Nutrient Recycling, Phytoplankton Grazing, and Associated Impacts of Limnoperna fortunei

Abstract

Laboratory and field experiments indicate that the presence of Limnoperna fortunei decreases concentrations of particulate organic matter and increases ammonia, nitrate, and especially phosphate. Long-term series of field data partially confirm these results. After having been colonized by the mussel, a 47 km2 reservoir developed higher concentrations of ammonia and phosphates, a higher P:N ratio, more transparency, less seston, and less phytoplankton and primary production. Phytoplankton clearance rates by the mussel vary widely, suggesting that “normal” values for adult organisms are around 100 mL/ind./h, or ca. 2–4 mL/mg DW/h. Data on grazing selectivity are inconclusive, but seem to indicate highest impacts on small (< 1 mm) particles. Large plankton are negatively selected, but they may account for greater proportions of total biomass in the diet. Studies on consumption of toxic cyanobacteria yield conflicting results, but large golden mussel populations significantly enhance blooms of colonial Microcystis spp. through changes in nutrient availability, size-selective grazing, promotion of colony formation, and reduced grazing of toxic cells. These toxic blooms, in turn, suppress reproduction of the mussel, most probably killing the larvae. Growth of periphyton and aquatic macrophytes are enhanced significantly by the golden mussel.