PHYTOPLANKTON DIVERSITY OF SHALLOW TIDAL LAKES: INFLUENCE OF PERIODIC SALINITY CHANGES ON DIVERSITY AND SPECIES NUMBER OF A NATURAL ASSEMBLAGE 1

The influence of periodic salinity changes was investigated for 42 days under semicontinuous culture conditions with phosphorus limitation using phytoplankton assemblages from Lake Waihola, a tidally influenced shallow lake. To simulate tidal effects on the phytoplankton community, salinity in the cultures was increased in pulses at different intervals (3.5, 7, and 14 days), and these cultures were compared with those that experienced constant freshwater conditions. Salinity pulses significantly affected competition and succession with a major loss in diversity during the first days of the experiment due to the initial pulse that caused a transition from freshwater to brackish conditions in the cultures. After this initial phase, diversity index (H') and species number (S corr ) decreased less rapidly. The loss in H' and S corr over time was highest under constant freshwater conditions and lowest in the treatment with an interval of 3.5 days between salinity pulses. At the end of the experiment, the combination of initial loss in H' and S corr and the time course of H' and S corr resulted in a U‐shaped relation between the interval length of salinity pulses and both H' and S corr temp1.txttemp1.txt. Our results indicate that salinity pulses at intervals of a few days tend to promote phytoplankton diversity. If saline intrusions in coastal freshwater systems occur only at spring tides, this will lead to decreases in diversity and species richness.