Abstract
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.
Highlights
Over the last centuries, atmospheric CO2 concentrations have increased due to human activities [1]
A multiple linear regression, testing the dependency of mean decalcified percentages to mean pHF and seawater temperature (n = 15), showed that the former was a significant predictor of calcium carbonate coverage on P. pavonica thalli (P,0.05, power = 0.969)
The submarine volcanic eruption off El Hierro Island, already described at an oceanographic scale [25,26], affected onshore areas of the entire island in November 2011, with increasing decalcification on P. pavonica thalli related to decreasing pHF values towards the submarine volcano
Summary
Atmospheric CO2 concentrations have increased due to human activities [1]. As a global net sink, oceans have absorbed almost one third of these anthropogenic CO2 emissions [2], causing readjustments in the carbonate chemistry and lowering the pH This phenomenon was originally known as Ocean Acidification (OA), a term that has been broadened to include other natural events (i.e., increased volcanic activity, methane hydrate releases, long-term changes in net respiration) and anthropogenic causes (i.e., release of nitrogen and sulphur compounds into the atmosphere) [3]. This chemical process has been observed by many oceanic long-term observatories belonging to the OceanSITES programme, such as BATS (Bermuda Atlantic Time-series Study), HOT (Hawaii Ocean Time-series station) and ESTOC (European Station for Time-series in the Ocean at the Canary Islands). These values have drawn scientific attention towards their future ecological and physiological impacts, especially on calcified benthic organisms such as corals, echinoderms, gastropods and several calcareous macroalgae [4,5,6]
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