Contrasting effects of acidification and warming on dimethylsulfide concentrations during a temperate estuarine fall bloom mesocosm experiment

The effects of ocean acidification and warming on the concentrations of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) were investigated during a mesocosm experiment in the Lower St. Lawrence Estuary (LSLE) in the fall of 2014. Twelve mesocosms covering a range of pHT (pH on the total hydrogen ion concentration scale) from 8.0 to 7.2, corresponding to a range of CO2 partial pressures (pCO2) from 440 to 2900µatm, at two temperatures (in situ and +5°C; 10°C and 15°C) was monitored during 13 days. All mesocosms were characterized by the rapid development of a diatom bloom dominated by Skeletonema costatum, followed by its decline upon the exhaustion of nitrate and silicic acid. Neither the acidification nor the warming resulted in a significant impact on the abundance of bacteria over the experiment. However, warming the water by 5°C resulted in a significant increase of the average bacterial production (BP) in all 15°C mesocosms as compared to 10°C, with no detectable effect of pCO2 on BP. Variations in total DMSP (DMSPt=particulate+dissolved DMSP) concentrations tracked the development of the bloom although the rise in DMSPt persisted for a few days after the peaks in chlorophyll a. Average concentrations of DMSPt were not affected by acidification or warming. Initially low concentrations of DMS (<1nmolL−1) increased to reach peak values ranging from 30 to 130nmolL−1 towards the end of the experiment. Increasing the pCO2 reduced the averaged DMS concentrations by 66% and 69% at 10°C and 15°C, respectively, over the duration of the experiment. On the other hand, a 5°C warming increased DMS concentrations by an average of 240% as compared to in situ temperature, resulting in a positive offset of the adverse pCO2 impact. Significant positive correlations found between bacterial production rates and concentrations of DMS throughout our experiment point towards temperature-associated enhancement of bacterial DMSP metabolism as a likely driver for the mitigating effect of warming on the negative impact of acidification on the net production of DMS in the LSLE and potentially the global ocean.

Bénard R., Levasseur M., Scarratt M., Michaud S., Starr M., Mucci A., Ferreyra G., Gosselin M., Tremblay J.-É., Lizotte M. & Yang G.-P., 2018. Contrasting effects of acidification and warming on dimethylsulfide concentrations during a temperate estuarine fall bloom mesocosm experiment. Biogeosciences Discussions. Article.

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