Contrasting effects of acidification and warming on dimethylsulfide 2 concentrations during a temperate estuarine fall bloom mesocosm 3 experiment

BENARD ROBIN; GUSTAVO FERREYRA

BIOGEOSCIENCES

COPERNICUS PUBLICATIONS

Lugar: Gottingen; Año: 2018

1726-4170

The effects of ocean acidification and warming on the concentrations of dimethylsulfoniopropionate (DMSP) and16 dimethylsulfide (DMS) were investigated during a mesocosm experiment in the Lower St. Lawrence Estuary (LSLE) in the17 fall of 2014. Twelve mesocosms covering a range of pHT (pH on the total hydrogen ion concentration scale) from 8.0 to 7.2,18 corresponding to a range of CO2 partial pressures (pCO2) from 440 to 2900 µatm, at two temperatures (in situ and +5 °C; 10 °C19 and 15 °C) was monitored during 13 days. All mesocosms were characterized by the rapid development of a diatom bloom20 dominated by Skeletonema costatum, followed by its decline upon the exhaustion of nitrate and silicic acid. Neither the21 acidification nor the warming resulted in a significant impact on the abundance of bacteria over the experiment. However,22 warming the water by 5 °C resulted in a significant increase of the average bacterial production (BP) in all 15 °C mesocosms23 as compared to 10 °C, with no detectable effect of pCO2 on BP. Variations in total DMSP (DMSPt = particulate + dissolved24 DMSP) concentrations tracked the development of the bloom although the rise in DMSPt persisted for a few days after the25 peaks in chlorophyll a. Average concentrations of DMSPt were not affected by acidification or warming. Initially lowconcentrations of DMS (< 1 nmol L-1) increased to reach peak values ranging from 30 to 130 nmol L-1 26 towards the end of the27 experiment. Increasing the pCO2 reduced the averaged DMS concentrations by 66 % and 69 % at 10 °C and 15 °C,28 respectively, over the duration of the experiment. On the other hand, a 5 °C warming increased DMS concentrations by an29 average of 240 % as compared to in situ temperature, resulting in a positive offset of the adverse pCO2 impact. Significant30 positive correlations found between bacterial production rates and concentrations of DMS throughout our experiment point31 towards temperature-associated enhancement of bacterial DMSP metabolism as a likely driver for the mitigating effect of32 warming on the negative impact of acidification on the net production of DMS in the LSLE and potentially the global ocean.