Nodularin accumulation during cyanobacterial blooms and experimental depuration in zooplankton

Cyanobacterial blooms are a common phenomenon in the Baltic Sea, and the hepatotoxin nodularin has been frequently detected in certain Baltic Sea fishes and mussels. However, there is no knowledge about the naturally occurring concentrations of nodularin in Baltic Sea zooplankton. The aim of this study was to survey the concentrations of nodularin in natural zooplankton assemblages, and to study the depuration of nodularin in one common copepod species, Eurytemora affinis, experimentally. The nodularin concentrations in common zooplankton species were determined from field-collected samples from the northern Baltic Proper in 2001 and 2002, during cyanobacterial blooms, and the samples were analysed by ELISA immunoassay. Nodularin could be detected from the field-collected zooplankton, suggesting that during a natural bloom event toxins accumulate in their tissues. The concentrations were relatively low (0.07±0.01 μg g⁻¹ ww), ranging from below detection limit to 0.62 μg g⁻¹ ww. Some variation occurred in the concentrations between species and years; generally concentrations were higher in 2001 than in 2002. In the depuration experiment E. affinis copepods were fed with toxic Nodularia spumigena for 24 h, and their toxin contents were monitored for 24 h after transferring them to filtered seawater. A rapid decrease in nodularin concentrations occurred during the first 0.5–3 h after the exposure. However, after a 24-h depuration period in filtered seawater, nodularin could be still detected in E. affinis tissues, indicating that part of the accumulated nodularin, or its derivatives, could be transferred to planktivores.