To understand the effects of predicted warming and changing salinity of marine ecosystems, it is important to have a good knowledge of their capacity to adapt to environmental changes. In this study we investigated how different populations of the copepod Eurytemora affinis from the Baltic Sea respond to varying temperatures and salinity conditions. We collected copepods in the Stockholm archipelago, Bothnian Bay, and Gulf of Riga and conducted common garden experiments.
Our main finding was that low salinity has a detrimental effect on development time, the additive effects of high temperature and low salinity have a negative effect on survival, and their interaction has a negative effect on hatching success. We observed no variation in survival and development within populations, and all genotypes had similar reaction norms with higher survival and faster development in higher salinities.
This suggests that there is no single genotype that performs better in low salinity or high salinity; instead, the best genotype in any given salinity is best in all salinities. Our results suggest that E. affinis can tolerate close to freshwater conditions also in high temperatures, but with a significant reduction in fitness.