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Since the industrial revolution, anthropogenic CO2 emissions have caused ocean acidification, which particularly affects calcified organisms. Given the fan-like calcified fronds of the brown alga Padina pavonica, we evaluated the acute (short-term) effects of a sudden pH drop due to a submarine volcanic eruption (October 2011–early March 2012) affecting offshore waters around El Hierro Island (Canary Islands, Spain). We further studied the chronic (long-term) effects of the continuous decrease in pH in the last decades around the Canarian waters. In both the observational and retrospective studies (using herbarium collections of P. pavonica thalli from the overall Canarian Archipelago), the percent of surface calcium carbonate coverage of P. pavonica thalli were contrasted with oceanographic data collected either in situ (volcanic eruption event) or from the ESTOC marine observatory data series (herbarium study). Results showed that this calcified alga is sensitive to acute and chronic environmental pH changes. In both cases, pH changes predicted surface thallus calcification, including a progressive decalcification over the last three decades. This result concurs with previous studies where calcareous organisms decalcify under more acidic conditions. Hence, Padina pavonica can be implemented as a bio-indicator of ocean acidification (at short and long time scales) for monitoring purposes over wide geographic ranges, as this macroalga is affected and thrives (unlike strict calcifiers) under more acidic conditions.

Human impacts accelerate the intensity and frequency of perturbations on ecosystems; approaches that integrate responses across organization levels are, however, lacking, particularly in the ocean. We experimentally simulated the frequency of fertilization ('chronic' versus 'pulse' events) in orthogonal combinations of two intensities ('large' versus 'moderate' fertilization) to determine physiological and biological responses by the seagrass Cymodocea nodosa and associated flora (epiphytes and green seaweeds, specifically Caulerpa prolifera), as well as functional changes (community primary and secondary productivity) at the ecosystem level. We predicted that the absence of recovery time from chronic perturbation would more severely affect responses at population and ecosystem levels relative to discrete events (pulses). Nutrient enrichment increased the biomass of C. prolifera irrespective of its frequency, whereas seagrass biomass and shoot density particularly decreased under a chronic scenario. These demographic responses were connected with varying photophysiological performance of both C. nodosa and C. prolifera. Fertilization, regardless of its intensity and frequency, decreased the maximum photosynthetic rate of C. nodosa, concomitant with increased pigments, particularly under chronic fertilization, and decreased photoprotective (phenols) compounds. In contrast, fertilization boosted the maximum photochemical yield of C. prolifera, in addition to an increase in pigments and photoprotective compounds. Community primary and secondary productivity, however, did not vary under fertilization of varying intensity and frequency. In summary, fertilization precipitated population-level changes in physiological and biological attributes of vegetation. However, fertilization effects did not entirely cascade into ecosystem-level processes, that is, ecosystem productivity, which suggests a functional compensation (that is, increased algal per-1455 formance to offset losses of seagrass production) during the initial stages of fertilization.

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