Mass mortality event in a small saline lake (Lake Rogoznica) caused by unusual holomictic conditions

The study of a small saline eutrophic lake was performed under meromictic and holomictic conditions. The meromictic period was characterised by anoxic conditions in the bottom layer with high sulphide and nutrient concentrations. The depth of the nutricline varied as affected by thermohaline conditions in the water column. In late summer 1997, holomictic conditions replaced ordinary meromitic conditions in the lake. This transformation caused anoxia to spread throughout the water column and mass mortality of almost all the organisms occured. Thereafter, oxic conditions were re-established slowly as the new phytoplankton population developed. As a consequence of the particulate organic matter decay, inorganic N and P concentrations increased and had doubled approximately six weeks after the mass mortality. The revitalisation of the phytoplankton population was initiated immediately after the mass mortality by the development of "microflagellate" species, which were favoured by the high ammonia concentrations. The larger phytoplankton species (diatoms and dinoflagellatae) developed at the expense of smaller ones when nitrate became the dominant nutrient in the water column. A year after the mass mortality the composition of the phytoplankton population was the same as before.