Recent works have begun to explore the magnitude and frequency of localized changes in seawater chemistry in shallow water systems, where the effects of warming and acidification are still unpredictable. In a Mediterranean shallow coastal site, we empirically characterized the diel, seasonal, and annual pattern of pH, temperature, and associated chemical changes on one Ellisolandia elongata ‘corniche’, (i.e. a coastal algal biogenic reef). Local benthic metabolism together with temperature, were two of the main drivers for the carbonate system variation over diel and seasonal cycles. During the 12-month study, a total of seven heatwave events were recorded (two of which occurred in winter) with the longest lasting 11 days. The present study highlights the importance of improving the knowledge of changes and dynamics occurring at local scale, by extending in situ data acquisition at shallow coastal sites, in order to better assess the impacts of climate change on both environment and ecosystems. © 2021 Elsevier B.V.

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