Martine Lizotte

Biological and physical processes influencing sea ice, under‐ice algae, and dimethylsulfoniopropionate during spring in the Canadian Arctic Archipelago

This study presents temporal variations in concentrations of chlorophyll a (Chl a), particulate and dissolved dimethylsulfoniopropionate (DMSPp and DMSPd) in the sea ice and underlying water column in the Canadian Arctic Archipelago during the spring of 2010 and 2011. During both years, bottom ice Chl a, DMSPp and DMSPd concentrations were high (up to 1328 µg L−1, 15,082 nmol L−1, and 6110 nmol L−1, respectively) in May and decreased thereafter. The release of bottom ice algae and DMSPp in the water column was gradual in 2010 and rapid (8 days) in 2011. Bottom brine drainage during the presnowmelt period in 2010 and a rapid loss of the snow cover in 2011 coinciding with rain events explain most of the difference between the 2 years. During both years, less than 13% of the DMSPd lost from the ice was detected in the water column, suggesting a rapid microbial consumption. An under-ice diatom bloom developed in both years. In 2010, the bloom was dominated by centric diatoms while in 2011 pennates dominated, likely reflecting seeding by ice algae following the faster snowmelt progression induced by rainfall events in 2011. Both under-ice blooms were associated with high DMSPp concentrations (up to 185 nmol L−1), but pennate diatoms showed DMSPp/Chl a ratios twice higher than centrics. These results highlight the key role of snowmelt and precipitation on the temporal pattern of ice-DMSP release to the water column and on the timing, taxonomic composition, and DMSP content of phytoplankton under-ice blooms in the Arctic.

Macroscale patterns of the biological cycling of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in the Northwest Atlantic

The influence of the seasonal development of microplankton communities on the cycling of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was investigated along a South–North gradient (36–59°N) in the Northwest (NW) Atlantic Ocean. Three surveys allowed the sampling of surface mixed layer (SML) waters at stations extending from the subtropical gyre to the Greenland Current during May, July and October 2003. Pools and transformation rates of DMSP and DMS were quantified and related to prevailing physical and biochemical conditions, phytoplankton abundance and taxonomic composition, as well as bacterioplankton abundance and leucine uptake. The South–North progression of the diatom bloom, a prominent feature in the NW Atlantic, did not influence the production of DMS whereas conditions in the N Atlantic Drift lead to a persistent bloom of DMSP-rich flagellate-dominated phytoplankton community and high net DMS production rates. Macroscale patterns of the observed variables were further explored using principal component analysis (PCA). The first axis of the PCA showed a strong association between the spatio-temporal distribution of DMSP and the abundance of several phytoplankton groups including dinoflagellates and prymnesiophytes, as well as with microbial-mediated DMSPd consumption and yields and rates of the conversion of DMSP into DMS. The second axis revealed a strong association between concentrations of DMS and SML depth and photosynthetically active radiation, a result supporting the prominent role of solar radiation as a driver of DMS dynamics.

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) cycling across contrasting biological hotspots of the New Zealand Subtropical Front

The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethyl sulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the subtropical front (STF). This study focused on two opposing short-term fates of DMSP-S following its uptake by microbial organisms (either its conversion into DMS or its assimilation into bacterial biomass) and has not considered dissolved non-volatile degradation products. Sampling took place in three phytoplankton blooms (B1, B2, and B3) with B1 and B3 occurring in relatively nitraterich, dinoflagellate-dominated subantarctic waters, and B2 occurring in nitrate-poor subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 and 14.5 nmol L−1, respectively. Pools of DMSPt showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs= 0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs= 0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs= 0.84). Estimates of the potential contribution of microbially mediated rates of DMS production (0.1–27 nmol L−1 day−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds.

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 15 dimethylsulfide (DMS) were investigated during a mesocosm experiment in the Lower St. Lawrence Estuary (LSLE) in the 16 fall of 2014. Twelve mesocosms covering a range of pHT (pH on the total hydrogen ion concentration scale) from 8.0 to 7.2, 17 corresponding to a range of CO2 partial pressures (pCO2) from 440 to 2900 μatm, at two temperatures (in situ and +5 °C; 10 °C 18 and 15 °C) was monitored during 13 days. All mesocosms were characterized by the rapid development of a diatom bloom 19 dominated by Skeletonema costatum, followed by its decline upon the exhaustion of nitrate and silicic acid. Neither the 20 acidification nor the warming resulted in a significant impact on the abundance of bacteria over the experiment. However, 21 warming the water by 5 °C resulted in a significant increase of the average bacterial production (BP) in all 15 °C mesocosms 22 as compared to 10 °C, with no detectable effect of pCO2 on BP. Variations in total DMSP (DMSPt = particulate + dissolved 23 DMSP) concentrations tracked the development of the bloom although the rise in DMSPt persisted for a few days after the 24 peaks in chlorophyll a. Average concentrations of DMSPt were not affected by acidification or warming. Initially low 25 concentrations of DMS (< 1 nmol L) increased to reach peak values ranging from 30 to 130 nmol L towards the end of the 26 experiment. Increasing the pCO2 reduced the averaged DMS concentrations by 66 % and 69 % at 10 °C and 15 °C, 27 respectively, over the duration of the experiment. On the other hand, a 5 °C warming increased DMS concentrations by an 28 average of 240 % as compared to in situ temperature, resulting in a positive offset of the adverse pCO2 impact. Significant 29 positive correlations found between bacterial production rates and concentrations of DMS throughout our experiment point 30 towards temperature-associated enhancement of bacterial DMSP metabolism as a likely driver for the mitigating effect of 31 warming on the negative impact of acidification on the net production of DMS in the LSLE and potentially the global ocean. 32 Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-338 Manuscript under review for journal Biogeosciences Discussion started: 22 August 2018 c