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 pHT (pH on the total proton scale) from 8.0 to 7.2 corresponding to a range of pCO2 from 440 to 2900 µatm, 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 pCO2 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 particulate primary production (PP) by 50 % and 160 %, respectively. During the declining phase of the bloom, warming accelerated the loss of diatom cells and negatively affected PP. Due to the countervailing responses of the plankton community to warming during the two phases of the experiment, the time-integrated primary production was not significantly affected over the full duration of the study. The diatom bloom was paralleled by a gradual decrease in the abundance of photosynthetic picoeukaryotes and followed by a bloom of picocyanobacteria. Increasing pCO2 and warming did not influence the abundance of picoeukaryotes, but warming stimulated picocyanobacteria proliferation. 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.
Bénard R., Levasseur M., Scarratt M. G., Blais M.-A., Mucci A., Ferreyra G., Starr M., Gosselin M., Tremblay J.-É. & Lizotte M., 2018. Experimental assessment of the sensitivity of an estuarine phytoplankton fall bloom to acidification and warming. Biogeosciences Discussions. doi:10.5194/bg-2018-31. Article.