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

Ante Baric,Zivana Nincevic,Ivona Marasovic,Branka Grbec,Grozdan Kuspilic,Ivana Grubelic

doi:10.3989/scimar.2003.67n2129

Abstract

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.

Highlights

Specific physical and chemical limnology cycles of small saline lakes are well documented in many parts of the world: for example, the mesothermic sulfatochloride Lake Mahega in Uganda (Melack and Kilhman, 1972), the hypersaline Solar Lake on the Sinai coast (Cohen, et al, 1977; Jørgensen et al, 1979), and a meromictic salt lake (MahoneyLake) in British Columbia (Northcote and Hall, CHARACTERISTIC OF A SALINE LAKE 1291983; Overmann et al, 1991), etc
The heterotrophic ebridian microflagellate Hermesinum Adriaticum and the diatom Eunotia spp were dominant in the phytoplankton population, while the zooplankton community was characterised by oligotrich ciliates and a monoculture of the copepod Acartia Italica
The aim of this study is to present the revitalisation of the phytoplankton community in the lake after the mass mortality of the entire community, which was coupled with the changes in the physicochemical conditions in the water column

Summary

Introduction

Specific physical and chemical limnology cycles of small saline lakes are well documented in many parts of the world: for example, the mesothermic sulfatochloride Lake Mahega in Uganda (Melack and Kilhman, 1972), the hypersaline Solar Lake on the Sinai coast (Cohen, et al, 1977; Jørgensen et al, 1979), and a meromictic salt lake (MahoneyLake) in British Columbia (Northcote and Hall, CHARACTERISTIC OF A SALINE LAKE 1291983; Overmann et al, 1991), etc. Specific physical and chemical limnology cycles of small saline lakes are well documented in many parts of the world: for example, the mesothermic sulfatochloride Lake Mahega in Uganda (Melack and Kilhman, 1972), the hypersaline Solar Lake on the Sinai coast (Cohen, et al, 1977; Jørgensen et al, 1979), and a meromictic salt lake A similar occurrence of sulphide and high concentrations of nutrients were for the first time recorded from the bottom layer of a salt lake All the other investigations of biological and chemical characteristics of the lake showed permanent stratification of the entire water column and the presence of sulphide in the bottom layer (Ciglenečki et al, 1996, 1998; Mihelčić et al, 1996; Stipaničev and Branica, 1996; Viličić et al, 1996). The community of the lake consisted of a limited number of species (adapted to the specific physical-chemical conditions in the lake). The heterotrophic ebridian microflagellate Hermesinum Adriaticum and the diatom Eunotia spp were dominant in the phytoplankton population, while the zooplankton community was characterised by oligotrich ciliates and a monoculture of the copepod Acartia Italica

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