Presence of Planktothrix sp. and cyanobacterial toxins in Lake Ammersee, Germany and their impact on whitefish (Coregonus lavaretus L.).

Abstract

Due to the increasing oligotrophy of Lake Ammersee, southern Germany, metalimnic Planktothrix have become one of the dominant planktonic species causing regular blooms. Whitefish (Coregonus lavaretus) is the dominant local fish species with great importance for the fishing industry. Recently, whole age groups of this fish species have disappeared and since 1991, average body-weight has decreased. The causes for this remain unclear. Planktothrix species produce the cyclic peptide toxin desmethyl microcystin-RR, which inhibits glycogen metabolism and has detrimental effects on the development of aquatic organisms. During blooms, gut contents of whitefish displayed a blue discoloration, possibly representing phycobiliproteins typical for cyanobacteria. This study aimed to elucidate the impact of Planktothrix blooms on fish population dynamics. Planktothrix cell counts, performed by epifluorescence microscopy, showed blooms to contain up to 80,000 cells/ml. Microcystin levels of 1-5 micrograms/mg dry weight in Planktothrix extracts and 0.08 microgram/l in water samples, were determined via HPLC and protein-phosphatase inhibition assay. Planktothrix filaments were detected in gut contents of whitefish. In addition, microcystins could be detected in gut contents via ELISA. Similarly, immunoprobing with microcystin-antibodies demonstrated microcystin-protein adducts in liver homogenates of whitefish caught during Planktothrix blooms. Furthermore, Planktothrix extracts proved cytotoxic to trout hepatocytes as determined by MTT reduction. Whitefish eggs and larvae, obtained during blooms in winter 1998 and 2000 in a Lake Ammersee hatchery, were studied for developmental progress. Malformations typical for cyanobacterial toxin exposure were observed.