Density of Limnoperna fortunei larval stages in a cascade of subtropical reservoirs: spatiotemporal variation and environmental influences

The golden mussel ( Limnoperna fortunei ) is an invasive bivalve that has established itself in several South American river systems, impacting ecosystem functioning. Reservoir cascades provide their larvae with the means of rapid dispersal, but the relationship between environmental variables and larval stage structure remains unclear. In this study, the density of four L. fortunei larval stages was assessed in a cascade of five reservoirs in the upper Uruguay River Basin and associated with spatiotemporal variation in environmental parameters. Larval density increases were more related to fluctuating environmental parameters than to the expected effect of a cascade system, as the highest larval densities did not occur in the downstream reservoirs. Smaller D-shaped larvae predominated, suggesting that larger umbonated larvae either attempted to settle on the substrate (prior to metamorphosis to pediveligers) or were naturally preyed upon. The relationships between environmental parameters were evaluated using a multivariate model. Water temperature and dissolved oxygen had a significant effect on larval density but did not respond to density variations independently. However, during winter, larval density decreased significantly. Other variables also contribute to species activity and survival in the upper Uruguay River Basin reservoirs and may be considered limiting factors. The interaction between basin area and rainfall strongly influenced the increase in larval density because runoff enriched the reservoirs with nutrients essential for the leading development of larval phases. Our findings provide insights on larval development in golden mussels, which can be used as a tool to manage further dispersal of this invasive species.