The extinction by sulfide—turnover and recovery of a naturally eutrophic, meromictic seawater lake

Since 1994 seasonal variations of temperature, salinity and vertical distribution of dissolved oxygen, nutrients, dissolved organic carbon (DOC), surface-active substances (SAS), reduced sulfur species (RSS), phyto- and zooplankton have been investigated in water column of the Rogoznica Lake. During the thermohaline stratification (spring and summer), the surface water is well-oxygenated while anoxia is occurring in the bottom layer. Anoxic deep water is characterized by high concentrations of RSS (up to 10−3 M, mainly in the form of sulfide); nutrients (NH4+, up to 150 μM; PO43−, up to 22 μM; SiO44−, up to 400 μM) and DOC (up to 6 mg/l) as a result of the pronounced remineralization of allochthonous organic matter produced in the surface water. The depth position of anoxic water layer changes seasonally and is greatly influenced by rainfall, the influence of which is visible from decreased salinity in deeper layers as well. As a result of autumn/winter mixing, bottom water rich with nutrients is coming to the surface, supporting new phytoplankton and oxygen productions. Natural eutrophication of the lake is strongly influenced by nutrient recycling under anaerobic conditions. Turnover of lake water layers in September 1997 occurred so quickly that it resulted in the appearance of total anoxia and sulfide presence throughout the water column. The aerobic flora and fauna died and added to the oxygen demand and the production of nutrients. Those long-term datasets help in recognizing the general trend from unique cases, such as the anoxic event in 1997, which was related to anthropogenic eutrophication.