Response of macroalgal assemblages from rockpools to climate change: effects of persistent increase in temperature and CO2

Abstract

Anthropogenically induced global climate change has important implications for marine ecosystems with unprecedented ecological and economic consequences. Climate change will include the simultaneous increase of temperature and CO 2 concentration in oceans. However, experimental manipulations of these factors at the community scale are rare. In this study, we used an experimental approach in mesocosms to analyse the combined eff ects of elevated CO 2 and temperature on macroalgal assemblages from intertidal rock pools. Our model systems were synthetic assemblages of varying diversity and understory component and canopy species identity. We used assemblages invaded by the non-indigenous canopy forming alga Sargassum muticum and assemblages with the native canopy species Cystoseira tamariscifolia . We examined the eff ects of both climate change factors on several ecosystem functioning variables (i.e. photosynthetic effi ciency, productivity, respiration and biomass) and how these eff ects could be shaped by the diversity and species identity of assemblages. CO 2 alone or in combination with temperature aff ected the performance of macroalgae at both individual and assemblage level. In particular, high CO 2 and high temperature (20 ° C) drastically reduced the biomass of macroalgal assemblages and aff ected their productivity and respiration rates. Th e identity of canopy species also played an important role in shaping assemblage responses, whereas species richness did not seem to aff ect such responses. Species belonging to the same functional eff ect group responded diff erently to the same environmental conditions. Data suggested that assemblages invaded with S. muticum might be more resistant in a future scenario of climate change. Th us, in a future scenario of increasing temperature and CO 2 concentration, macroalgal assemblages invaded with canopy-forming species sharing response traits similar to those of S. muticum could be favoured.