Consequences of increased temperature on the abundances of epilithic cyanobacteria and macroalgae in the Baltic Sea

Abstract

AbstractThe ongoing global warming is likely to cause changes in the growth and species composition of aquatic epilithic communities. To be able to predict such changes, a field study was conducted in the cooling water discharge area of a nuclear power plant that releases brackish cooling water to the Baltic Sea at Forsmark. The power plant creates a temperature gradient in the field from 0 to ca. 8 °C above normal temperature. The project was concentrated on the growth and distribution of four dominant taxa in the upper littoral zone: the colonial cyanobacterium Rivularia atra and the three filamentous macroalgae Cladophora glomerata (green), Ulva spp. (green) and Ceramium tenuicorne (red). Overall, Cladophora increased strongly, Ulva spp. increased slightly and Ceramium decreased strongly in abundance with higher water temperature. For Rivularia these results were corroborated in a laboratory experiment and studied in more detail. The colonies collected at 18 °C in the field were experimentally exposed to 18 °C, 22 °C and 26 °C in climate chambers for 50 days. Growth rates were highest in 18 °C and lowest in 22 and 26 °C. Addition of extra stress in the form of a heavy metal (copper) decreased growth in all temperatures, but highest growth rates still occurred in 18 °C. Rivularia was less stressed in 18 °C than in 22 °C and 26 °C, which was shown by, higher chlorophyll a (chla) concentration, higher photosynthetic performance (measured as electron transport rate by pulse amplitude modulated fluorescence), lower zeaxanthin concentration and a lower carotenoids:chla ratio.