Seagrasses play an essential ecological role within coastal habitats and their worldwide population decline has been linked to different types of anthropogenic forces. We investigated, for the first time, the combined effects of future ocean warming and acidification on fundamental biological processes of Zostera noltii, including shoot density, leaf coloration, photophysiology (electron transport rate, ETR; maximum PSII quantum yield, Fv/Fm) and photosynthetic pigments. Shoot density was severely affected under warming conditions, with a concomitant increase in the frequency of brownish colored leaves (seagrass die-off). Warming was responsible for a significant decrease in ETR and Fv/Fm (particularly under control pH conditions), while promoting the highest ETR variability (among experimental treatments). Warming also elicited a significant increase in pheophytin and carotenoid levels, alongside an increase in carotenoid/chlorophyll ratio and De-Epoxidation State (DES). Acidification significantly affected photosynthetic pigments content (antheraxanthin, β-carotene, violaxanthin and zeaxanthin), with a significant decrease being recorded under the warming scenario. No significant interaction between ocean acidification and warming was observed. Our findings suggest that future ocean warming will be a foremost determinant stressor influencing Z. noltii survival and physiological performance. Additionally, acidification conditions to occur in the future will be unable to counteract deleterious effects posed by ocean warming.

The macrobenthic production of an estuarine system was evaluated over a 14-year 12 study period in a seagrass bed and in a sandflat. Over this period, the estuary suffered severe 13 eutrophication and extreme weather events with important impacts on the community, 14impairing system functioning and ultimately the goods and services provided by the estuary 15 (decline in the seagrass bed, decreased community production and/or a boost in the 16 production by opportunist species, such as Hydrobia ulvae). Following the anthropogenic 17 impacts, management measures were introduced which allowed a gradual recovery of the 18 seagrass bed and a new macrobenthic community structure manifested by production 19 increases of slow-growing species, such as Scrobicularia plana and Hediste diversicolor. 20There was a gradual re-orientation of energy into population biomass instead of population 21 density but this was not translated into higher community production, mainly due to the 22 decreased production of opportunist species (H. ulvae and several polychaetes). Several 23 weather extreme events occurred during this post-mitigation phase -floods, heatwaves and 24 droughts, all of which had negative impacts on macrobenthic dynamics and production. The 25 33

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