Influence of abiotic and biotic factors on microcystin content in Microcystis aeruginosa cells in a eutrophic temperate reservoir

The Sulejow Reservoir (Poland) was sampled on a weekly basis between May and September in 2003 and 2004 to examine changes in weight-specific microcystin content of Microcystis aeruginosa and to examine which abiotic or biotic factors may be the key factors governing microcystin content. The variables examined in this study included: temperature; total and dissolved inorganic phosphorus and nitrogen; and Daphnia biomass. Comparing summertime from both years, despite the similar levels of cyanobacterial biomass, a significant difference between microcystin concentrations was observed which resulted from a difference in microcystin content. Differences in weight-specific microcystin content were discussed in relation to different Daphnia biomass (r = 0.34, n = 40, P < 0.05). It is possible that exposure to Daphnia and/or chemical signals released by predators may have resulted in increased microcystin content of M. aeruginosa. Nevertheless, the influence of weight-specific microcystin content on microcystin concentrations (r = 0.56, n = 40, P < 0.05) was less than that of cyanobacterial biomass (r = 0.82, n = 40, P < 0.05), which was strongly correlated with temperature and phosphorus ability. This study indicated that not only abiotic factors, but also the presence of herbivorous zooplankton, may determine microcystin content of M. aeruginosa.