Experimental assessment of the sensitivity of an estuarine phytoplankton fall bloom to acidification and warming

• Published in: Biogeosciences
• Main Authors: Bénard, Robin, Levasseur, Maurice, Scarratt, Michael, Blais, Marie-Amélie, Mucci, Alfonso, Ferreyra, Gustavo, Starr, Michel, Gosselin, Michel, Tremblay, Jean-Éric, Lizotte, Martine
• Format: Text
• Language: English
• Published: 2019
• Subjects: Canada; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-15-4883-2018; https://www.biogeosciences.net/15/4883/2018/

We investigated the combined effect of ocean acidification and warming on the dynamics of the phytoplankton fall bloom in the Lower St. Lawrence Estuary (LSLE), Canada. Twelve 2600 L mesocosms were set to initially cover a wide range of pH T (pH on the total proton scale) from 8.0 to 7.2 corresponding to a range of p CO 2 from 440 to 2900 µ atm, and two temperatures (in situ and +5 ∘ C). The 13-day experiment captured the development and decline of a nanophytoplankton bloom dominated by the chain-forming diatom Skeletonema costatum . During the development phase of the bloom, increasing p CO 2 influenced neither the magnitude nor the net growth rate of the nanophytoplankton bloom, whereas increasing the temperature by 5 ∘ C stimulated the chlorophyll a (Chl a ) growth rate and maximal particulate primary production ( P P ) by 76 % and 63 %, respectively. During the declining phase of the bloom, warming accelerated the loss of diatom cells, paralleled by a gradual decrease in the abundance of photosynthetic picoeukaryotes and a bloom of picocyanobacteria. Increasing p CO 2 and warming did not influence the abundance of picoeukaryotes, while picocyanobacteria abundance was reduced by the increase in p CO 2 when combined with warming in the latter phase of the experiment. Over the full duration of the experiment, the time-integrated net primary production was not significantly affected by the p CO 2 treatments or warming. Overall, our results suggest that warming, rather than acidification, is more likely to alter phytoplankton autumnal bloom development in the LSLE in the decades to come. Future studies examining a broader gradient of temperatures should be conducted over a larger seasonal window in order to better constrain the potential effect of warming on the development of blooms in the LSLE and its impact on the fate of primary production.