Development of Baltic Sea zooplankton in the presence of a toxic cyanobacterium:: a mesocosm approach

Cyanobacteria blooms are common in the Baltic Sea and are considered to be a poor food source and sometimes toxic to zooplankton. Most experiments demonstrating harmful effects have been short-term incubations with monocultures or simple mixtures of food. In this study, a mesocosm approach was used to examine zooplankton responses over generation timescales. A toxic strain of the cyanobacterium Nodularia spumigena was added to bag enclosures of ambient water. The initial mesozooplankton concentration was either reduced by prescreening the water or enriched with locally caught zooplankton. Experiments ran for 15 days, long enough to monitor reproductive success and development of the next mesozooplankton generations. There was no major harmful effect on the zooplankton assemblage, even though the concentration of the toxin nodularin was in the upper range of field observations. The copepod Eurytemora affinis, rotifers Synchaeta spp. and the cladoceran Bosmina longispina maritima were able to develop and reproduce successfully in the presence of N. spumigena. The only species showing impaired recruitment was the copepod Acartia bifilosa. The general lack of population level effects from N. spumigena in this study can be reconciled with previous observations of adverse effects. Cyanobacteria alone may be poor food and toxic to zooplankton, but in the mesocosms a rich assemblage of microbiota developed, similar to that associated with blooms in the field. We suggest that, in the context of otherwise food-depleted summer situations in the open Baltic Sea, zooplankton can derive benefit from cyanobacteria bloom assemblages.