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 pH_T (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 2900 µatm, at two temperatures (in situ and +5 ^∘C; 10 and 15 ^∘C) were 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 in 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 (DMSP_t = particulate + dissolved DMSP) concentrations tracked the development of the bloom, although the rise in DMSP_t persisted for a few days after the peaks in chlorophyll a. Average concentrations of DMSP_t were not affected by acidification or warming. Initially low concentrations of DMS (<1 nmol L⁻¹) increased to reach peak values ranging from 30 to 130 nmol L⁻¹ towards the end of the experiment. Increasing the pCO₂ reduced the averaged DMS concentrations by 66 % and 69 % at 10 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 and concentrations of DMS throughout our experiment point towards temperature-associated enhancement of bacterial DMSP metabolism as a likely driver of 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.