Sonia Michaud

A new source of dimethylsulfide (DMS) for the arctic atmosphere: ice diatoms

We report the first evidence that pennate diatoms growing within the bottom layer of first-year ice in the Arctic produce significant amounts of particulate dimethylsulfoniopropionate (DMSPp) and dissolved DMSP+DMS. In 1992 in Resolute Passage, a tributary of Barrow Strait, DMSPp concentrations within the bottom layer of ice reached 1055 mg S m-3 at the end of the vernal bloom, a value one order of magnitude higher than the maximum value reported in antarctic ice. Bottom-ice concentrations in DMSPp and DMSPd+DMS were significantly correlated with the abundance of the dominant pennate diatom Nitzschia frigida. Intracellular concentration in DMSP of ice algae was very low (0.001 pg cell-1) at the end of April when algae were light-limited and reached 1.17 pg cell-1 in mid-May following an increase in light and algal growth. We calculate that the rapid release of the dissolved DMSP+DMS from the ice into surface waters following the ice break-up will generate a sea-to-air DMS flux of 0.7 mg S m-2 d-1, a pulse ten times higher than the mean arctic summer flux. We estimate that this 1-d pulse represents up to 5% of the annual DMS emission in the Arctic.

Production of DMSP and DMS during a mesocosm study of an Emiliania huxleyi bloom: influence of bacteria and Calanus finmarchicus grazing

We investigated the influence of bacteria and metazooplankton on the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) during blooms of Emiliania huxleyi (Lohmann) Hay and Mohler in seawater mesocosms. The phytoplankton succession was marked by the rapid collapse of an initial Skeletonema costatum (Greville) Cleve bloom followed by a small E. huxleyi bloom. The collapse of the diatom bloom was accompanied by an increase in concentrations of dissolved DMSP (DMSPd) and bacterial abundance and activity (as determined by the thymidine incorporation technique). The increase in bacterial activity was followed by a rapid decrease in DMSPd concentrations which remained low for the rest of the experiment, even during the subsequent collapse of the E. huxleyi blooms. The absence of DMSPd and DMS peaks during the declining phase of the E. huxleyi blooms was attributed to the high bacterial activity prevailing at that time. The influence of metazooplankton grazing on DMSP and DMS production was investigated by adding moderate (24 mg dry weight m-3) and high (520 mg dry weight m-3) concentrations of Copepodite Stage V and adults of Calanus finmarchicus to two of four filtered (200 μm mesh net) enclosures during the E. huxleyi blooms. The addition of C. finmarchicus, even in high concentrations, had no apparent effect on the dynamics of E. huxleyi, suggesting that the copepods were not grazing significantly on nanophytoplankton. The addition of copepods in high concentrations favored an accumulation of chlorophyll a and particulate DMSP. These results suggest that copepods were preying on the herbivorous microzooplankton which, in turn, was controlling the biomass of nanophytoplankton. DMS production was also enhanced in the enclosure with maximum metazooplankton biomass, suggesting that the grazing of C. finmarchicus on microzooplankton containing DMSP may contribute to DMS production. These results provide strong support to the emerging idea that bacteria and metazooplankton grazing play a dominant role in determining the timing and magnitude of DMS pulses following phytoplankton blooms.

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.

Biogenic sulfur emissions from the Gulf of Saint Lawrence and assessment of its impact on the Canadian east coast

We measured dimethylsulfide (DMS) concentrations over a grid of 79 stations covering the central part of the Gulf of Saint Lawrence and during a 24-hour sampling period at a fixed station, in August 1993. Surface water DMS concentrations ranged from <0.9 to 9 nM over the sampling grid. DMS sea-to-air fluxes ranged from <0,1 to 23 μmol m−2 d−1, with a mean of 4.7 μmol m−2 d−1 for August. Atmospheric DMS levels measured during the cruise ranged from less than 0.5 nmol m−3 (12 parts per trillion by volume, or pptv) to 25 nmol m−3 (625 pptv) with a mean value of 11.9 nmol m−3 (291 pptv). Results from the 24-hour sampling at the fixed station show that ventilation may represent the most important sink for surface DMS when wind speeds exceed approximately 8 m s−1. The relative magnitude of the biogenic and anthropogenic S components was estimated by comparing sulfur emissions from the gulf to the sulfur emissions from surrounding continental regions and to a sulfur budget calculated by using a Lagrangian box model. The first approach suggests that DMS emissions from the gulf represented at least 4% of total (biogenic plus anthropogenic) sulfur emissions from the Atlantic Provinces in August 1993. A Lagrangian forward motion trajectory shows that these biogenic emissions would affect all the Maritime Provinces and a portion of eastern Quebec. The second approach suggests that the contribution of the gulf to the total sulfur flux may reach 13% when compared with the anthropogenic inflow of sulfur from the Atlantic Provinces impacting the Maritimes. When the transboundary inflow of sulfur from the United States is also taken into account, the contribution of the gulf becomes 8%. Thus DMS emissions from coastal waters may represent a significant source of sulfur to the atmosphere.

Modeling analysis of the effect of iron enrichment on dimethyl sulfide dynamics in the NE Pacific (SERIES experiment)

The large-scale iron enrichment conducted in the NE Pacific during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES) triggered a phytoplankton bloom dominated successively by nanophytoplankton and large diatoms. During the first 14 days, surface dimethyl sulfide (DMS) levels increased both inside (up to 22 nmol L-1) and outside (up to 19 nmol L-1) the patch, with no consistent Fe effect. Later, DMS concentrations became sixfold lower inside the patch than outside. In this study, we used a DMS budget module embedded in a one-dimensional ocean turbulence model to investigate the contribution of the interacting physical, photochemical, and biological processes to this particular DMS response. Temporal variations in biological net DMS production were reconstructed using an inverse modeling approach. Our results show that short-term (days) variations in both the physical processes (i.e., turbulent mixing and ventilation) and the biological cycling of DMS are needed to explain the time evolution of DMS concentrations both outside and inside the Fe-enriched patch. The biological net DMS production was generally high (up to 0.35 nmol L-1 h-1) and comparable outside and inside the patch during the first 10 days, corresponding to the observed accumulation of DMS inside and outside the patch. Later, it became negative (net DMS biological consumption) inside the patch, suggesting a change in dimethylsulfoniopropionate bacterial metabolism. This study stresses the importance of short-term variations in biological processes and their sensitivity to the physical environment in shaping the DMS response to iron enrichment.

DMSP and DMS in the Northwest Atlantic : Late-summer distributions, production rates and sea-air fluxes

Abstract.DMSP and DMS were measured along a set of transects in the Northwest Atlantic during September, 1999. Six 24 h Lagrangian stations were occupied between 36° and 61° N latitude, covering subtropical to polar water types. Profiles of total DMSP (DMSPt), DMS, chl a, and oceanographic variables were determined at each station. Phytoplankton abundance and species assemblage were determined in surface waters and at the depth of the Chl a maximum in all profiles. Between profile stations, DMSPt and DMS samples were collected by a pump while the vessel was moving. Chl a and DMSPt were most abundant in the northern regions, with very low levels in subtropical waters. There was no direct correlation between DMSPt and Chl a. Maximum DMSPt concentrations reached 203 nM in coastal waters and 112 nM in the open ocean. A strong correlation was observed between DMSPt and the abundance of dinoflagellates (Spearman r=0.91; p <0.0001; n=13) and prymnesiophytes (Spearman r=0.91; p<0.0001; n=13). Cryptophytes also showed a weak but significant correlation (Spearman r=0.58; p=0.039; n=13). The waters around Greenland were the only site dominated by diatoms and their abundance was not correlated with DMSPt concentrations. DMS concentrations were low and fairly uniform, with maximum levels of 4.7 nM in coastal waters and 2.2 nM in the open ocean. DMS fluxes from surface waters were calculated based on observed sea-surface concentrations and wind speeds and showed a strong peak associated with a storm event, although no depletion of DMS resulting from the storm was observed. In situ incubation experiments showed DMSP consumption and DMS production rates to be relatively high, notwithstanding the generally low phytoplankton biomass.

Multispecies mass mortality of marine fauna linked to a toxic dinoflagellate bloom

Following heavy precipitation, we observed an intense algal bloom in the St. Lawrence Estuary (SLE) that coincided with an unusually high mortality of several species of marine fish, birds and mammals, including species designated at risk. The algal species was identified as Alexandrium tamarense and was determined to contain a potent mixture of paralytic shellfish toxins (PST). Significant levels of PST were found in the liver and/or gastrointestinal contents of several carcasses tested as well as in live planktivorous fish, molluscs and plankton samples collected during the bloom. This provided strong evidence for the trophic transfer of PST resulting in mortalities of multiple wildlife species. This conclusion was strengthened by the sequence of mortalities, which followed the drift of the bloom along the coast of the St. Lawrence Estuary. No other cause of mortality was identified in the majority of animals examined at necropsy. Reports of marine fauna presenting signs of neurological dysfunction were also supportive of exposure to these neurotoxins. The event reported here represents the first well-documented case of multispecies mass mortality of marine fish, birds and mammals linked to a PST-producing algal bloom.

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