Dominance of harmful algae, Microcystis spp. and Micrasterias hardyi, has negative consequences for bivalves in a freshwater lake

The proliferation of cyanobacteria Microcystis spp. and the invading green alga Micrasterias hardyi in Lake Biwa has been increasing. However, the available knowledge on the dietary utilization of these cyanobacterial and algal species by bivalves, which are key species in lake ecosystems, is limited. In this study, we examined the dietary quality and utilization of these species by freshwater bivalves of the Corbicula spp., which are important fishery resources, by performing feeding experiments and field investigations based on fatty acid profiles and stomach content analysis. Although a significant increase in the dry weight and condition factor of the Corbicula spp. individuals fed on diatom was observed at the end of the experiment, for the individuals fed on Microcystis aeruginosa or M. hardyi, a dry weight increase was not observed and their condition factor decreased. Moreover, the fatty acid profile of the Corbicula spp. individuals fed on M. aeruginosa or M. hardyi indicated that they did not assimilate these diets, even though filtration was confirmed during the experiments. However, the stomach contents of wild Corbicula spp. specimens, collected from six sampling sites in Lake Biwa on four sampling occasions, showed that Microcystis spp. were the most abundant dietary items in all sites and on all occasions. Moreover, M. hardyi was detected during the analysis of stomach contents; this alga was the third most abundant algal species. As shown in the feeding experiments, they do not contribute to bivalve growth, indicating that the high occupation of Microcystis spp. and M. hardyi in the consumer's stomach may inhibit effective carbon transfer. The expansion of these unsuitable dietary organisms may affect the stability of lake ecosystems.