Evidence for control of microcystin dynamics in Bautzen Reservoir (Germany) by cyanobacterial population growth rates and dissolved inorganic carbon

Abstract

Laboratory experiments with Microcystis aeruginosa (strain PCC 7806) showed (1) that microcystins (MCYSTs) were synthesized only during the exponential growth phase of the producer, and (2) that MCYST production started when pH exceeded the value of 8.4 indicating a lack of free CO 2 and a beginning decline in HCO 3 - . Both findings were included as basic considerations in a model to simulate MCYST concentrations during a four-year period in the hypertrophic Bautzen Reservoir. The model calculates the particular MCYST production rates of five different species of cyanobacteria which occurred in Bautzen Reservoir. The model simulations show a good correlation between measured and calculated concentrations (r = 0.74). correlation being worse (1 = 0.62) when the pH-switch (no MCYST formation below pH 8.2) is excluded from the model. Since MCYST production is correlated with the growth rates and growth rates usually are controlled by changing environmental variables, no correlation between MCYST concentration and single abiotic or biotic factors can be expected. Consequently, MCYST concentration can be influenced only indirectly by the environmental factors. We conclude that the MCYST production under field conditions is controlled simultaneously by (1) the growth phase of a particular MCYST producer, (2) the species (and/or strain) composition of the cyanobacterial community, and (3) the availability of dissolved inorganic carbon. This complex control pattern may also explain many inconsistent results hitherto published regarding the influence of environmental factors on MCYST production.