Warming and acidification effects on biofilm/grazer interaction in an early-stage rocky intertidal community

Anthropogenic carbon dioxide (CO2) emissions are increasing greenhouse effects and changing seawater carbonate chemistry, leading to ocean warming and acidification. Increasing temperature and decreasing seawater pH can potentially interact to cause changes in species interactions and community shifts. To understand how ocean warming and acidification may affect the early stages of an intertidal community in the Mediterranean, we set up an in situ experiment and assessed potential changes in grazer density andbiofilm composition by manipulating temperature on bare rock plots (10x10cm)in two locations along a natural coastal CO2 gradient (Low pH: 7.8 pH, and Ambient:8.1 pH) off Vulcano Island.A total of 36black and white-framed plots were positioned and randomly assigned to closed cage (exclusion), open cage (artefact) and no cage (control) treatments. ‘Robo-limpets’ revealed that black plots were on average +0.25°C warmer and the max temperature was +2°C higher than white plots. Density of limpets did not differ between pH and temperature treatments, and was significantly lower in exclusion plots. CCA% was significantly higher in Ambient than Low pH. No differences in grazing effects were recorded in all pHxT combinations. Numbers of early-stage phytobenthic taxa was higher in white than black plots. Our findings – representing the first in situ evidence after manipulations of multiple biotic and abiotic drivers of change – suggest that warming and/or acidification might affect early-stage intertidal assemblages in different ways. DNA-based analyses are underway to provide a more detailed understanding on phytobenthos composition and will help further in disentangling these effects.