Multiannual trends (2004–2019) in the abundance of larvae of the invasive mussel Limnoperna fortunei and crustacean zooplankton in a large South American reservoir

Abstract

The behavior of non-indigenous species with time since introduction varies widely and is a major issue in the understanding of their long-term effects on the habitats colonised. We analysed the long-term trends in the densities of larvae of the invasive mussel Limnoperna fortunei and planktonic Copepoda and Cladocera in Salto Grande, a large South American reservoir in the Uruguay River (Argentina-Uruguay), on the basis of weekly samples collected between June 2004 and November 2019. All three groups depicted variable interannual abundances. While mussel veligers showed no significant interannual trends (overall mean: 2552 larvae L−1), crustaceans decreased significantly in the time elapsed (Copepoda: from ˜1000 to 100 ind. L−1; Cladocera: from ˜500 to 100 ind. L−1). We conclude that increasing discharge rates by the Uruguay River and the concomitant decreases in reservoir water residence times are the most likely cause for these drops in their populations. As opposed to many other invasive species elsewhere, and to the same mussel in other South American waterbodies, L. fortunei larvae did not change in abundance with time since introduction. Our results underscore the assumption that each biological introduction is a particular and highly context-dependent event and the dynamics of biological invasions in general elude broad generalizations.