Interaction of marine heatwaves and grazing on two canopy-forming algae

Brown canopy-forming macroalgae species form complex assemblages known as marine forests, which are highly productive systems and provide multiple ecosystem services. In many regions worldwide, these key foundation species are being replaced by generalist species due to numerous underlying impacts acting at local, regional and global scales (i.e. overgrazing, marine heatwaves (MHWs), rising temperatures, among others). In Madeira Island (Macaronesia, NE Atlantic Ocean), rocky bottoms on the south coast are dominated by bare rocks, sediments and high densities of sea urchins and turfs. One of the most common macroalgae in these degraded systems is the cryptogenic red algae Asparagopsis taxiformis, naturally competing with other canopy-forming algae such as Cystoseira foeniculacea. In this study, we analysed 37-year high-resolution sea surface temperature data on the occurrence of MHWs in Macaronesia, finding a significant increase in the frequency of moderate and extreme events in the last 20 years. We performed a mesocosm experiment to understand how these two foundation species respond to heat stress and grazing activity by the sea urchin Paracentrotus lividus under different scenarios simulating moderate and extreme MHWs. Biomass and productivity of C. foeniculacea showed a significant reduction under the interaction of MHWs and grazing, with higher loss of biomass at the moderate event. The biomass of A. taxiformis decreased significantly by the increase in temperatures, independetly of the grazing pressure. The productivity of A. taxiformis was significantly reduced at extreme MHWs, where respiration rate was higher than C. foeniculacea, which outperformed in the dark phase. Results suggest that under control conditions and moderate MHWs, A. taxiformis is not impacted by grazing and is possibly more efficient in resource utilisation than C. foeniculacea, but losing this potential advantage under extreme MHWs. Under the context of future climate change, marine forests in Madeira are, thus, expected to be threatened by the combined effects of more common extreme climatic events (e.g. MHWs) and local changes in the biotic interactions (e.g. grazing and competition).