Consequences of a cyanobacteria bloom for copepod reproduction, mortality and sex ratio

The aim of the study was to measure copepod reproduction, mortality and sex ratio in the field before, during and after a cyanobacteria bloom during the summer in the western Gulf of Finland. Environment and zooplankton samples were collected every fortnight, and the copepod Acartia spp. was incubated in the laboratory for reproductive output, i.e. egg production and egg hatching success. Other responses monitored were female: male ratio, mortality and body condition. In addition, molecular analyses of the nodularin-producing cyanobacterium Nodularia in Acartia gut contents (GCs) were assessed. Egg production and body condition decreased with increasing Nodularia GCs. During the bloom, hatching decreased as a response to Nodularia in the copepod gut. Although not related to cyanobacteria variables, male mortality was higher than female mortality, resulting in a female-biased sex ratio over most of the summer. The study demonstrates that Acartia reproductive output is constrained by cyanobacteria blooms in the Baltic Sea, and more generally that copepod population dynamics may be negatively affected by such blooms. This is especially significant considering that toxin-producing blooms are predicted to increase due to warming.