Toxic Microcystis in shallow lake Muggelsee (Germany) -: dynamics, distribution, diversity

The occurrence of toxic blooms of cyanobacteria in due course of the eutrophication of most inland waters has become an increasingly important issue of scientific and public health concern. One objective of many field studies is to assess the predictability of bloom toxicity. Here we present data of a four-year monitoring of microcystins in lake Muggelsee together with data on spatial distribution and diversity of the predominant toxin-producer Microcystis sp. Microcystins were detectable in all seasons when Microcystis-biovolumes were detectable with seasonal maximum concentrations of 1-3 mug/L. In spatially resolved samples, the microcystin concentrations spanned more than an order of magnitude between up-wind and down-wind sites with highest concentrations at the latter ones. The bloom development of Microcystis and Aphanizomen was significantly correlated to the ratio of DIN: SRP and it is discussed whether this reflects a causal relationship or rather similar trends in both variables related to third one, like water temperature. The microcystin content of Microcystis was negatively correlated to Microcystis abundance, thus blooms were relatively less toxic during their peak. A metabolic typing of strains isolated from a single bloom sample revealed that the Microcystis bloom was composed of a multitude of toxin-producing and non-producing strains that differed in their production of microcystins and other non-ribosomal peptides. The number of distinguishable chemotypes clearly exceeded the number of microscopically determined morphotypes. Given the high clonal diversity of the Microcystis community we argue that a quantitative prediction of bloom toxicity is not possible without a better understanding of the molecular ecology of Microcystis and that an avoidance of toxic blooms of Microcystis is best to be done by appropriate restoration measures.