Species invasions shift microbial phenology in a two-decade freshwater time series.

Abstract

Invasive species impart abrupt changes on ecosystems, but their impacts on microbial communities are often overlooked. We paired a 20 y freshwater microbial community time series with zooplankton and phytoplankton counts, rich environmental data, and a 6 y cyanotoxin time series. We observed strong microbial phenological patterns that were disrupted by the invasions of spiny water flea (<i>Bythotrephes cederströmii</i>) and zebra mussels (<i>Dreissena polymorpha</i>). First, we detected shifts in <i>Cyanobacteria</i> phenology. After the spiny water flea invasion, <i>Cyanobacteria</i> dominance crept earlier into clearwater; and after the zebra mussel invasion, <i>Cyanobacteria</i> abundance crept even earlier into the diatom-dominated spring. During summer, the spiny water flea invasion sparked a cascade of shifting diversity where zooplankton diversity decreased and <i>Cyanobacteria</i> diversity increased. Second, we detected shifts in cyanotoxin phenology. After the zebra mussel invasion, microcystin increased in early summer and the duration of toxin production increased by over a month. Third, we observed shifts in heterotrophic bacteria phenology. The <i>Bacteroidota</i> phylum and members of the acI <i>Nanopelagicales</i> lineage were differentially more abundant. The proportion of the bacterial community that changed differed by season; spring and clearwater communities changed most following the spiny water flea invasion that lessened clearwater intensity, while summer communities changed least following the zebra mussel invasion despite the shifts in <i>Cyanobacteria</i> diversity and toxicity. A modeling framework identified the invasions as primary drivers of the observed phenological changes. These long-term invasion-mediated shifts in microbial phenology demonstrate the interconnectedness of microbes with the broader food web and their susceptibility to long-term environmental change.