Interspecific trait variability and plasticity of the Baltic Sea phytoplankton species along a salinity gradient

In the face of changing climate and global water cycle, the plastic response of phytoplankton species to salinity fluctuations is increasingly important. This study used a multivariate approach to determine interspecific trait variability and plasticity of 10 Baltic Sea phytoplankton species along the salinity gradient. Phytoplankton species representing a broad range of sizes and taxonomic groups were grown at six salinity conditions (0, 5, 15, 20, 30 and 35 psu), and 15 different traits were measured at the end of the experiment. Results showed species-specific salinity preferences. Nutrient uptake and resource use efficiency (RUE) explained interspecific trait variability among the species. Variability in nutrient uptake reflected species-specific differences in cell size. RUE and cellular elemental content were the most plastic traits across the salinity gradient and did not scale with cell size. Interestingly, low trait plasticity did not always translate into low biomass production, as a diatom <i>Phaeodactylum tricornutum</i> exemplified. As expected, the salinity range between 5 and 20 psu was optimal for most phytoplankton species, corresponding to the brackish Baltic Sea where they were isolated. Many species survive in salinities above this range, but not in freshwater, which can have consequences for the plankton community functioning with predicted Baltic Sea freshening.