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11 pages, 6 figures, 1 table, The effects of harmful algae on bivalve physiology are complex and involve both physiological and behavioural responses. Studying those responses is essential to better describe and predict their impact on shellfish aquaculture and health risk for humans. In this study we recorded for two months the physiological response of the blue mussel Mytilus edulis from Eastern Canada to a one-week exposure to a paralytic shellfish poisoning producing dinoflagellate strain of Alexandrium catenella, isolated from the St Lawrence estuary, Canada. Mussels in a ‘control’ treatment were fed continuously with a non-toxic diet, while mussels in a ‘starvation’ treatment were fed the same non-toxic diet the first week and subsequently starved for seven weeks. Mussels in a ‘toxic’ treatment received A. catenella for one week before being starved until the end of the experiment. Over a two-month experiment we monitored shell and tissue growth, filtration capacity, respiration rate, byssal attachment strength, valve opening behaviour, and toxin content in tissues. Mussels fed normally on the toxic dinoflagellate and accumulated an average of 51.6 µg STXeq 100 g–1 after one week of exposure. After seven weeks of depuration, about half of the specimen showed levels around 18 µg STXeq 100 g–1. The condition index of exposed mussels (‘toxic’ treatment) decreased rapidly from the start as compared to mussels that received a one-week non-toxic diet (‘starvation’ treatment). Oxygen consumption rates increased in the ‘toxic’ treatment before leveling out with that of mussels from the ‘starvation’ treatment. Valve opening amplitude was lower in the ‘toxic’ treatment during and following the exposure. Average valve closure duration was higher right after the exposure, during the peak of mussel tissue intoxication. No significant change in byssal thread strength was observed through time in each treatment but less force was required to detach mussels from the ‘toxic’ and ‘starvation’ treatments. The number of byssus threads produced by mussels exposed to the toxic dinoflagellate was also lower than in the control group. These results represent advancements in our understanding of the impacts of harmful algae on bivalves and contribute to the development of mitigation measures necessary to both the safety of consumers and the sustainability of aquaculture operations, This project was undertaken with the financial support of the Government of Canada, specifically by Environment and Climate Change Canada (FDE-CA-2017i002) and Fisheries and Oceans Canada

The objective of this study was to assess experimentally the potential impact of anthropogenic pH perturbation (ApHP) on concentrations of dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP), as well as processes governing the microbial cycling of sulfur compounds. A summer planktonic community from surface waters of the Lower St. Lawrence Estuary was monitored in microcosms over 12 days under three pCO2 targets: 1 × pCO2 (775 µatm), 2 × pCO2 (1,850 µatm), and 3 × pCO2 (2,700 µatm). A mixed phytoplankton bloom comprised of diatoms and unidentified flagellates developed over the course of the experiment. The magnitude and timing of biomass buildup, measured by chlorophyll a concentration, changed in the 3 × pCO2 treatment, reaching about half the peak chlorophyll a concentration measured in the 1 × pCO2 treatment, with a 2-day lag. Doubling and tripling the pCO2 resulted in a 15% and 40% decline in average concentrations of DMS compared to the control. Results from 35S-DMSPd uptake assays indicated that neither concentrations nor microbial scavenging efficiency of dissolved DMSP was affected by increased pCO2. However, our results show a reduction of the mean microbial yield of DMS by 34% and 61% in the 2 × pCO2 and 3 × pCO2 treatments, respectively. DMS concentrations correlated positively with microbial yields of DMS (Spearman’s ρ = 0.65; P < 0.001), suggesting that the impact of ApHP on concentrations of DMS in diatom-dominated systems may be strongly linked with alterations of the microbial breakdown of dissolved DMSP. Findings from this study provide further empirical evidence of the sensitivity of the microbial DMSP switch under ApHP. Because even small modifications in microbial regulatory mechanisms of DMSP can elicit changes in atmospheric chemistry via dampened efflux of DMS, results from this study may contribute to a better comprehension of Earth’s future climate.

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 pHT (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 C5 °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 (DMSPt DparticulateCdissolved DMSP) concentrations tracked the development of the bloom, although the rise in DMSPt persisted for a few days after the peaks in chlorophyll a. Average concentrations of DMSPt 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 temperatureassociated 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. Fil: Benard, Robin. Laval University; Canadá Fil: Ferreyra, Gustavo Adolfo. Laval University; Canadá Fil: Michael, Scarratt. Maurice Lamontagne Institute, Fisheries And Oceans; Canadá Fil: Sonia, Michaud. Maurice Lamontagne Institute, Fisheries And Oceans ; Canadá Fil: Michel, Starr. Maurice Lamontagne Institute, Fisheries And Oceans; Canadá Fil: Alfonso, Mucci. Université Mcgill; Canadá Fil: Ferreyra, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Gosselin, Michel. Institut Des Sciences de la Mer de Rimouski; Canadá Fil: Tremblay, Jean-Éric. Laval University; Canadá Fil: Lizotte, Martine. Laval University; Canadá Fil: Yang, Gui Peng. Ocean University; China

The ecological conditions and cellular mechanisms which affect the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in marine ecosystems are still enigmatic. This information is crucial for deriving accurate oceanic ecosystem models for the dynamics of these major players in the Earth’s sulfur cycle and climate. In the present study, we examined the effect of short-term increases in photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) on the production of DMSP and DMS and on DMSP lyase potential activity (DLPA) in an axenic culture of the coccolithophore Emiliania huxleyi (CCMP 1742). Algal cells were subjected to a rapid shift from a low intensity of PAR (50 μE m-2 s-1; low light, LL) to a high intensity of PAR (198 μE m-2 s-1) and elevated UVR conditions (high light, HL), simulating what may occur during upward mixing in the surface mixed layer or during changes in cloud cover. During the 4.5 h exposure to HL, the intracellular DMSP normalized to cell volume increased by ca. 30%, and dissolved DMSP doubled relative to control values. However, the DLPA per unit of cell volume decreased by ~45% compared to the control value. The up-regulation of cellular DMSP concentration is consistent with an antioxidant and/or energy dissipation role for DMSP. The decrease in DLPA may indicate that the DMSP lyase enzyme plays no role in antioxidant protection in this algal species, but rather serves some other cellular function, such as grazing protection.

Royer, S.-J. ... et. al.-- 13 pages, 5 figures, 3 tables, 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, by the American Society of Limnology and Oceanography, Inc., This project was supported by grants to M.L. from the Natural Sciences and Engineering Research Council (NSERC). S.-J.R. received scholarships from Québec-Océan and the Biology department of Universite´ Laval. The Line P program is sponsored by Fisheries and Oceans Canada

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.

The impact of the decline of the vernal bloom on the bacterial metabolism of dimethyl- sulfoniopropionate (DMSP), the precursor of dimethyl- sulfide (DMS), was investigated during a 7 d Lagran- gian study conducted in the Northwest Atlantic in spring 2003. Daily variations in bacterial leucine incor- poration, 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 35 S-DMSPd, and the prod- ucts 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 assem- blage 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 pre- sent study provides new field evidence for the po- tential uncoupling between bacterial production and DMS dynamics likely due to variations in the availabil- ity of other S-containing organic compounds released during the decay of phytoplankton blooms.

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 DMSP p 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 DMSP p at the DCM. The decaying bloom gave rise to an initial release of dissolved DMSP (DMSP d ) in the upper water column, which was rapidly consumed by the growing bacterial community. The rapid loss of DMSP d 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 DMSP d -consuming processes.

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

Weekly variations in total dimethylsulfoniopropionate (DMSPt) and dimethylsulfide (DMS) were investigated in relation to the phytoplankton assemblage from spring to fall 1994 at a coastal fixed station in the St. Lawrence Estuary. DMSPt and DMS concentrations showed a strong seasonality and were tightly coupled in time. Maximum concentrations of DMSPt and DMS were observed in July and August, during a period of warm water and low nutrient concentrations. Seasonal maxima of 365.4 nmol l � 1 for DMSPt and 14.2 nmol l � 1 for DMS in early August coincided with the presence of many phytoplankton species, such as Alexandrium tamarense, Dinophysis acuminata, Gymnodinium sp., Heterocapsa rotundata, Protoperidinium ovatum, Scrippsiella trochoidea, Chrysochromulina sp. (6 mm), Cryptomonas sp. (6 mm), a group of microflagellates smaller than 5 mm (mf < 5), many tintinnids, and Mesodinium rubrum. The abundance of mf < 5 followed the general trend of DMS concentrations. The temporal occurrence of high P. ovatum abundance and DMSPt concentrations suggests that this heterotrophic dinoflagellate can either synthesize DMSP or acquire it from DMSP-rich prey. The calculated sea-to-air DMS flux reached a maximum of 8.36 mmol � 2 d � 1 on August 1. The estimated annual emission from the St. Lawrence Estuary is 77.2 tons of biogenic sulfur to the atmosphere.

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 μg C L−1 d−1 and 14.2 nmol S L−1 d−1, respectively. During this period of high bacterial production, bacterial DMSPd consumption was also very high (6 nmol L−1 h−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.

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 15 m 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−3 d−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.

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 (

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.

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.

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.

En mai 1998, nous avons mesure les profils des concentrations ambiantes et des changements nets de dimethylsulfoniopropionate particulaire (DMSP p ), de dimethylsulfoniopropionate dissous (DMSP d ) et de sulfure de dimethyle (DMS) dans trois provinces biogeographiques du nord-ouest de l'Atlantique, soit le plateau continental (Grands Bancs), la derive nord-atlantique et la gyre subtropicale de l'Atlantique nord (mer des Sargasses). Toutes les stations/profondeurs affichent d'importantes pertes de DMSP p (jusqu'a 18,0 nmol.L - 1 .jour - 1 ). Le recyclage du DMSP et du DMS varie en fonction de la composition des communautes planctoniques et de leur stade de developpement. Les conditions qui suivent l'efflorescence des diatomees sur les Grands Bancs sont associees a une utilisation efficace du DMSP par le microzooplancton et les bacteries. La production bacterienne de DMS contrebalance la consommation bacterienne de DMS, ce qui explique la faible production nette de DMS (0,3 nmol.L - 1 .jour - 1 ). La situation est tres differente dans les stations de la derive nord-atlantique et de la mer des Sargasses ou les flagelles foisonnent et ou la plus grande partie de la perte de DMSP p est recuperee dans le pool de DMSP dissous, ce qui indique que le metabolisme bacterien du DMSP y est moins actif. Dans ces situations, la scission du DMSP d est importante et elle depasse la consommation bacterienne de DMS, ce qui permet une production nette de DMS (jusqu'a 1,8 nmol.L - 1 .jour - 1 ). Ces resultats indiquent que la production nette maximale de DMS se produit dans les systemes d'algues en croissance, dans lesquels la production de DMSP d due au broutage du microzooplancton surpasse les besoins en carbone et en soufre des bacteries.

The variations in abundance of Prorocentrum lima (Ehrenberg) Stein, an epiphythic species implicated in diarrhetic shellfish poisoning (DSP), were studied during the summer of 1999 in the Magdalen Islands, an archipelago located in the southern Gulf of St. Lawrence, eastern Canada. This investigation was initiated as a result of an incident in 1998, when 20 people became ill and exhibited DSP-like symptoms following the consumption of blue mussels collected from one of the Magdalen Islands lagoons. P. lima was regularly found as an epiphyte on the epibionts growing on the mussel socks at the 2 aquaculture sites investigated between June and October 1999. P. lima abundance on the epibionts varied from 100 to 9600 cells g -1 dry wt epibiont, with maximum abun- dances observed in July and August. P. lima was found in low abundance in the water column as well as in the digestive glands of the mussels. There was no correlation between cell abundances in the digestive glands, the water column and the cells attached to the epibionts. At the same sites, P. mex- icanum Tafall was identified for the first time in eastern Canada. P. mexicanum abundances were low throughout the summer and began to increase both in the water column and in mussel digestive glands by early September. A significant correlation was found between the P. mexicanum abun- dances in the water column and in the mussel digestive glands at both sampling sites. The identifi- cation of these 2 potentially toxic species in the St. Lawrence highlights the importance of Prorocen- trum spp. as toxin producers in coastal waters, as well as the need for adequate monitoring of non-pelagic harmful algal bloom species (HABs).

Blooms of the toxic dinoflagellate Alexandrium tamarense, which is responsible for paralytic shellfish poisoning, are annually recurrent events in the Estuary and Gulf of St. Lawrence, Québec, Canada. The analysis of abundance data for this algal species between 1989 and 1998 at Sept-Îles, a presumed initiation site in the north western Gulf of St. Lawrence, revealed yearly fluctuations in the onset, duration, and magnitude of toxic A. tamarense blooms. Hydrological and meteorological data for the region indicate that rainfall, Moisie River runoff, and wind are highly related to the pattern of bloom development each year. Results from the 10-year data set reveal that in this system: (i) high Moisie River runoff from a prolonged spring freshet or from heavy rainfall events in the summer and fall can initiate A. tamarense blooms; (ii) high Moisie River runoff combined with prolonged periods of weak winds (–1) favour the continued development of blooms; and (iii) winds >8 m·s–1 disrupt blooms. Salinity, which reflects the general state of the water column in terms of freshwater input and stability, had a strong negative correlation with the probability of observing A. tamarense cells at this station and could thus be used as a predictive tool for the presence of cells in this system.

Distributions of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) were surveyed in surface waters of the NW Atlantic in May 1998, a few weeks after the spring bloom. A tri- angular transect extending from Nova Scotia to Bermuda and northeast toward Newfoundland encompassed 4 major oceanic biogeochemical provinces: Northwest Atlantic Continental Shelves (NWCS), Gulf Stream (GFST), North Atlantic Subtropical Gyre (NAST) and the North Atlantic Drift (NADR). Surface concentrations of DMS and DMSP were highest in the NADR, with peaks up to 8.9 nM DMS, 44.1 nM dissolved DMSP (DMSPd) and 240 nM particulate DMSP (DMSPp). The phyto- plankton assemblage throughout the study area was dominated by dinoflagellates and prymnesio- phytes. Statistically significant correlations were observed between the abundance of dinoflagellates and prymnesiophytes and the concentrations of DMS and DMSP along the transect. Size-fractionation of DMSPp revealed the 2 to 11 µm fraction to be the most important contributor to total DMSP (mean 72%, range 27% to 91% of total). In the region of the highest DMS(P) concentrations, the phyto- plankton assemblage was dominated by prymnesiophytes and dinoflagellates, with Chrysochro- mulina spp. and Gyrodinium flagellare being the most abundant. The abundance of these taxa showed a marked correlation with total DMSPp and with the 2 to 11 µm size fraction of DMSP in which these cells are found. This plankton assemblage was observed both early and late in the transect, which may indicate that it is a persistent feature along the northern side of the Gulf Stream at this time of year. Sea-air flux of DMS was calculated based on 2 different models. The results showed peaks in flux corresponding to the peaks in DMS concentration in surface water. Pooling and averaging the values for each biogeochemical province reveals DMS concentrations lower than the average values of an existing global DMS database for the same regions and times.

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.

We adapted the dilution technique to study microzooplankton grazing of algal dimethylsulfoniopropionate (DMSP) vs. Chl a , and to estimate the impact of microzooplankton grazing on dimethyl sulfide (DMS) production in the Labrador Sea. Phytoplankton numbers were dominated by autotrophic nanoflagellates in the Labrador basin, but diatoms and colonial Phaeocystis pouchetii contributed significantly to phytomass at several high chlorophyll stations and on the Newfoundland and Greenland shelfs. Throughout the region, growth of algal Chl a and DMSP was generally high (0.2–1 d − 1 ), but grazing rates were lower and more variable, characteristic of the early spring bloom period. Production and consumption of Chl a vs. DMSP followed no clear pattern, and sometimes diverged greatly, likely because of their differing distributions among algal prey taxa and size class. In several experiments where Phaeocystis was abundant, we observed DMS production proportional to grazing rate, and we found clear evidence of DMS production by this haptophyte following physical stress such as sparging or filtration. It is possible that grazing-activated DMSP cleavage by Phaeocystis contributes to grazer deterrence: protozoa and copepods apparently avoided healthy colonies (as judged by relative growth and grazing rates of Chl a and DMSP), and grazing of Phaeocystis was significant only at one station where cells were in poor condition. Although we hoped to examine selective grazing on or against DMSP-containing algal prey, the dilution technique cannot differentiate selective ingestion and varying digestion rates of Chl a and DMSP. We also found that the dilution method alone was poorly suited for assessing the impact of grazing on dissolved sulfur pools, because of rapid microbial consumption and the artifactual release of DMSP and DMS during filtration. Measuring and understanding the many processes affecting organosulfur cycling by the microbial food web in natural populations remain a technical challenge that will likely require a combination of techniques to address.

The dynamics of the cleavage of dissolved dimethylsulfoniopropionate (DMSP d ) to dimethylsulfide (DMS) were measured experimentally in the surface waters of the Labrador Sea in spring 1997. At in situ DMSP d concentrations, DMS production and consumption processes were generally in balance. Two stations in the central Labrador Sea displayed net DMS production of approximately 2 nmol l -1 h -1 , DMSP d net consumption of 3.48 nmol l -1 h -1 and a net DMS production yield from DMSP d of 60 % at near in situ DMSP d concentrations. Similar to general bacterial substrate utilization in cold waters, DMS production in the Labrador Sea seemed to be temperature and substrate limited. Following DMSP d additions, linear and non-linear net DMS production were observed. The non-linear response was characterized by a lag in DMS production and was associated with the cold, polar waters of the Labrador and West Greenland Currents. Net DMS production rates measured after DMSP d addition were proportional to the added amount of DMSP d , No saturation of the net DMS production rate was observed for concentrations up to 5000 nmol DMSP d l -1 , First order rate constants determined for these DMS production kinetics suggest an average turnover time of DMSP d by cleavage to DMS of 3.8 d (2.7 to 5.2 d). At water temperatures of -1.3 to 8°C, potential net DMS production rates measured following DMSP d additions were comparable and even higher than those previously published for temperate and warm oceanic and coastal regions. The net DMS production potential varied by 1 order of magnitude (1.7 to 18.4 nmol DMS l -1 h -1 ) throughout the study area. Causal links established with path analysis indicate that this potential seemed to be controlled by water temperature and chlorophyll a concentrations.

Production and consumption of dimethylsulfide (DMS) were studied in surface waters of the northwest Atlantic between latitude 32° and 45° N during May 1998. The kinetics of DMS production by the whole planktonic community were studied in short-term (3 h) experiments using additions of dissolved dimethylsulfoniopropionate (DMSP d ) (0 to 3000 nM). Measurements of DMS production and DMSP d consumption showed that the DMS production rate increased in direct proportion to the concentration of added DMSP d . This rate relationship did not saturate, suggesting acclimation of the microbial community to DMSP d concentrations much higher than the average for bulk seawater. Longer-term experiments were performed in which DMSP d consumption and DMS production were measured over 48 to 60 h. The DMSP d consumption rate decreased as the concentration of DMSP d decreased during the incubations. However, the DMS production rate was initially constant for the first 36 h. When DMSP d concentrations fell below 50 nM, DMS production stopped, even though DMSP d consumption continued. A possible explanation is that DMSP cleavage might dominate the total DMSP consumption at high DMSP d concentrations while DMSP demethylation and other processes dominate at low DMSP d concentrations. DMS consumption was measured both directly and by using the DMS consumption inhibitors dimethyl disulfide (DMDS) and methyl butyl ether (MBE). DMS consumption was generally undetectable except at 1 station dominated by a dense population of the DMSP-producing phytoplankton Chrysochromulina sp. and where ambient DMS concentrations were high. This suggests that the potential for DMS consumption is highest where ambient DMS levels are elevated. Pooling results from these experiments with earlier results from more northerly waters revealed an inverse exponential relationship (r 2 = 0.75, p < 0.0001) between the potential rate constant and chlorophyll a standing stock across a wide area of the Northwest Atlantic. This finding is potentially useful for the development of DMS production models.

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

Atlantic mackerel Scomber scombrus are known to be lethal vectors of paralytic shellfish poisoning (PSP) toxins to predators. To elucidate dynamics of PSP toxin accumulation in this species, mackerel were sampled in the Gulf of St Lawrence from May to October 1993. Mackerel appear to retain toxins (saxitoxin, gonyautoxins 2 and 3) year-round. The toxin content of the liver, as determined by high performance liquid chromatography, increased significantly with fish age (r2=0.40) and length (r2=0.52), suggesting that mackerel progressively accumulate PSP toxins throughout their life. The toxin content of the liver also increased significantly during the summer feeding sojourn in the Gulf of St Lawrence. Comparison of profiles of saxitoxin derivatives indicated that zooplankton were the likely source of PSP toxins found in mackerel. The mean ± S.D toxin content was 17.4 ± 10.6 nmol liver−1 and the mean ± S.D. PSP toxicity was 112.4 ± 67.0 μg saxitoxin equivalents 100 g−1 liver wet weight (n=247).

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.

We investigated the influence of mesozooplankton grazing on the distribution of dimethylsulfide (DMS) and its metabolic precursor dimethylsulfoniopropionate (DMSP) in the Gulf of St. Lawrence in August 1993. The horizontal distributions of DMS. DMSP, phytoplankton, microzooplankton, and mesozooplankton were determined over a grid of 79 stations covering 41 000 km2 Phytoplankton biomass was low [mean = 0.37 pg chl a I-') and the community was dominated by unidentified flagellates, Prymnes~ophyceae, Chrysophyceae, and Dlnophyceae. Maximum plankton biomass and DMSP were found in the western portion of the grid influenced by the outflow of the St. Lawrence Estuary vla the Gaspe Current. Surface concentrations of particulate DMSP (DMSP,,), dissolved DMSP (DMSP,), and DMS ranged from 6 to l17 nM, 1.7 to 23.1 nM, and

[1] Mixing ratios of gas-phase dimethyl sulfide (DMS), acetone and methanol were measured for the first time in the Canadian Archipelago using a proton-transfer-reaction mass spectrometer (PTR-MS) onboard the CCGSAmundsen from 23 August 2008 to 12 September 2008. For the entire measurement period, there existed moderate negative correlations between both acetone (R = 0.37, p < 10−4) and methanol (R = 0.26, p < 10−4) with DMS. When elevated DMS mixing ratios were observed on a transect between Kugaaruk and Resolute, during which elevated oceanic DMS was also observed, the anticorrelations of acetone and methanol mixing ratios with DMS were stronger (R = 0.61, p < 10−4 and 0.45, p < 10−4 respectively). There were no significant correlations of the aromatic species (benzene and toluene) with DMS, and the relatively low values of benzene (mean = 12 pptv) and toluene (mean = 4 pptv) indicate little significant local or regional influence of anthropogenic sources on these observations. When the recent air parcel history was predominantly of marine influence, the DMS mixing ratios were positively correlated with wind speed whereas acetone and methanol were negatively correlated. These observations suggest that biologically active waters in the high Arctic act as a sink for acetone and methanol. While speculative, continued changes in the Arctic climate have the potential to affect levels of acetone and methanol in the polar atmosphere, either through increased levels of open ocean or increased biological productivity.

[1] We determined the distribution and bacterial metabolism of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in the two dominant surface water masses in northern Baffin Bay/Lancaster Sound during September 2008. Concentrations of particulate DMSP (DMSPp; 5–70 nmol L−1) and the DMSPp:Chl a ratios (15–229 nmol μg−1) were relatively high, suggesting the presence of DMSP-rich phytoplankton taxa. Photosynthetic picoeukaryotes and total prokaryotes were tenfold and threefold more abundant in Baffin Bay surface water (BBS) than in Arctic surface water (AS), respectively. Heterotrophic bacterial production (0.07–2.5μC L−1 d−1) and bacterial turnover rate constants for dissolved DMSP (DMSPd) were low (0.03–0.11 h−1) compared with the values previously reported in warmer and more productive environments. Nonetheless, a relatively large proportion (12%–31%) of the DMSP metabolized by the bacteria was converted into DMS. Additionally, between 40% and 65% of the total bacterial cells incorporated sulfur from DMSPd, with Gammaproteobacteriaand non-Roseobacter Alphaproteobacteria (AlfR-) contributing proportionally more to total DMSP-incorporating cells. The contribution of AlfR- to the total prokaryotic community was 50% higher in BBS than in AS, while the bacterial rate constants for DMSPd turnover were 78% higher in BBS than in AS. These results show that the two different Arctic water masses host specific microbial assemblages that result in distinct affinity for DMSP.

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 DMSP d 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. © 2012 Springer Science+Business Media B.V.

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.

[1] 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.

[1] 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.

[1] 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.

Dimethylsulfide (DMS) is a gas that forms in some biologically productive regions of the surface ocean. Only a few percent of the total DMS in surface waters is released to the atmosphere but, because it has the potential to form new CCN on oxidation to sulfate, this flux may be an important variable to understand in a changing climate scenario. DMS fluxes in this study were used in a comparative approach to constrain DMS oxidation pathways. A traditional approach to calculate DMS flux (bottom-up) used DMS concentrations in surface waters and the parameterization of Wanninkhof and McGillis (1999) for piston velocity. The bottom-up surface water DMS fluxes were calculated for the SOLAS SABINA Spring cruise over the North Atlantic: 30–60°N from April 25–May 14, 2003. These fluxes were compared to a top-down approach using a steady-state oxidation model where corresponding DMS mixing ratios in the air over the Atlantic, and boundary layer heights from the NARCM model were incorporated. In the simplest scheme, using OH alone as an oxidant, the top-down and bottom-up approaches showed poor agreement in terms of both the magnitude of DMS flux and its temporal variations. Better sensitivity for DMS flux was achieved using OH plus NO 3 oxidation, but temporal variations were still poorly represented. Adding very small amounts of BrO to the oxidation scheme improved the agreement between the top-down and bottom-up approaches, but only slightly. Transport of DMS was examined during a high atmospheric DMS concentration event using estimates for the spatial distribution of atmospheric DMS from the NARCM model using the Kettle et al. (1999) database and could be responsible for the poor agreement between flux estimates during high wind regimes. However, the poor agreement between the two approaches during low wind regimes suggests that the oxidation pathway for DMS over the North Atlantic is more complex than at low latitudes where OH oxidation is dominant.

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.