Thermal discharge effects on microbial populations and copepod gut content in the Baltic Sea

Climate change is expected to increase water temperatures, potentially reshaping microbial communities and their interactions. This study assessed microbial composition and abundances in two Baltic Sea bays (warm and control) from winter to late spring, and investigated the prokaryotic gut microbiomes and ingested prey of individual copepods (Acartia and Eurytemora) from the same sites. The warm bay has experienced approximately 50 years of artificial warming due to thermal discharge from a nuclear power plant, serving as a natural laboratory for studying temperature effects on ecosystems. The prokaryotic and eukaryotic community composition, assessed using 16S and 18S rRNA amplicon sequencing, differed significantly between the bays. The abundances of small photoautotrophs (nanoplankton, picoeukaryotes, and phycocyanin-rich picocyanobacteria) were generally higher in the control bay. Bay-specific differences of ingested photoautotrophic prey were observed in the copepod guts, driven by compositional differences in seawater. Bay-specific differences in Actinobacteriota in the copepod gut microbiome suggest host regulation rather than seawater compositional influence, whereas Proteobacteria likely reflect both. Our findings illustrate how warming reshapes coastal microbial communities and highlight the need to assess interactions to predict climate change impacts.