Evidence of Effect of Short Term Increase in Seawater Temperature on Composition of Benthic Communities in NE Baltic Sea

Baltic Sea is a unique, semi enclosed brackish water system characterized by stable south-north and west-east directional salinity gradient, spatio-temporal variations in thermohaline stratification and strong seasonality. Baltic Sea hosts a mixture of benthic species of marine, brackish and fresh-water origin, living often close to their physiological salinity tolerance limit, and therefore sensitive to even small changes in the abiotic environment. In this study, changes in benthic communities in relation to extreme weather conditions like occurrence of heat wave and upwelling events were analyzed and described in temperate north-eastern Baltic Sea. In summer of 2018, the maximum water temperatures along exposed coast of NE Baltic Sea varied between 20 and 25°C at the bottom of 5 m depth and values were up to 7°C higher compared to long term mean (according to longterm national monitoring dataset). However, the daily water temperatures showed remarkable variation due to multiple occasions of upwelling events, the extreme temperature shifts up to 13°C within few days were recorded. In NE Baltic Sea salinity gradient is up to 7 g kg-1 in outer areas and falls to zero in bays and inner archipelago with riverine impact. Salinity has been proven to have significant importance on abundance of benthic species richness and distribution in the Baltic Sea. During upwelling events rapid increase of salinity in magnitude up to 2 g kg-1 occurred in observed stations. Described extraordinal heat wave accompanied with upwelling event caused the increase of zoobenthic biodiversity and abundance of herbivores and deposit feeders along the monitored coastline. At the same time effect of described events on longlived macrovegetation species composition and proportion of fast growing opportunistic species remained negligible while increase in abundance of herbivores can possibly influence the species composition in the case of future events.