Response of Phytoplankton Assemblages from Naturally Acidic Coastal Ecosystems off Chile to Ocean Acidification

The interplay of coastal oceanographic processes usually results in partial pressures of CO₂ (pCO₂) higher than expected from the equilibrium with the atmosphere and even higher than those expected by the end of the century caused by ocean acidification (OA). In spite of this, the natural variability of the seawater chemistry at the locations where the organisms are collected is seldomly considered in OA experiments. In this work, we aimed to evaluate the role of the carbonate chemistry dynamics in shaping the response of coastal phytoplankton communities to increased pCO₂ levels. The study was conducted at two coastal ecosystems off Chile, the Valdivia River estuary and the coastal upwelling ecosystem in the Gulf of Arauco. We characterized the seasonal variability (winter/summer) of the hydrographic conditions, the carbonate system parameters and the phytoplankton community structure at both sites. The results showed that carbonate chemistry dynamics in the estuary were mainly related to seasonal changes in freshwater discharges, with acidic and corrosive conditions dominating in winter. In the Gulf of Arauco, these conditions were observed in summer associated with the upwelling of high pCO₂ (>1000 μatm) waters. Both phytoplankton communities were dominated by diatoms, yet the one in Valdivia was more diverse. Only certain phytoplankton groups in this latter ecosystem showed a significant correlation with the carbonate system parameters. When the impact of elevated pCO₂ levels was investigated by CO₂ manipulation experiments, we did not observe any significant effect on either of the two communities. Changes in the phytoplankton species composition and abundance during the incubations were related to other factors, such as competition and growth phases. Our findings highlight the importance of the natural variability of coastal ecosystems to determine the potential for local adaptation of coastal phytoplankton communities to face predicted scenarios of OA.