Effects of zebra mussels (Dreissena polymorpha) on phytoplankton community structure under eutrophic conditions

Abstract

We conducted two mesocosm experiments to determine how invasive zebra mussels affected phytoplankton community and size structure in the summers of 2014 and 2017. Water containing natural phytoplankton and zooplankton communities was collected from the eutrophic Lake Mikołajskie (Masurian Lake District, northeastern Poland) and used to start each experiment. We analyzed how the introduction of zebra mussels impacted chlorophyll and the biomass of individual algae taxa, with particular interest on cyanobacteria. The starting phytoplankton communities differed in the two study years, whereby cyanobacteria dominated in 2014 and dinoflagellates and diatoms dominated in 2017. The biomass of crustacean zooplankton (cladocerans and copepods) was not affected by zebra mussels; therefore, we attributed differences in phytoplankton between treatments to the presence of zebra mussels. Zebra mussels had contrasting effects on cyanobacteria in the 2014 and 2017 experiments. In 2014, zebra mussels decreased cyanobacteria, while in 2017 they had a positive effect on cyanobacteria chlorophyll. The contrasting effects of zebra mussels on cyanobacteria could be related to differences in starting nutrient concentrations and phytoplankton communities between the two experiments. Filamentous green macroalgae dominated at the end of the both experiments in the zebra mussel treatments while they were not very abundant in the controls. Therefore, zebra mussels can have indirect negative effects on higher trophic levels even when they decrease cyanobacteria because they shift the autotrophic structure in favor of attached green macroalgae, which are a poor resource for zooplankton.