relationships between temporal nutrient variations in the surface layer of an anoxic basin with the changes on its water column physicochemical characteristics, the changes on the bottom water phosphorus and nitrogen concentration and their effect on the basin's primary productivity, were studied.

In coastal environments, storm events could result in water column total mixing. This disturbance affects almost all the ecosystem's physical, chemical and biological parameters. The basin becomes anoxic, massive fish kills occur and H₂S, PO₄³⁻ and NH₄⁺ release from bottom waters to the interface and surface waters promoting algal blooms. Bottom layer can supply the surface waters with nutrients, even during periods of high water column stratification. Small scale, usually subtle, changes in physicochemical and hydrological basin's characteristics promote this supply, affecting the ecosystem's primary production and shifting its quality character.