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Bibliography of the Maurice Lamontagne Institute

Michael G. SCARRATT

LIZOTTE, M., M. LEVASSEUR, S. MICHAUD, M.G. SCARRATT, A. MERZOUK, M. GOSSELIN, J. POMMIER, R.B. RIVKIN, R.P. KIENE, 2012. Macroscale patterns of the biological cycling of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in the Northwest Atlantic. Biogeochemistry, XX(X): XX-XX Article in press .

LUCE, M., M. LEVASSEUR, M.G. SCARRATT, S. MICHAUD, S.-J. ROYER, R. KIENE, C. LOVEJOY, M. GOSSELIN, M. POULIN, Y. GRATTON, M. LIZOTTE, 2011. Distribution and microbial metabolism of dimethylsulfoniopropionate and dimethylsulfide during the 2007 Arctic ice minimum. J. Geophys. Res. (C Oceans), 116(11). Art. no C00G06, 8 p .

The distribution and biological cycling of the climate active trace gas dimethylsulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP) were characterized at 20 stations across the Canadian High Arctic during fall 2007. Transformation rates of DMSP and production rates of DMS from dissolved DMSP (DMSPd) were measured during 3 h onboard incubations with radioactively labeled 35S-DMSP. Particulate DMSP (DMSPp) in surface waters varied between 2 and 39 nmol L-1 and increased with chlorophyll a (Chl a) concentrations (r = 0.84). DMS concentrations in surface waters ranged from 0.05 to 0.8 nmol L-1 and were positively correlated with DMSPp (r = 0.89) and Chl a (r = 0.74). The DMSPd loss rate constant varied from 0.01 to 0.14 h-1 and was also positively correlated with Chl a concentrations (r = 0.67). The turnover time of the DMSPd pool varied between 0.3 and 3.4 days (mean = 0.96 day). Bacterial DMS production varied between 0.01 and 0.51 nmol L-1 d-1 (mean = 0.14 nmol L-1 d-1). Assuming local steady state conditions at the time scale of a day, the turnover time of the DMS pool based only on production from DMSPd was ˜6 days at the sampling stations. This long turnover time suggests that DMS production was dominated by nonbacterial processes during our study. Our results show that DMS production could persist at low rates in late fall under ice-free conditions. The magnitude of this production appears to be limited by the low algal and bacterial production prevailing at that time.© 2011 American Geophysical Union

REMPILLO, O., A.M. SEGUIN, A.-L. NORMAN, M. SCARRATT, S. MICHAUD, R. CHANG, S. SJOSTEDT, J. ABBATT, B. ELSE, T. PAPAKYRIAKOU, S. SHARMA, S. GRASBY, M. LEVASSEUR, 2011. Dimethyl sulfide air-sea fluxes and biogenic sulfur as a source of new aerosols in the Arctic fall. J. Geophys. Res. (D Atmospheres), 116(24): art. no. D00S04 .

Dimethyl sulfide (DMS) and its oxidation products, which have been proposed to provide a climate feedback mechanism by affecting aerosol and cloud radiative properties, were measured on board the Canadian Coast Guard ship Amundsen in sampling campaigns in the Arctic in the fall of 2007 and 2008. DMS flux was calculated based on the surface water measurements and yielded 0.1–2.6 μmol m-2 d-1 along the Northwest Passage in 2007 and 0.2–1.3 μmol m-2 d-1 along Baffin Bay in 2008. DMS oxidation products, sulfur dioxide (SO2), methane sulfonic acid (MSA), and sulfate in aerosols were also measured. The amounts of biogenic SO2 and sulfate were approximated using stable isotope apportionment techniques. Calculating the threshold amount of SO2 needed for significant new particle formation from the formulation by Pirjola et al. (1999), the study suggests that instances of elevated biogenic SO2 concentrations (between 8 and 9 September 2008) derived using conservative assumptions may have been sufficient to form new aerosols in clean air conditions in the Arctic region.© 2011 American Geophysical Union

CHANG, R.Y.-W., S.J. SJOSTEDT, J.R. PIERCE, T.N. PAPAKYRIAKOU, M.G. SCARRATT, S. MICHAUD, M. LEVASSEUR, W.R. LEAITCH, J.P.D. ABBATT, 2011. Relating atmospheric and oceanic DMS levels to particle nucleation events in the Canadian Arctic. J. Geophys. Res. (D Atmospheres), 116(21): art. no. D00S03 .

Measurements of ocean surface and atmospheric dimethyl sulfide (DMS) and particle size distributions were made in the Canadian Arctic Archipelago during the fall of 2007 and the late summer of 2008 aboard the Canadian Coast Guard Ship Amundsen. Nucleation-mode particles were observed during the 2008 cruise, which took place in the eastern Arctic from August to September when the atmosphere and ocean were more photo-active as compared to the October 2007 transit in the Beaufort Sea during which no nucleation/growth events were observed. The observed nucleation periods in 2008 coincided with high atmospheric and ocean surface DMS concentrations, suggesting that the particles originated from marine biogenic sources. An aerosol microphysics box model was used to simulate nucleation given the measured conditions in the marine boundary layer. Although other sources may have contributed, we find that the newly formed particles can be accounted for by a marine biogenic DMS source for combinations of the following parameters: [OH] ≥ 3 × 105 molecules cm-3, DMS mixing ratio is ≥ 100 pptv, the activation coefficient is ≥ 10-7 and the background particle concentration is ≥ 100 cm-3.©2011American Geophysical Union

ROYER, S.-J., M. LEVASSEUR, M. LIZOTTE, M. ARYCHUK, M.G. SCARRATT, C.S. WONG, C. LOVEJOY, M. ROBERT, K. JOHNSON, A. PENA, S. MICHAUD, R.P. KIENED, 2010. Microbial dimethylsulfoniopropionate (DMSP) dynamics along a natural iron gradient in the northeast subarctic Pacific. Limnol. Oceanogr., 55(4): 1614-1626 .

We characterized the effect of an inshore–offshore gradient in Fe in the northeast subarctic Pacific on the bacterioplankton and phytoplankton assemblages and on the microbial cycling of particulate and dissolved dimethylsulfoniopropionate (DMSPp and DMSPd) and dimethylsulfide (DMS). Averaged concentrations of total dissolved Fe (TDFe) decreased linearly with increasing water density along the transect, from 3.4 nmol L-1 at the two inshore stations to 1.0 nmol L-1 at the offshore stations, as a result of the vertical and lateral mixing between the Fe-rich coastal water and the Fe-poor Alaska Current. The Fe-rich inshore stations were dominated by diatoms and characterized by low DMSPp : chlorophyll a (Chl a) ratios (ca. 26 nmol μg-1) and bacterial DMS yield (< 4 %). In contrast, the Fe-poor offshore stations were dominated by prymnesiophytes and exhibited high DMSPp : Chl a ratios (ca. 84 nmol μg-1) and bacterial DMS yield (8 %). Chl a, DMSPp, and the abundance of total bacteria and three bacterial clades (Gammaproteobacteria, Roseobacter, and Betaproteobacteria) were positively correlated with the TDFe gradient. At the Fe-poor offshore stations, the positive correlation found between TDFe and the DMSPp : Chl a ratios suggests that Fe supplied by mixing stimulated DMSP production in the prymnesiophyte-dominated assemblage, a response similar to that generally observed during the first days of most of the large-scale ocean iron fertilizations (OIFs). These results suggest that the stimulation of DMSP production takes place whatever the Fe supply mode: atmospheric dust deposition, as simulated by OIFs, or mixing, as reported in this study.©2010 American Society of Limnology and Oceanography, Inc.

LIZOTTE, M., M. LEVASSEUR, I. KUDO, K. SUZUKI, K., TSUDA, A., R.P. KIENE, M.G. SCARRATT, 2009. Iron-induced alterations of bacterial DMSP metabolism in the western subarctic Pacific during SEEDS-II. Deep-Sea Res., Part II, Top. Stud. Oceanogr., 56(26): 2889-2898 .

The effect of added iron on bacterial cycling of the climate-active gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was tested during the second Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS II) from 19 July to 21 August 2004 aboard the R/V Hakuho-Maru. The study area in the northwest Pacific Ocean (48°N 165°E) was enriched with Fe and the conservative tracer, SF6, allowing the fertilized patch to be tracked. Microbial DMSP cycling rates were determined in the surface mixed layer (5 m) during incubations using the 35S-DMSP technique. The addition of iron resulted in a 4-fold increase in concentrations of chlorophyll a (chl a) within the surface mixed layer (5 m depth), and the length of the sampling period allowed the observation of both bloom and post-bloom conditions. Inside the fertilized patch, the alleviation of resource limitation gave rise to the concurrent increase in bacterial abundance and production. Changes in the phytoplankton community within the Fe-enriched patch translated into a sustained decrease in chl a-normalized particulate DMSP (DMSPp) concentrations, suggesting a preferential stimulation of the growth of DMSPp-poor phytoplankton species. Despite short-lived peaks of DMSPp within the Fe-enriched area, concentrations of DMSPp generally remained stable during the entire sampling period inside and outside the fertilized patch. During the Fe-induced bloom, microbial DMSP-sulfur (DMSP-S) assimilation efficiency increased 2.6-fold inside the Fe-enriched area, which indicated that as bacterial production increased, a greater proportion of DMSP-S was assimilated and possibly diverted away from the bacterial cleavage pathway (i.e. production of DMS). Our results suggest that iron-induced stimulation of weak DMSPp-producers and DMSP-assimilating bacteria may diminish the potential production of DMS and thus limit its flux towards the atmosphere over the subarctic Pacific Ocean.©2009 Elsevier Ltd.

YANG, G.-P., M. LEVASSEUR, S. MICHAUD, A. MERZOUK, M. LIZOTTE, M. SCARRATT, 2009. Distribution of dimethylsulfide and dimethylsulfoniopropionate and its relation with phytoneuston in the surface microlayer of the western North Atlantic during summer. Biogeochemistry, 94(3): 243-254 .

One of the key steps towards predicting dimethylsulfide (DMS) emission to the atmosphere is to understand the distribution and cycling of biogenic sulfur in the microlayer. In this study, we examined the distribution of DMS and dissolved and particulate fractions of dimethylsulfoniopropionate (DMSPd and DMSPp) in the surface microlayer and bulk water of the western North Atlantic during July 2003. DMS concentrations in the bulk water varied from 0.71 to 7.65 nM. In contrast, DMS concentrations in the surface microlayer were fairly low (0.17–1.33 nM). Average concentrations of DMSPd and DMSPp in the bulk water were 2.09 (1.87–6.25) and 44.1 (8.06–119.8) nM, respectively, and those in the surface microlayer were 15.4 (4.06–54.3) and 29.9 (7.32–97.0) nM. In general, DMS was depleted in the microlayer (mean concentration: 0.60 nM) relative to the bulk water (mean concentration: 2.38 nM) with enrichment factors (the ratio of the microlayer concentration to bulk water concentration) ranging from 0.13 to 0.54. There was no consistent enrichment of DMSPp and chlorophyll a in the microlayer. On the contrary, DMSPd appeared to be highly enriched in the microlayer with an average EF of 4.89. The concentration of phaeopigments was also generally greater in the microlayer than in the bulk water, presumably due to enhanced photo-oxidation of chlorophyll a under high surface light intensities in the microlayer. In the study area, the concentration of DMSPp was significantly correlated with the abundance of dinoflagellates in the microlayer. Moreover, a significant correlation between the distributions of DMS, DMSPp, chlorophyll a and phaeopigment concentrations in the microlayer and the bulk water demonstrated that the biogenic materials in the microlayer come primarily from the bulk water below.©2009 Springer Science+Business Media B.V.

MERZOUK, A., M. LEVASSEUR, M. SCARRATT, S. MICHAUD, M. LIZOTTE, R.B. RIVKIN, R.P. KIENE, 2008. Bacterial DMSP metabolism during the senescence of the spring diatom bloom in the Northwest Atlantic. Mar. Ecol. Prog. Ser., 369: 1-11 .

The impact of the decline of the vernal bloom on the bacterial metabolism of dimethylsulfoniopropionate (DMSP), the precursor of dimethylsulfide (DMS), was investigated during a 7 d Lagrangian study conducted in the Northwest Atlantic in spring 2003. Daily variations in bacterial leucine incorporation, dissolved DMSP (DMSPd) uptake and DMS production rates were measured in the surface mixed layer (SML) and in the deep chlorophyll a maximum (DCM) that formed as the bloom collapsed. Seawater samples were amended with 35S-DMSPd, and the products of bacterial DMSPd degradation were measured during 3 h on-board incubations. The gradual decrease in phytoplankton biomass and diatom abundance measured in the SML was accompanied by a sharp doubling of the bacterial abundance and a peak in leucine incorporation rate on Day 2, suggesting that bacteria responded to a transient pulse in dissolved organic matter. Bacterial DMSPd uptake and DMS production were highest on Days 1 and 2 (1.2 and 0.10 nmol l-1 h-1, respectively), but rapidly decreased by Day 3, suggesting that DMSPd was becoming a less important substrate for the growing bacterial assemblage as other substrates became available. Bacterial DMSPd uptake and DMS production rates were also low in the DCM despite very high DMS yields (40 to 50 %), showing that neither the decline of the diatom spring bloom in the SML nor the accumulation of cells in the DCM resulted in a stimulation of bacterial DMSP metabolism or accumulation of DMS. The present study provides new field evidence for the potential uncoupling between bacterial production and DMS dynamics likely due to variations in the availability of other S-containing organic compounds released during the decay of phytoplankton blooms.©2008 Inter-Research

LIZOTTE, M., M. LEVASSEUR, M.G. SCARRATT, S. MICHAUD, A. MERZOUK, M. GOSSELIN, J. POMMIER, 2008. Fate of dimethylsulfoniopropionate (DMSP) during the decline of the northwest Atlantic Ocean spring diatom bloom. Aquat. Microb. Ecol., 52(2): 159-173 .

A 7 d Lagrangian process study of the biogeochemical cycling of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) was conducted within a decaying diatom bloom in the northwest Atlantic Ocean in spring 2003. Ambient profiles of DMSP and DMS were surveyed daily in the water column and were used to estimate in situ net transformation rates. Phytoplankton and bacterioplankton abundance were determined within the surface mixed layer (SML) as well as at the deep chlorophyll maximum (DCM), and sinking fluxes of particulate DMSP (DMSPp) below 75 to 100 m were assessed using free-drifting particle interceptor traps. Chlorophyll a (chl a) concentration and diatom abundance declined in the SML over the course of the study period, and the phytoplankton chl a biomass progressively settled above the nitracline forming the DCM. The decline of the diatom bloom coincided with the settling of DMSPp out of the SML and the formation of a DMSP-rich layer at the DCM. The low daily sinking loss rate of DMSPp at 75 m (<2 % d-1) provided confirmation of the efficient retention of DMSPp at the DCM. The decaying bloom gave rise to an initial release of dissolved DMSP (DMSPd) in the upper water column, which was rapidly consumed by the growing bacterial community. The rapid loss of DMSPd was accompanied by significant increases in net production of DMS in the SML and fluxes of DMS to the atmosphere. Despite this increase in DMS dynamics, overall in situ net production rates remained fairly low during the 7 d period (≤0.4 nmol DMS l-1 d-1), suggesting that demethylation by the developing bacterial community dominated DMSPd-consuming processes.©2008 Inter-Research

SCARRATT, M.G., M. LEVASSEUR, S. MICHAUD, S. ROY, 2007. DMSP and DMS in the Northwest Atlantic: late-summer distributions, production rates and sea-air fluxes. Aquat. Sci., 69(3): 292-304 .

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 DMSP t 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.©2007 Eawag, Dübendorf

LE CLAINCHE, Y., M. LEVASSEUR, A. VÉZINA, R.-C. BOUILLON, A. MERZOUK, S. MICHAUD, M. SCARRATT, C.S. WONG, R.B. RIVKIN, P.W. BOYD, P.J. HARRISON, W.L. MILLER, C.S. LAW, F.J. SAUCIER, 2006. Modeling analysis of the effect of iron enrichment on dimethyl sulfide dynamics in the NE Pacific (SERIES experiment). J. Geophys. Res. (C Oceans), 111, art. no C01011, 15 p .

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.©2006 American Geophysical Union

MERZOUK, A., M. LEVASSEUR, M.G. SCARRATT, S. MICHAUD, R.B. RIVKIN, M.S. HALE, R.P. KIENE, N.M. PRICE, W.K.W. LI, 2006. DMSP and DMS dynamics during a mesoscale iron fertilization experiment in the Northeast Pacific-Part II: biological cycling. Deep-Sea Res., Part II, Top. Stud. Oceanogr., 53: 2370-2383 .

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) biological cycling rates were determined during SERIES, a mesoscale iron-fertilization experiment conducted in the high-nutrient low-chlorophyll (HNLC) waters of the northeast subarctic Pacific. The iron fertilization resulted in the rapid development of a nanoplankton assemblage that persisted for 11 days before abruptly crashing. The nanoplankton bloom was followed by a diatom bloom, accompanied by an important increase in bacterial abundance and production. These iron-induced alterations of the plankton assemblage coincided with changes in the size and biological cycling of the DMSP and DMS pools. The initial nanoplankton bloom resulted in increases in particulate DMSP (DMSPp; 77-180 nmol L-1), dissolved DMSP (DMSPd; 1-24 nmol L-1), and biological gross (0.11-0.78 nmol L-1 h-1) and net (0.04-0.74 nmol L-1 h-1) DMS production rates. During the nanoplankton bloom, DMSPd consumption by bacteria exceeded their sulfur demand and the excess sulfur was probably released as DMS, consistent with the high gross DMS production rates observed during that period. The crash of the nanoplankton bloom was marked by the rapid decline of DMSPp, DMSPd, and gross DMS production to their initial values. Following the crash of the nanoplankton bloom, bacterial production and estimated sulfur demand reached transient maxima of 9.3 μCL-1d-1 and 14.2 nmol S L-1d-1, respectively. During this period of high bacterial production, bacterial DMSPd consumption was also very high (6 nmol L-1h-1), but none of the consumed DMSPd was converted into DMS and a net biological DMS consumption was measured. This transient period initiated a rapid decrease in DMS concentrations inside the iron-enriched patch, which persisted during the following diatom bloom due to low biological gross and net DMS production that prevented the replenishment of DMS. Our results show that the impact of Fe fertilization on DMS production in HNLC waters result from a complex interplay between the dynamics of the algal blooms and their influence on bacterial DMSP and DMS metabolism. ©2006 Elsevier Ltd.

BOUILLON, R.-C., W.L. MILLER, M. LEVASSEUR, M. SCARRATT, A. MERZOUK, S. MICHAUD, L. ZIOLKOWSKI, 2006. The effect of mesoscale iron enrichment on the marine photochemistry of dimethylsulfide in the NE subarctic Pacific. Deep-Sea Res., Part II, Top. Stud. Oceanogr., 53: 2384-2397 .

Measurements of underwater light fields and available quantum yield spectra were used to calculate photochemical removal rates of DMS for surface waters of the northeast subarctic Pacific during the SERIES mesoscale iron-fertilization experiment in July 2002. We observed that the UV portion of the solar spectrum was most important in inducing DMS photo-oxidation, and calculated that UV-B accounted for more than 20 % and UV-A for more than 68 % of the total DMS photo-oxidation near the sea surface. Vertically resolved rates showed that most (>90 %) of the DMS photo-oxidation occurs in the upper 15m of the water column. During the study, calculated rates of DMS photo-oxidation, just below the ocean’s surface ranged from 0.34 to 5.9 μmol m-3d-1. As the study progressed, an initial increase in photo-oxidation rates occurred within the iron-enriched patch and this was followed by a dramatic decrease in rates, whereas little change was observed outside the patch. Changes in DMS concentrations and decreases in the photochemical removal efficiency for DMS were the dominant factors explaining the variation in the DMS photo-oxidation rates. The turnover rate constants for DMS photo-oxidation, calculated for the upper mixed layer (UML) of the water column, (0.03-0.25 d-1) were in the range of those previously published and were at times higher than those calculated for biological consumption of DMS during SERIES. Our results suggest that iron fertilization of an oceanic patch in the northeast Pacific Ocean considerably altered the photochemical removal rates and turnover rate constants of DMS.©2006 Elsevier Ltd.

LEVASSEUR, M., M.G. SCARRATT, S. MICHAUD, A. MERZOUK, C.S. WONG, M. ARYCHUK, W. RICHARDSON, R.B. RIVKIN, M. HALE, E. WONG, A. MARCHETTI, H. KIYOSAWA, 2006. DMSP and DMS dynamics during a mesoscale iron fertilization experiment in the Northeast Pacific-Part I: temporal and vertical distributions. Deep-Sea Res., Part II, Top. Stud. Oceanogr., 53: 2353-2369 .

This paper reports on the influence of the Fe fertilization conducted during the subarctic ecosystem response to iron enrichment study (SERIES) on the distribution of the biogenic sulfur compounds dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in the context of changes in plankton composition. The Fe enrichment resulted in a rapid increase in the abundance of a nanoplankton assemblage dominated by Prymnesiophyceae, Prasinophyceae, small diatoms (<5 μm), heterotrophic dinoflagellates, and zooflagellates. This first assemblage persisted for 8 days before collapsing abruptly due to an increase in microzooplankton herbivory. The abundance of large diatoms started to increase shortly after the initial Fe fertilization but peaked 1-2 days after the crash of the nanoplankton bloom. Inside the Fe patch, particulate DMSP (DMSPp) increased from 100 to 285 nmol L-1 during the nanoplankton bloom, decreased rapidly back to initial level as this bloom collapsed, and remained low during the bloom of large diatoms. Outside the patch, phytoplankton and protists abundance and DMSPp concentrations remained low and relatively stable throughout the experiment. DMS concentrations were elevated at the onset of the experiment outside the patch (maximum of 15.7 nmol L-1 on day 1), increased up to 26.5 nmol L-1 10 days after the enrichment, and decreased to ca. 6 nmol L-1 by the end of the experiment. This large natural pulse in DMS coincided with conditions of high irradiance and decreasing wind speed. Inside the Fe patch, DMS concentrations exhibited the same general pattern, but with distinctive features related to the Fe fertilization. First, DMS concentrations tended to increase more rapidly inside the patch during the initial nanoplankton bloom, leading to DMS concentrations ca. 2 times higher inside the patch than outside on day 6. Second, DMS concentrations became consistently lower inside the patch (often below our limit of quantification of 0.03 nmol L-1) than outside (ca. 6 nmol L-1) during the peak of the diatom bloom. Our results thus confirm the rapid increase in nanoplankton and DMSPp reported during all previous Fe-fertilization experiments. On the other hand, the decrease in DMS concentrations measured inside the Fe patch during SERIES is unique and shows that adding Fe to HNLC waters may not always lead to conditions that could mitigate climate warming.©2006 Elsevier Ltd.

SCARRATT, M.G., A. MARCHETTI, M.S. HALE, R.B. RIVKIN, S. MICHAUD, P. MATTHEWS, M. LEVASSEUR, N. SHERRY, A. MERZOUK, W.K.W. LI, H. KIYOSAWA, 2006. Assessing microbial responses to iron enrichment in the Subarctic Northeast Pacific: Do microcosms reproduce the in situ condition?. Deep-Sea Res., Part II, Top. Stud. Oceanogr., 53: 2182-2200 .

A microcosm experiment was conducted in the NE Pacific in July 2002 to compare the microbial response between microcosms and the Subarctic Ecosystem Response to Iron-Enrichment Study (SERIES) in situ iron-enrichment experiment. Seawater microcosms (20 L) were incubated aboard ship under natural light using three treatments: (1) low-iron seawater amended with 4 nmol l-1 FeSO4 (+Fe); (2) low-iron seawater amended with 4 nmol l-1 FeSO4 and 86 nmol l-1 GeO2 (+Fe+Ge); (3) seawater collected from the in situ Fe-enriched patch (PW). The +Fe+Ge treatment used germanium to control diatom growth to assess the role of diatoms in dimethylsulfoniopropionate (DMSP) production. The following variables were measured in the microcosms and in situ: chlorophyll a (chl a), nitrate (NO‾3), silicic acid (Si(OH)4), phytoplankton abundance and species identification, bacterial abundance (including estimates of low- and high-DNA bacteria), bacterial production, bacterial specific growth rate, particulate and dissolved DMSP and dimethylsulfide (DMS) concentrations. There was little or no significant difference (ANCOVA) in the response of most variables between the +Fe and PW microcosms, but large differences were observed between both these treatments and the in situ data from the enriched patch. Chl a in all microcosms increased from ambient levels (approx. 0.5-1 μg l-1) to approx. 4.5-6.2 μg l-1 after 11 d incubation, when NO‾3 was fully depleted from all microcosms. During this same period, in situ chl a increased more slowly to a maximum of 2.9 μg l-1 on day 11. Nanophytoplankton and picophytoplankton were more abundant in the microcosms relative to the in situ community, which became dominated by large diatoms. Bacterial abundance was similar in the microcosms and in situ, but bacterial production was significantly higher in the microcosms. While neither DMSPd nor DMS accumulation showed significant differences between the microcosms and in situ, particulate DMSP concentrations increased significantly faster in the +Fe and PW treatments. These differences represent bottle effects resulting from the containment of the microcosms, which suppresses grazing, alters community and food web structure, enhances iron and nutrient regeneration, and isolates the community from physical transport and export processes including sinking. Thus during this experiment, the microcosms were not a good model for the in situ system in terms of the effects of iron on the phytoplankton biomass, nutrient uptake, bacterial dynamics and DMSPp production. In the germanium-amended treatment, the inhibition of diatom growth resulted in enhanced growth of other taxa and a suppression of bacterial production, leading to increased production of DMSP and DMS and strong correlations between DMSP, DMS and non-diatom phytoplankton taxa. Diatoms did not contribute significantly to particulate DMSP concentrations. Crown Copyright © 2006 Published by Elsevier Ltd.

YANG, G.-P., M. LEVASSEUR, S. MICHAUD, M. SCARRATT, 2005. Biogeochemistry of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the surface microlayer and subsurface water of the western North Atlantic during spring. Mar. Chem., 96(3-4): 315-329 .

Sixteen surface microlayer samples and corresponding subsurface water samples were collected in the western North Atlantic during April-May 2003 to study the distribution and cycling of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) and the factors influencing them. In the surface microlayer, high concentrations of DMS appeared mostly in the samples containing high levels of chlorophyll a, and a significant correlation was found between DMS and chlorophyll a concentrations. In addition, microlayer DMS concentrations were correlated with microlayer DMSPd (dissolved) concentrations. DMSPd was found to be enriched in the microlayer with an average enrichment factor (EF) of 5.19. However, no microlayer enrichment of DMS was found for most samples collected. Interestingly, the DMS production rates in the microlayer were much higher than those in the subsurface water. Enhanced DMS production in the microlayer was likely due to the higher concentrations of DMSPd in the microlayer. A consistent pattern was observed in this study in which the concentrations of DMS, DMSPd, DMSPp (particulate) and chlorophyll a in the microlayer were closely related to their corresponding subsurface water concentrations, suggesting that these constituents in the microlayer were directly dependent on the transport from the bulk liquid below. Enhanced DMS production in the microlayer further reinforces the conclusion that the surface microlayer has greater biological activity relative to the underlying water.©2005 Elsevier B.V.

MERZOUK, A., M. LEVASSEUR, M. SCARRATT, S. MICHAUD, M. GOSSELIN, 2004. Influence of dinoflagellate diurnal vertical migrations on dimethylsulfoniopropionate and dimethylsulfide distribution and dynamics (St. Lawrence Estuary, Canada). Can. J. Fish. Aquat. Sci., 61: 712-720 .

The influence of the diurnal vertical migration of the dinoflagellates Alexandrium tamarense and Scrippsiella trochoidea on dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) dynamics was studied during a 34-h Lagrangian experiment in the St. Lawrence Estuary in July 2000. Particulate DMSP (DMSPp), dissolved DMSP (DMSPd), and DMS exhibited diel patterns with minimum concentrations during the night and maximum concentrations around noon. DMSPp concentrations were correlated with the abundance of the two vertically migrating DMSP-rich dinoflagellates. The DMSPp:Chl a ratio exhibited similar diel variations, suggesting a light-induced de novo DMSP synthesis during the day. Diel variations of the DMS:Chl a ratio suggest that the accumulation of DMS around noon resulted from physiological responses of the algae and (or) bacteria to light. Biological gross DMS production and bacterial DMS consumption were decoupled, leading to rapid fluctuations in DMS. These results show that in systems dominated by DMSP-rich dinoflagellates containing DMSP lyases, DMS concentrations may vary by as much as a factor of 10 over a 24-h period. Such diel variations must be considered when estimating the contribution of such systems to the DMS sea to air flux.©2004 NRC Canada

LEVASSEUR, M., M. SCARRATT, S. ROY, D. LAROCHE, S. MICHAUD, G. CANTIN, M. GOSSELIN, A. VÉZINA, 2004. Vertically resolved cycling of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in the Northwest Atlantic in spring. Can. J. Fish. Aquat. Sci., 61: 744-757 .

In May 1998, profiles of ambient concentration and net changes of particulate dimethylsulfoniopropionate (DMSPp), dissolved dimethylsulfoniopropionate (DMSPd), and dimethylsulfide (DMS) were measured in three bio geographic provinces of the Northwest Atlantic: Northwest Atlantic Continental Shelf (Grand Banks), North Atlantic Drift, and North Atlantic Subtropical Gyre (Sargasso Sea). All stations/depths exhibited large losses of DMSPp (up to 18.0 nmol·L-1·day-1). DMSP and DMS cycling varied in relation to the type and development stage of the plankton assemblages. The postdiatom bloom conditions on the Grand Banks were associated with an efficient utilization of DMSP by microzooplankton and bacteria. Bacterial DMS production balanced the DMS bacterial consumption, resulting in little net DMS production (0.3 nmol·L-1·day-1). This contrasted with the North Atlantic Drift and Sargasso Sea stations where flagellates were thriving and most of the DMSPp loss was recovered in the dissolved pool, indicating a less active microbial DMSP metabolism. DMSPd cleavage was high in these latter cases and exceeded DMS bacterial consumption, allowing a net production of DMS (up to 1.8 nmol·L-1·day-1). These results indicate that maximum DMS net production occurs in growing algal systems where the production of DMSPd resulting from microzooplankton grazing exceeds the bacterial requirement in carbon and sulfur.©2004 NRC Canada

SCARRATT, M.G., M. LEVASSEUR, S. MICHAUD, G. CANTIN, M. GOSSELIN, S.J. DE MORA, 2002. Influence of phytoplankton taxonomic profile on the distribution of dimethylsulfide and dimethylsulfoniopropionate in the northwest Atlantic. Mar. Ecol. Prog. Ser., 244: 49-61 .

SCARRATT, M.G., M. LEVASSEUR, S. SCHULTES, S. MICHAUD, G. CANTIN, A. VÉZINA, M. GOSSELIN, S.J. DE MORA, 2000. Production and consumption of dimethylsulfide (DMS) in North Atlantic waters. Mar. Ecol. Prog. Ser., 204: 13-26 .

SCARRATT, M., G. CANTIN, M. LEVASSEUR, S. MICHAUD, 2000. Particle size-fractionated kinetics of DMS production : where does DMSP cleavage occur at the microscale?. J. Sea Res., 43(3-4): 245-252 .

SCARRATT, M.G., R.M. MOORE, 1999. Production of chlorinated hydrocarbons and methyl iodide by the red microalga Porphyridium purpureum. Limnol. Oceanogr., 44: 703-707 .