Impacts of CO₂ Perturbation on the Ecology and Biogeochemistry of Plankton Communities During a Simulated Upwelling Event: A Mesocosm Experiment in Oligotrophic Subtropical Waters

The ocean is a major sink for anthropogenic carbon dioxide (CO₂), taking up one third of fossil fuel CO₂ annually. This causes pronounced shifts in marine carbonate chemistry, including decreasing seawater pH and carbonate saturation states. A growing body of scientific evidence indicates that these changes – summarized by the term ocean acidification (OA) – can significantly affect marine life, with potential consequences for food webs and biogeochemical cycles. Our current understanding of OA effects is largely based on laboratory experiments under rather artificial environmental conditions and with cultures of single species, thereby neglecting ecological interactions. Studies on the response of natural communities are still relatively rare, with the few existing community-level studies mostly conducted in eutrophic environments.

To close this knowledge gap and better understand how natural communities and food webs in oligotrophic environments respond to ocean acidification, an in situ mesocosm experiment was conducted in the subtropical northeast Atlantic Ocean, off the island of Gran Canaria. To investigate how OA effects might differ between oligotrophic conditions and phases of high biological productivity, which regularly occur in response to upwelling of nutrient-rich deep water in the study region, a deep-water upwelling event was simulated in the mesocosms three weeks into the experiment.