Effects of heat shock and salinity changes on coastal Mediterranean phytoplankton in a mesocosm experiment

Decreasing biodiversity is projected as one of the most consistent effects of warming on marine microbial communities. It is predicted that low biodiversity will consequently influence the community sensitivity to additional environmental alterations. Mesocosms were used to study the response of natural Mediterranean phytoplankton communities (control and heat shock +6 degrees C) to salinity variations (-5psu, control, +5psu). We examined the effect on species composition, species richness as well as phytoplankton biomass and resource use efficiency. Heat shock was coupled with decreased species richness (30 species in control community while 26 in heat shock) and slightly reduced phytoplankton biomass. Changes in salinity altered the phytoplankton species composition (dinoflagellates were absent in decreased salinity treatments) and significantly reduced the phytoplankton species richness. The phytoplankton biomass and the resource use efficiency also decreased with exception of the increased salinity treatment in the non-heated community. In general, decreased salinity had stronger negative effects compared to increased salinity as displayed by the lowest species richness and lowest phytoplankton biomass in those treatments. Most notably, we identified a synergistic negative effect of heat shock with increased/decreased salinity which can be attributed to the lower species richness and, thus, decreased stability in the heated community.