engleski

The extiction 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, surface-active substances, reduced sulfur species, phyto- and zooplankton have been investigated in water column of the Rogoznica Lake. During the thermohaline stratification (spring, summer) the surface water is well-oxygenated while anoxia is occurring in the bottom layer. Anoxic deep water is characterized by high concentrations of reduced sulfur compounds (up to 10-3 M, mainly in the form of sulfide) ; nutrients (NH4+, up to 150 ? ; ; ; M ; PO43-, up to 22 ? ; ; ; M ; SiO44-, up to 400 ? ; ; ; M) and dissolved organic matter as a result of the pronounced remineralization of organic matter in this water layer. The depth 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 winter mixing, bottom water enriched 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 production of nutrients. Special conditions leading to and following after the disastrous anoxia are illustrated and discussed.

Rogoznica Lake; eutrophication; meromixis; anoxia; reduced sulfur species; nutrients; dissolved organic carbon

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