Population dynamic of bloom-forming Microcystis aeruginosa in the presence of the invasive bivalve Limnoperna fortunei.

Aquatic ecosystems have been severely altered by invasive species and a connection has been observed between bivalve invasions and an increase in frequency of cyanobacteria toxic blooms. In South America, the invasive golden mussel, Limnoperna fortunei, has caused serious environmental and economic impacts, because of its high filtration rates, high population densities and rapid dispersion. Changes in the plankton community have been detected at sites invaded by this species. The aim of the present study was to investigate the possible effects of this mussel on the growth of the bloom-forming cyanobacterium Microcystis aeruginosa, in laboratory experiments under controlled light and temperature conditions. The cyanobacterium M. aeruginosa and a chlorophyte (Pseudokirchneriella sp.) were used either together or alone as food for the mussels. Density measurements of M. aeruginosa and Pseudokirchneriella sp. growing in vessels with and without mussels were performed daily, and nutrient concentrations in the water were assessed at the beginning and end of the experiment. The results indicated that Pseudokirchneriella sp. acted as a competitor, contributing to reduce densities and growth rates of M. aeruginosa. These effects, however, did not occur in the presence of the golden mussel, when the densities of Pseudokirchneriella sp. were significantly lower, possibly due to selective grazing. Phosphate and nitrate concentrations were always higher in the presence of the bivalves. As previously observed for other invasive mollusks, our results suggest that the golden mussel may have a positive effect on M. aeruginosa by the exclusion of potential competitors as well as by increasing nutrient supply.