Long-term spatio-temporal dynamics of the meroplankton community in the southern Baltic Sea

Meroplankton, the pelagic larvae of benthic invertebrates, are an understudied component of zooplankton communities with particularly large knowledge gaps on interannual variation and long-term changes in meroplankton phenology. This study presents the first long-term assessment of meroplankton in the brackish southern Baltic Sea, spanning 17 years (2005–2021). Data from 11 stations across 33 cruises reveal significant temporal changes in community composition and biomass and encompass three distinct periods. Early in the time series, polychaete larvae typically dominated the spring meroplankton community, contributing most biomass and frequently persisting throughout the summer (June–August). This pattern became rare after 2014, when bivalve larvae dominated during summer. A spring community was present in multiple samples collected in June, only in years following cold winters. A second major shift occurred in 2019, characterized by a dramatic, order of magnitude increase in both meroplankton abundance and biomass, whereas the relative community composition remained largely unchanged. These transitions likely reflect rising water temperatures, and in particular winter sea surface temperatures above the temperature of maximum density for water within our study region. Our study links winter sea surface temperature and meroplankton phenology, with potentially strong implications for benthic recruitment and ecosystem functioning.