Bénard, Robin (1)*, Levasseur, Maurice (1), Ferreyra, Gustavo (2), Mucci, Alfonso (3), Scarratt, Michael G. (4), Starr, Michel (4), Blais, Marie-Amélie (1), Tremblay, Jean-Éric (1)
1 Université Laval, Québec, Québec, G1V 0A6, Canada
2 Université du Québec à Rimouski, Rimouski, Québec, G5L 3A1, Canada
3 McGill University, Montréal, Québec, H3A 0G4, Canada
4 Maurice-Lamontagne Institute, Mont-Joli, Québec, G5H 3Z4
Anthropogenic carbon dioxide (CO2) emissions will result in concomitant warming and acidification of oceanic waters. There is still limited information on how changes in these two drivers will affect ocean ecosystems. To start filling this knowledge gap, we conducted a mesocosm experiment where we exposed a natural plankton community of the St. Lawrence Estuary to two temperatures (in situ and +5°C) and to a range of decreasing pH conditions (from -0.2 to -0.6 unit). The pH was manipulated by addition of CO2-bubbled artificial seawater to ~3m3 mesocosms and was kept constant for 14 days. A phytoplankton bloom dominated by the diatom Skeletonema costatum developed in all twelve enclosures resulting in a complete nutrients drawdown. Phytoplankton growth was higher at the highest temperature but was not affected by the decreasing pH. Dimethylsufide (DMS) concentrations remained relatively low during the development of the bloom but increased significantly during the declining phase. DMS net production was enhanced by 250% due to the increasing temperature and slightly negatively impacted by acidification. These results suggest that the potential negative impact of ocean acidification on DMS ocean production may be cancelled by the stronger positive effect of warming on the recycling of the most important source of sulfur of the atmosphere.