Impact of ocean acidification and warming on the functioning of a North Eastern Atlantic maerl bed community

Coralline red calcareous algae are especially vulnerable to OA due to the high solubility of their magnesium calcite skeleton. In communities dominated by coralline algae like maerl beds, OA and warming can lead to significant changes in species interactions and community structure and functioning with drastic effects on ecosystems. This work aims to assess the impact of OA and warming on the response of a maerl bed community, by considering interactions between calcareous and non-calcareous algae and interactions between herbivores and macroalgae. Experimental assemblages of the coralline algae <i>Lithothamnion corallioides</i> (living and dead maerl), and associated epiphytic fleshy macroalgae and dominant grazers (gastropods <i>Jujubinus exasperatus</i> and <i>Gibbula magus</i> and echinoderm <i>Psammechinus miliaris</i>) were maintained in mesocosms in winter and summer conditions under controlled pH (ambient and -0.32 pH units [RCP8.5, IPCC 2014]) and temperature (ambient and + 3.7°C [RCP8.5]) conditions. Measurements of metabolic rates (primary production, respiration, and/or calcification) were performed at both species and community scales. In winter, the respiration rates of the grazers were mainly influenced by temperature and/or pH, while their calcification rates remained unaffected. Primary production and respiration rates in living maerl were affected by temperature but not pH. In contrast, its calcification rates were negatively affected by pH decrease but increased with temperature. OA also induced significant dissolution rates in dead maerl. At the community scale, there was no effect of OA and warming on community primary production and respiration. Warming enhanced community calcification rates, whereas OA lead to a significant drop in calcification with high dissolution rates observed in the dark. The differences observed here between community and species responses highlight the importance of species interactions in the community functioning and its response to global changes.

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