ABSTRACT The effects of elevated temperature are pronounced in high latitudes where warming is faster than the global average. It is unclear how warming and other simultaneous stressors will affect the functioning of trophic levels. To study the effects of increasing temperature and decreasing salinity on diversity, functional traits, and biomass of Baltic Sea benthic diatoms and zooplankton, we conducted an indoor mesocosm experiment. We hypothesized that higher trophic levels, that is, zooplankton, would be more vulnerable to climate change than lower trophic levels, that is, benthic diatoms, because benthic diatoms typically have a high species richness that promotes high resilience and functional redundancy. We also hypothesized that the diversity and biomass of both trophic levels would decline when subjected to warming and decreasing salinity. The study organisms were subjected to ambient temperature and +5°C warming, and five salinity treatments with targeted salinity levels between 3 and 7. Main results showed that warming affects the diversity of both taxa, as warming decreased the species and trait diversity of benthic diatoms but increased the species and trait diversity of zooplankton. The interaction of warming and decreasing salinity had significant effects on benthic diatoms. No effect of salinity on zooplankton was found. Our results emphasized the effect of multiple stressors on aquatic communities but did not support our hypothesis of stronger effects of warming and decreasing salinity on higher trophic levels. This could be due to Baltic zooplankton being well adapted to the salinities present in the area, whereas salinity ~5 is a threshold for benthic diatoms.
Virta et al. (Mon,) studied this question.