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Coccolithophores are typically thought of as photoautotrophs, yet a few genera inhabit sub-euphotic environments with insufficient light for photosynthesis, suggesting that other carbon acquisition strategies are likely. Field experiments were performed in the northwest Atlantic (a region with potentially abundant coccolithophores). Phytoplankton populations were incubated with 14 C-labeled dissolved organic carbon (DOC) compounds, acetate, mannitol, and glycerol. Coccolithophores were sorted from these populations 24 hours later using flow cytometry, and DOC uptake was measured. DOC uptake rates were as high as 10 −15 moles cell −1 day −1 , slow relative to photosynthesis rates (10 −12 moles cell −1 day −1 ). Growth rates on the organic compounds were low, suggesting that osmotrophy plays more of a survival strategy in low-light situations. Assimilated DOC was found in both particulate organic carbon and calcite coccoliths (particulate inorganic carbon), suggesting that osmotrophic uptake of DOC into coccolithophore calcite is a small but notable part of the biological carbon pump and alkalinity pump paradigms.

Dry deposition to the surface is one of the main removal pathways of tropospheric ozone (O3). We quantified for the first time the impact of O3 deposition to the Arctic sea ice on the planetary boundary layer (PBL) O3 concentration and budget using year-round flux and concentration observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign and simulations with a single-column atmospheric chemistry and meteorological model (SCM). Based on eddy-covariance O3 surface flux observations, we find a median surface resistance on the order of 20,000 s m−1, resulting in a dry deposition velocity of approximately 0.005 cm s−1. This surface resistance is up to an order of magnitude larger than traditionally used values in many atmospheric chemistry and transport models. The SCM is able to accurately represent the yearly cycle, with maxima above 40 ppb in the winter and minima around 15 ppb at the end of summer. However, the observed springtime ozone depletion events are not captured by the SCM. In winter, the modelled PBL O3 budget is governed by dry deposition at the surface mostly compensated by downward turbulent transport of O3 towards the surface. Advection, which is accounted for implicitly by nudging to reanalysis data, poses a substantial, mostly negative, contribution to the simulated PBL O3 budget in summer. During episodes with low wind speed (

Near-surface mercury and ozone depletion events occur in the lowest part of the atmosphere during Arctic spring. Mercury depletion is the first step in a process that transforms long-lived elemental mercury to more reactive forms within the Arctic that are deposited to the cryosphere, ocean, and other surfaces, which can ultimately get integrated into the Arctic food web. Depletion of both mercury and ozone occur due to the presence of reactive halogen radicals that are released from snow, ice, and aerosols. In this work, we added a detailed description of the Arctic atmospheric mercury cycle to our recently published version of the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem 4.3.3) that includes Arctic bromine and chlorine chemistry and activation/recycling on snow and aerosols. The major advantage of our modelling approach is the online calculation of bromine concentrations and emission/recycling that is required to simulate the hourly and daily variability of Arctic mercury depletion. We used this model to study coupling between reactive cycling of mercury, ozone, and bromine during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) spring season in 2020 and evaluated results compared to land-based, ship-based, and remote sensing observations. The model predicts that elemental mercury oxidation is driven largely by bromine chemistry and that particulate mercury is the major form of oxidized mercury. The model predicts that the majority (74%) of oxidized mercury deposited to land-based snow is re-emitted to the atmosphere as gaseous elemental mercury, while a minor fraction (4%) of oxidized mercury that is deposited to sea ice is re-emitted during spring. Our work demonstrates that hourly differences in bromine/ozone chemistry in the atmosphere must be considered to capture the springtime Arctic mercury cycle, including its integration into the cryosphere and ocean.

6 pags., 2 figs., Unlike bromine, the effect of iodine chemistry on the Arctic surface ozone budget is poorly constrained. We present ship-based measurements of halogen oxides in the high Arctic boundary layer from the sunlit period of March to October 2020 and show that iodine enhances springtime tropospheric ozone depletion. We find that chemical reactions between iodine and ozone are the second highest contributor to ozone loss over the study period, after ozone photolysis-initiated loss and ahead of bromine., This study received funding from the European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation Program (project ERC‐2016‐COG 726349 CLIMAHAL and ERC-2016-STG 714621 GASPARCON) and the European Commission via the EMME-CARE project and was supported by the Consejo Superior de Investigaciones Científicas of Spain. This work was supported by the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 856612 and the Academy of Finland (project no. 334514). The Indian Institute of Tropical Meteorology is funded by the Ministry of Earth Sciences, Government of India. Ozone, CO, CH4 and AMS measurements were funded by the Swiss National Science Foundation (grant 200021_188478), the Swiss Polar Institute and U.S. National Science Foundation grants 1914781 and 1807163. J.S. holds the Ingvar Kamprad chair for extreme environments research, sponsored by Ferring Pharmaceuticals. Data reported in this manuscript were produced as part of the international MOSAiC expedition with tag MOSAiC20192020, with activities supported by Polarstern expedition AWI-PS122_00. H.S. was funded by the European ERA-PLANET projects iGOSP and iCUPE (consortium agreement no. 689443 for both projects). We thank FORMAS and the Swedish Polar Research Secretariat for support. We gratefully acknowledge funding by the Deutsche Forschungsgemeinschaft (project no. 268020496 – TRR 172) within the Transregional Collaborative Research Center ‘ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3’ in subproject C03. We thank I. Bourgeois (NOAA/CIRES) for providing the ATom NOx data.

The methylamines form part of the marine organic nitrogen pool. Although they are ubiquitous in marine systems, little is known about their distribution, production and fate in the marine environment. Analytical methods have been developed for their analysis in marine waters, but these have employed custom-made apparatus, limiting data to a few studies. Here, we report a method developed for the determination of methylamines in seawater which combines headspace solid phase micro-extraction (SPME) and gas chromatography with selective nitrogen detection. Gaseous methylamines were pre-concentrated from water samples (approximately 1 L) heated to 60 oC, onto a polydimethylsiloxane/divinylbenzene SPME fibre located in the headspace of the extraction flask. The analytes were then thermally desorbed from the fibre in a gas chromatograph containing a CP-Volamine column and a nitrogen-phosphorus detector. The method was routinely calibrated down to 1 nM for all three analytes using matrix-matched external standards. Limits of detection were determined operationally and ranged from 0.4-2.9 nM during two sampling campaigns. The analytes were detected in seawater samples from the English Channel and Southern Ocean where concentrations up to 20 and 5.6 nM were measured, respectively, consistent with previously reported levels. We propose this method for the analysis of methylamines, and other low molecular weight amines, as it is sensitive, robust, and the necessary equipment is widely-available.

Predictions of the ocean-atmosphere flux of dimethyl sulfide will be improved by understanding what controls seasonal and regional variations in dimethylsulfoniopropionate (DMSP) production. To investigate the influence of high levels of irradiance including ultraviolet radiation (UVR), on DMSP synthesis rates (mu DMSP) and inorganic carbon fixation (mu POC) by natural phytoplankton communities, nine experiments were carried out at different locations in the low nutrient, high light environment of the northeastern Tropical Atlantic. Rates of mu DMSP and mu POC were determined by measuring the incorporation of inorganic C-13 into DMSP and particulate organic carbon. Based on measurements over discrete time intervals during the day, a unique mu DMSP vs. irradiance (P vs. E) relationship was established. Comparison is made with the P vs. E relationship for mu POC, indicating that light saturation of DMSP occurs at similar irradiance to mu POC and is closely coupled to carbon fixation on a diel basis. Photoinhibition during the middle of the day was exacerbated by exposure to UVR, causing an additional 55-60% inhibition of both mu DMSP and mu POC at the highest light levels. In addition, decreased production of DMSP in response to UVR-induced photoxidative stress, contrasted with the increased net synthesis of photoprotective xanthophyll pigments. Together these results indicate that DMSP production by phytoplankton in the tropical ocean is not regulated in the short term by the necessity to control increasing photooxidative stress as irradiance increases during the day. The study provides new insight into the regulation of resource allocation into this biogeochemically important, multi-functional compatible solute.

Surface ocean biogeochemistry and photochemistry regulate ocean–atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or p CO 2 ) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N 2 O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes.

Long-term time series data show that ocean acidification is occurring in the subtropical oceans. As a component of an in situ mesocosm experiment carried out off Gran Canaria in the subtropical North Atlantic, we examined the influence of ocean acidification on the net production of dimethylsulfide (DMS). Over 23 days under oligotrophic conditions, time-integrated DMS concentrations showed an inverse relationship of −0.21 ± 0.02 nmol DMS nmol−1 H+ across the gradient of H+ concentration of 8.8–23.3 nmol l−1, equivalent to a range of pCO2 of 400–1,252 atm. Proportionally similar decreases in the concentrations of both dissolved and particulate dimethylsulfoniopropionate (DMSP) were observed in relation to increasing H+ concentration between the mesocosms. The reduced net production of DMSP with increased acidity appeared to result from a decrease in abundance of a DMSP-rich nanophytoplankton population. A 35S-DMSP tracer approach was used to determine rates of dissolved DMSP catabolism, including DMS production, across the mesocosm treatments. Over a phase of increasing DMS concentrations during the experiment, the specific rates of DMS production were significantly reduced at elevated H+ concentration. These rates were closely correlated to the rates of net DMS production indicating that transformation of dissolved DMSP to DMS by bacteria was a major component of DMS production. It was not possible to resolve whether catabolism of DMSP was directly influenced by H+ concentrations or was an indirect response in the bacterial community composition associated with reduced DMSP availability. There is a pressing need to understand how subtropical planktonic communities respond to the predicted gradual prolonged ocean acidification, as alterations in the emission of DMS from the vast subtropical oceans could influence atmospheric chemistry and potentially climate, over a large proportion of the Earth's surface.

The coccolithophore Emiliania huxleyi forms some of the largest phytoplankton blooms in the ocean. The rapid demise of these blooms has been linked to viral infections. E. huxleyi abundance, distribution, and nutritional status make them an important food source for the heterotrophic protists which are classified as microzooplankton in marine food webs. In this study we investigated the fate of E. huxleyi (CCMP 374) infected with virus strain EhV-86 in a simple predator-prey interaction. The ingestion rates of Oxyrrhis marina were significantly lower (between 26.9 and 50.4%) when fed virus-infected E. huxleyi cells compared to non-infected cells. Despite the lower ingestion rates, O. marina showed significantly higher growth rates (between 30 and 91.3%) when fed infected E. huxleyi cells, suggesting higher nutritional value and/or greater assimilation of infected E. huxleyi cells. No significant differences were found in O. marina cell volumes or fatty acids profiles. These results show that virally infected E. huxleyi support higher growth rates of single celled heterotrophs and in addition to the “viral shunt” hypothesis, viral infections may also divert more carbon to mesozooplankton grazers.

The coccolithophore Emiliania huxleyi forms some of the largest phytoplankton blooms in the ocean. The rapid demise of these blooms has been linked to viral infections. E. huxleyi abundance, distribution, and nutritional status make them an important food source for the heterotrophic protists which are classified as microzooplankton in marine food webs. In this study we investigated the fate of E. huxleyi (CCMP 374) infected with virus strain EhV-86 in a simple predator-prey interaction. The ingestion rates of Oxyrrhis marina were significantly lower (between 26.9 and 50.4%) when fed virus-infected E. huxleyi cells compared to non-infected cells. Despite the lower ingestion rates, O. marina showed significantly higher growth rates (between 30 and 91.3%) when fed infected E. huxleyi cells, suggesting higher nutritional value and/or greater assimilation of infected E. huxleyi cells. No significant differences were found in O. marina cell volumes or fatty acids profiles. These results show that virally infected E. huxleyi support higher growth rates of single celled heterotrophs and in addition to the “viral shunt” hypothesis, viral infections may also divert more carbon to mesozooplankton grazers.

Emissions of dimethylsulfide (DMS) from the polar oceans play a key role in atmospheric processes and climate. Therefore, it is important we increase our understanding of how DMS production in these regions may respond to environmental change. The polar oceans are particularly vulnerable to ocean acidification (OA). However, our understanding of the polar DMS response is limited to two studies conducted in Arctic waters, where in both cases DMS concentrations decreased with increasing acidity. Here, we report on our findings from seven summertime shipboard microcosm experiments undertaken in a variety of locations in the Arctic Ocean and Southern Ocean. These experiments reveal no significant effects of short term OA on the net production of DMS by planktonic communities. This is in contrast to identical experiments from temperate NW European shelf waters where surface ocean communities responded to OA with significant increases in dissolved DMS concentrations. A meta-analysis of the findings from both temperate and polar waters (n = 18 experiments) reveals clear regional differences in the DMS response to OA. We suggest that these regional differences in DMS response reflect the natural variability in carbonate chemistry to which the respective communities may already be adapted. Future temperate oceans could be more sensitive to OA resulting in a change in DMS emissions to the atmosphere, whilst perhaps surprisingly DMS emissions from the polar oceans may remain relatively unchanged. By demonstrating that DMS emissions from geographically distinct regions may vary in response to OA, our results may facilitate a better understanding of Earth's future climate. Our study suggests that the way in which processes that generate DMS respond to OA may be regionally distinct and this should be taken into account in predicting future DMS emissions and their influence on Earth's climate.

© 2015, The Oceanography Society. All rights reserved. Ocean acidification (OA) research seeks to understand how marine ecosystems and global elemental cycles will respond to changes in seawater carbonate chemistry in combination with other environmental perturbations such as warming, eutrophication, and deoxygenation. Here, we discuss the effectiveness and limitations of current research approaches used to address this goal. A diverse combination of approaches is essential to decipher the consequences of OA to marine organisms, communities, and ecosystems. Consequently, the benefits and limitations of each approach must be considered carefully. Major research challenges involve experimentally addressing the effects of OA in the context of large natural variability in seawater carbonate system parameters and other interactive variables, integrating the results from different research approaches, and scaling results across different temporal and spatial scales.

The ubiquitous marine trace gas dimethyl sulfide (DMS) comprises the greatest natural source of sulfur to the atmosphere and is a key player in atmospheric chemistry and climate. We explore the short-term response of DMS production and cycling and that of its algal precursor dimethyl sulfoniopropionate (DMSP) to elevated carbon dioxide (CO2) and ocean acidification (OA) in five 96 h shipboard bioassay experiments. Experiments were performed in June and July 2011, using water collected from contrasting sites in NW European waters (Outer Hebrides, Irish Sea, Bay of Biscay, North Sea). Concentrations of DMS and DMSP, alongside rates of DMSP synthesis and DMS production and consumption, were determined during all experiments for ambient CO2 and three high-CO2 treatments (550, 750, 1000 μatm). In general, the response to OA throughout this region showed little variation, despite encompassing a range of biological and biogeochemical conditions. We observed consistent and marked increases in DMS concentrations relative to ambient controls (110% (28–223%) at 550 μatm, 153% (56–295%) at 750 μatm and 225% (79–413%) at 1000 μatm), and decreases in DMSP concentrations (28% (18–40%) at 550 μatm, 44% (18–64%) at 750 μatm and 52% (24–72%) at 1000 μatm). Significant decreases in DMSP synthesis rate constants (μDMSP, d−1) and DMSP production rates (nmol d−1) were observed in two experiments (7–90% decrease), whilst the response under high CO2 from the remaining experiments was generally indistinguishable from ambient controls. Rates of bacterial DMS gross consumption and production gave weak and inconsistent responses to high CO2. The variables and rates we report increase our understanding of the processes behind the response to OA. This could provide the opportunity to improve upon mesocosm-derived empirical modelling relationships and to move towards a mechanistic approach for predicting future DMS concentrations.

[1] Concentrations of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are highly variable in time and space. What is driving the variability in DMS(P), and can those variability be explained by physical processes and changes in the biological community? During the Southern Ocean Gas Exchange Experiment (SO GasEx) in the austral fall of 2008, two 3He/SF6 labeled patches were created in the surface water. SF6 and DMS were surveyed continuously in a Lagrangian framework, while direct measurements of air-sea exchange further constrained the gas budgets. Turbulent diffusivity at the base of the mixed layer was estimated from SF6 profiles and used to calculate the vertical fluxes of DMS and nutrients. Increasing mixed layer nutrient concentrations due to mixing were associated with a shift in the phytoplankton community structure, which in turned likely affected the sulfur dynamics on timescales of days. DMS concentration as well as air-sea DMS flux appeared to be decoupled from the DMSP concentration, possibly due to grazing and bacterial DMS production. Contrary to expectations, in an environment with high winds and modest productivity, physical processes (air-sea exchange, photochemistry, vertical mixing) only accounted for a small fraction of DMS loss from the surface water. Among the DMS sinks, inferred biological consumption most likely dominated during SO GasEx.

Using computer simulations, this study quantifies the potential effect of ocean acidification on marine biogenic sulphur emissions and the resulting feedback mechanisms on future climate. It emphasizes that a reduction of anthropogenic carbon dioxide emissions is not only necessary to limit the negative effects of ocean acidification on marine life, but also to avoid amplified climate warming due to changes in biogenic sulphur production.

The global surface seawater dimethylsulphide (DMS) database (http://saga.pmel.noaa.gov/dms/) contains > 50,000 data points and is the second largest trace gas database after carbon dioxide. However, there has been relatively little quality control on the data that have been collated to date. Furthermore, the recent development of technologies capable of high frequency (> 1 Hz) DMS measurements will have a disproportionate effect on the database in future years. At this juncture, the comparability of analytical techniques, sample handling methodologies and standards are pressing issues that the DMS community needs to address. In October 2010, during the Fifth International Symposium on Biological and Environmental Chemistry of DMS(P) and Related Compounds held in Goa, India, attendees participated in a discussion concerning the current DMS database and its future development. We develop some of the ideas from that session and combine them with available data. From the few inter-comparison exercises that have been conducted we show that variability between existing measurements within the DMS database is likely to be a parts per thousand currency sign25%. Tests comparing different DMSP center dot HCl standards demonstrate that a reference calibration standard would be beneficial for the DMS community. Confidence in future data collation would be substantially improved with a comprehensive inter-comparison experiment between new analytical techniques and sampling methodologies (e.g., mass spectrometers with equilibrators attached to a continuous flow of seawater) and more established methods (i.e., filtered samples analysed with purge and trap gas chromatography). We conclude with recommendations for the future expansion of the DMS database and its data quality control.

In the Sargasso Sea, maximum dimethylsulfide (DMS) accumulation occurs in summer, concomitant with the minimum of chlorophyll and 2 months later than its precursor, dimethylsulfoniopropionate (DMSP). This phenomenon is often referred to as the DMS “summer paradox”. It has been previously suggested that the main agent triggering this pattern is increasing irradiance leading to light stress-induced DMS release from phytoplankton cells. We have developed a new model describing DMS(P) dynamics in the water column and used it to investigate how and to what extent processes other than light induced DMS exudation from phytoplankton, may contribute to the DMS summer paradox. To do this, we have conceptually divided the DMS “summer paradox” into two components: (1) the temporal decoupling between chlorophyll and DMSP and (2) the temporal decoupling between DMSP and DMS. Our results suggest that it is possible to explain the above cited patterns by means of two different dynamics, respectively: (1) a succession of phytoplankton types in the surface water and (2) the bacterially mediated DMSP(d) to DMS conversion, seasonally varying as a function of nutrient limitation. This work differs from previous modelling studies in that the presented model suggests that phytoplankton light-stress induced processes may only partially explain the summer paradox, not being able to explain the decoupling between DMSP and DMS, which is possibly the more challenging aspect of this phenomenon. Our study, therefore, provides an “alternative” explanation to the summer paradox further underlining the major role that bacteria potentially play in DMS production and fate.

The SOLAS air–sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive subpolar zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment was conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a Lagrangian study, quantifying key biological and physical drivers influencing the air–sea gas exchange processes of CO2, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom. A dual tracer SF6/3He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air–sea tracer exchange at tenths of kilometer scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describe air–sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties and windspeed were quantified to further assist the development of gas exchange models for high-wind environments. There was a significant increase in the photosynthetic competence (Fv/Fm) of resident phytoplankton within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary production and column-integrated chlorophyll a concentrations had only doubled relative to the unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions totalling 1.1 ton Fe2+, this was a very modest response compared to other mesoscale iron enrichment experiments. An investigation of the factors limiting bloom development considered co-limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst incident light levels and the initial Si:N ratio were the lowest recorded in all FeAXs to date, there was only a small seed-stock of diatoms (less than 1% of biomass) and the main response to iron addition was by the picophytoplankton. A high rate of dilution of the fertilised patch relative to phytoplankton growth rate, the greater than expected depth of the surface mixed layer and microzooplankton grazing were all considered as factors that prevented significant biomass accumulation. In line with the limited response, the enhanced biological draw-down of pCO2 was small and masked by a general increase in pCO2 due to mixing with higher pCO2 waters. The DMS precursor DMSP was kept in check through grazing activity and in contrast to most FeAX’s dissolved dimethylsulfide (DMS) concentration declined through the experiment. SAGE is an important low-end member in the range of responses to iron addition in FeAX’s. In the context of iron fertilisation as a geoengineering tool for atmospheric CO2 removal, SAGE has clearly demonstrated that a significant proportion of the low iron ocean may not produce a phytoplankton bloom in response to iron addition.

The impact of in situ iron fertilisation on the production of particulate dimethylsulphoniopropionate (DMSPp) and its breakdown product dimethyl sulphide (DMS) was monitored during the SOLAS air–sea gas exchange experiment (SAGE). The experiment was conducted in the high nitrate, low chlorophyll (HNLC) waters of the sub-Antarctic Southern Ocean (46.7°S 172.5°E) to the south-east of New Zealand, during March–April, 2004. In addition to monitoring net changes in the standing stocks of DMSPp and DMS, a series of dilution experiments were used to determine the DMSPp production and consumption rates in relation to increased iron availability. In contrast to previous experiments in the Southern Ocean, DMS concentrations decreased over the course of the 15-d iron-fertilisation experiment, from an integrated volume-specific concentration in the mixed layer on day 0 of 0.78 nM (measured values 0.65–0.91 nM) to 0.46 nM (measured values 0.42–0.47 nM) by day 15, in parallel with the surrounding waters. DMSPp, chlorophyll a and the abundance of photosynthetic picoeukaryotes exhibited indiscernible or only moderate increases in response to the raised iron availability, despite an obvious physiological response by the phytoplankton. High specific growth rates of DMSPp, equivalent to 0.8–1.2 doublings d−1, occurred at the simulated 60% light level of the dilution experiments. Despite the high production rates, DMSPp accumulation was suppressed in part by microzooplankton grazers who consumed between 61% d−1 and 126% d−1 of the DMSPp production. Temporal trends in the rates of production and consumption illustrated a close coupling between the DMSP-producing phytoplankton and their microzooplankton grazers. Similar grazing and production rates were observed for the eukaryotic picophytoplankton that dominated the phytoplankton biomass, partial evidence that picoeukaryotes contributed a substantial proportion of the DMSP synthesis. These rates for DMSPp and picoeukaryotes were considerably higher than for chlorophyll a, indicating higher cycling rates of the DMSP-producing taxa than for the bulk phytoplankton community. When compared to the total phytoplankton community, there was no evidence of selection against the DMSP-containing phytoplankton by the microzooplankton grazers; the opposite appeared to be the case. SAGE demonstrated that increased iron availability in the HNLC waters of the Southern Ocean does not invariably lead to enhanced DMS sea–air flux. The potential suppression of DMSPp accumulation by grazers needs to be taken into account in future attempts to elevate DMS emission through in situ iron fertilisation and in understanding the hypothesised link between levels of Aeolian iron deposition in the Southern Ocean, DMS emission and global albedo.

An approach combining flow-cytometric fluorescence-activated cell sorting (FACS) of specific phytoplankton groups and subsequent quantification of dimethylsulphoniopropionate (DMSP) content by gas chromatography (GC) is introduced and applied to cultivated microalgae and natural seawater samples. The FACS-GC based method produces statistically robust estimates of the intracellular DMSP content of a variety of phytoplankton groups ranging from prokaryotic Synechococcus spp. to eukaryotic nanophytoplankton. Comparison between FACS and traditional filtration approaches to determine intracellular DMSP content of cultivated microalgae illustrates variable responses between species to the 2 methods. Natural photosynthetic nanoeukaryote populations contained high intracellular DMSP content; comparable to many cultivated prymnesiophyte taxa. A surprisingly high DMSP content was found in natural populations of Synechococcus spp., in contrast to previous values measured in cultivated strains, but possibly confirming the uptake and retention of DMSP by this photosynthetic prokaryote. When combined, the DMSP contributions of each phytoplankton group amounted to between 40 and 57 % of total DMSP in each water sample, with the bulk of the total comprising nanoeukaryote-DMSP. The approach has high potential to provide information that enables better parameterisation of phytoplankton functional types in mechanistic dimethyl sulphide models and to help elucidate the environmental controls that govern the extent and diversity of DMSP production in the oceans.

Microzooplankton are ecologically important grazers of phytoplankton and bacteria in aquatic systems. Inhabiting a three-dimensional realm, the ability of a microzooplankter to detect and respond to infochemical cues from its prey will confer a strong selective advantage. The heterotrophic dinoflagellate Oxyrrhis marina has been repeatedly used to study the effect of infochemicals on foraging success and the results from such investigations have stimulated further research on other microzooplankton. Here, we present a case study that highlights the utility of O. marina for infochemistry research and review current knowledge on its behavioural response to dissolved infochemicals. Through a series of microcapillary assays, we show that 0.1–100 µM source concentrations of the algal secondary metabolite dimethylsulphoniopropionate elicit a motile chemosensory response in O. marina. We outline some of the technical limitations in infochemistry research and provide examples where studies on O. marina resulted in recent progress, before emphasizing the role of the diffusion limited boundary layer or “phycosphere” in chemodetection. The ability of O. marina to detect prey exudates in the phycosphere effectively increases the predator–prey encounter radius and has significant implications to the outcome of encounter rate models. Finally, we introduce a new direction of research that utilizes O. marina as a model microzooplankter to elucidate the function of infochemicals in multi-trophic interactions.

The response in intracellular dimethyl sulfoniopropionate (DMSP) and dissolved DMSP and dimethyl sulfide (DMS) concentrations when Emiliania huxleyi was exposed to acute (1-h) increases in photon flux densities of photosynthetically active radiation (PAR) and ultraviolet (UV) radiation was examined in cells acclimated to low light (LL, 30 mmol photons m22 s21) and high light (HL, 300 mmol photons m22 s21). LL-acclimated cells displayed greater photoinhibition, assessed as a decrease in maximum photochemical efficiency (Fv:Fm). Photoinhibition was increased by exposure to UV wavelengths. LL-acclimated cells also exhibited more light dissipation through the xanthophyll cycle, evident as changes in de-epoxidation state. Greater photoinhibition in LL-acclimated cells corresponded with increased accumulation of DMSP of 21% 6 4% relative to initial concentrations, contrasting with a slight decrease of 5% 6 6% in HL-acclimated cells. Exposure to UV appeared to decrease the rates of intracellular accumulation of DMSP. Conversely, PAR + UV exposure stimulated the net production of dissolved DMSP and DMS in both HL-acclimated and LL-acclimated cultures, compared with high PAR alone. The results indicate a direct link between acute photo-oxidative stress and DMSP synthesis by E. huxleyi. The physiological basis for increased release of DMSP and DMS from cells due to high PAR + UV exposure is unclear. However, the timescales of changes in intracellular DMSP, dissolved DMSP, and DMS are consistent with variations in light intensity experienced by phytoplankton in a turbulent mixed layer and are similar to rates of change in photosynthetic parameters associated with photoacclimation. Emission of dimethyl sulfide (DMS) from the oceans may contribute to the nucleation and growth of aerosol particles, enhancing formation of cloud condensation nuclei in the marine boundary layer; and thereby increasing the Earth’s albedo (Shaw 1983; Charlson et al. 1987). DMS is generally supersaturated in oceanic waters and its sea-toair flux is dictated to a large extent by surface seawater concentration and the gas transfer velocity. Understanding

Using a steady-state diffusion model, we calculated the concentration of the ecologically relevant algal metabolite dimethylsulphoniopropionate (DMSP) in the phycosphere. Incorporating diffusive losses, the concentration was predicted to be 106-fold lower than previously suggested.

The influences of physico-chemical and biological variables on the concentrations of di- methyl sulphide (DMS) and its precursor β-dimethylsulphoniopropionate (DMSP) were investigated through an annual cycle in the temperate shelf seas of the western English Channel. Total DMSP to chlorophyll a ratios (DMSPt/chl a) varied seasonally by 40-fold, and DMS and DMSP concentrations became temporally uncoupled, with elevated relative DMS concentrations during spring and mid- summer. Taxonomic succession of high DMSP-producing phytoplankton, including Phaeocystis pouchetii, Scrippsiella trochoidea and Prorocentrum minimum, is apparent in the seasonal pattern of DMSPt concentrations. Peridinin and DMSPt concentrations showed similar seasonal trends (p < 0.0001), illustrating the substantial contribution by dinoflagellate taxa to DMSP production. Summer- time stratification of the water column coincided with increased mixed layer doses of photosyntheti- cally active radiation (PAR), increased surface ultraviolet-B (UVB) irradiance relative to PAR and a de- crease in nitrate and phosphate availability. PAR dose explained 68% of the variability in DMSP/chl a during the seasonal study; whilst nitrate concentrations were inversely related to DMSP/chl a and explained 64% of the variability in log-transformed DMSP/chl a. PAR dose explained only 25% of the variation in DMS concentration, whilst nitrate concentration was inversely related to DMS and explained 49% of the variation in log-transformed DMS concentration. The highly significant relation- ship between DMSP/chl a and PAR dose was similar to those observed for the chlorophyll-specific accumulation of the photoprotective xanthophyll compounds diadinoxanthin and diatoxanthin and the chlorophyll-specific concentrations of UV-absorbing mycosporine-like amino acids. These results lend further, indirect evidence for a photoprotective role of DMSP, possibly associated with physiological stress caused by high PAR and UV radiation and intensified by nutrient limitation.

This document is a supplement to “Physical Exchanges at the Air–Sea Interface: UK–SOLAS Field Measurements,” by Ian M. Brooks, Margaret J. Yelland, Robert C. Upstill-Goddard, Philip D. Nightingale, Steve Archer, Eric d’Asaro, Rachael Beale, Cory Beatty, Byron Blomquist, A. Anthony Bloom, Barbara J. Brooks, John Cluderay, David Coles, John Dacey, Michael DeGrandpre, Jo Dixon, William M. Drennan, Joseph Gabriele, Laura Goldson, Nick Hardman-Mountford, Martin K. Hill, Matt Horn, Ping-Chang Hsueh, Barry Huebert, Gerrit de Leeuw, Timothy G. Leighton, Malcolm Liddicoat, Justin J. N. Lingard, Craig McNeil, James B. McQuaid, Ben I. Moat, Gerald Moore, Craig Neill, Sarah J. Norris, Simon O’Doherty, Robin W. Pascal, John Prytherch, Mike Rebozo, Erik Sahlee, Matt Salter, Ute Schuster, Ingunn Skjelvan, Hans Slagter, Michael H. Smith, Paul D. Smith, Meric Srokosz, John A. Stephens, Peter K. Taylor, Maciej Telszewski, Roisin Walsh, Brian Ward, David K. Woolf, Dickon Young, and Henk Zemmelink (Bull. Amer. Meteor. Soc., 90, 629–644) • ©2009 American Meteorological Society • Corresponding author: Ian M. Brooks, Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom • E-mail: i.brooks@see.leeds.ac.uk • DOI:10.1175/2008BAMS2578.2

This study tested the validity of the modified dilution method approach to partition nat- ural phytoplankton mortality into grazer versus virus-induced fractions. A series of experiments was conducted weekly between August and October 2005 from an offshore site in the western English Channel. The phytoplankton community during this period was dominated numerically by Syne- chococcus spp. and picoeukaryotes. Despite highly significant regression results, no discernible viral lysis of Synechococcus spp. or picoeukaryote populations was apparent during the 3 mo period. Microzooplankton grazing was the major source of mortality for Synechococcus spp. and picoeukary- otes during this time. Statistical analysis indicates that, using our experimental design, the modified dilution approach was not sensitive enough to be able to measure significant viral lysis during the experimental period. Sensitivity analysis indicated that viral mortality occurred at rates of

Notre etude de modelisation vise a placer dans un cycle annuel nos connaissances de la biogeochimie du sulfure de dimethyle (DMS), un gaz qui affecte le climat, dans les eaux marines superficielles. Nous avons incorpore les processus impliques dans la production et le sort du DMS et de son precurseur le β-dimethylsulfoniopropionate (DMSP), un produit de synthese du phytoplancton, dans le modele ERSEM (« European Regional Seas Ecosystem Model »), un modele physique, complexe, unidimensionel et couple de l'ecosysteme et dans le modele GOTM (« General Ocean Turbulence Model »). Nous avons ainsi mis au point un modele de la biogeochimie du DMS applique a des donnees de 1989 provenant d'un site a stratification saisonniere de la mer du Nord. Nous l'avons valide avec des donnees de concentrations de nutriments, de biomasses biologiques, de production biologique, de concentrations de DMS et de DMSP et de flux de DMS de la mer a l'atmosphere obtenues en 1989 dans le cadre du Projet de la mer du Nord (« North Sea Project »). Les resultats de cette validation demontrent que des modeles complexes de l'ecosysteme, combines a un reseau complet des processus qui controlent la production et le sort du DMSP et du DMS, peuvent representer de facon exacte la biogeochimie du DMS. Le modele fournit des perspectives interessantes sur l'impact du forcage physique, qui varie d'une saison a l'autre, sur les contributions relatives des divers processus particuliers a la production du DMS et a son passage de la mer a l'atmosphere.

The impact of microzooplankton grazing upon phytoplankton production was quantified in surface waters of the Southern Bight of the North Sea, during April 1998. Two sites were studied in order to examine the impact of microzooplankton grazing on phytoplankton communities dominated by either Phaeocystis globosa and large phytoplankton or small phytoplankton taxa. The nearshore site was characterised by a phytoplankton community comprised mainly of P. globosa and chains of diatoms with high productivity (av. 346 F 185 AgC l � 1 d � 1 ) and biomass (280 F 171 AgC l � 1 d � 1 ). In contrast, in the offshore waters relatively small diatoms dominated the phytoplankton where productivity and biomass were more than ca. five times lower than in nearshore waters. Contrary to expectations, the nearshore site supported a high biomass of microzooplankton (av. 22.4 F 10.6 AgC l � 1 d � 1 ) which was dominated by large heterotrophic dinoflagellates, mostly Gyrodinium cf. spirale. Offshore the microzooplankton community contained one-third the biomass of the nearshore community and was dominated by smaller individuals, in particular oligotrich ciliates, Strombidium spp. Dilution experiments were conducted in order to quantify phytoplankton growth and losses due to microzooplankton grazing in the

Microzooplankton herbivory is considered to be a key process by which dimethylsulphoniopropionate (DMSP) in phytoplankton is transformed to climatically active dimethyl sulphide (DMS). However, there is little firm evidence to show that this occurs in natural waters. We used direct measurements of microzooplankton grazing rates and net DMS production in the southern North Sea to examine the impact of herbivory on DMS production. Estimates of the particulate DMSP ingested by microzooplankton in the form of Phaeocystis sp. were found to account for the DMS production rates observed.

During a mesocosm study in Raunefjorden, Norway, a Micromonas spp. population, ini- tially showing exponential net growth, dramatically declined after Day 4 of the experiment. Using a modification of the dilution approach originally developed to quantify grazing by microzooplankton on phytoplankton, it was possible to partition the mortality of Micromonas spp. between grazing and viral lysis on Days 5, 6 and 7 during the population decline. Parallel dilution experiments were car- ried out in which 0.2 µm- and 10 kDa-filtered water was used as the diluents. In this way, gradients of grazing pressure (0.2 µm series) and grazing pressure + viral concentration (10 kDa series) were produced. Model 1 linear regression of the fraction of whole water versus the apparent growth rate of chlorophyll a and Micromonas spp. provided an estimate of mortality in the 0.2 µm and 10 kDa dilution series. On Days 5, 6 and 7, the slopes of the linear regressions of 0.2 µm and 10 kDa dilution series were significantly different at p = 0.083, 0.001 and 0.093 respectively. From the differences in slope between the series, estimates of viral mortality amounted to a turnover rate of the Micromonas spp. standing stocks of 10, 25 and 9% d -1 . This compares to a turnover rate by the microzooplankton of 48, 26 and 23% d -1 . On all 3 d the combined viral lysis and grazing mortality exceeded estimates of the potential production of Micromonas spp., in part accounting for the population decline. This study demonstrates that the dilution approach can be adapted to directly determine virus-induced mortality rates of specific phytoplankton. However, further work is required to determine how the specificity of viral infection and variety of viral infection cycles affect the results of this modified dilution approach when applied to other phytoplankton taxa and communities.

During an experiment in the North Atlantic in June 1998, water samples were collected approximately 400 km south of Iceland inside and outside of a bloom of the coccolithophorid Emilia- nia huxleyi. In vitro dimethylsulphoniopropionate (DMSP) lyase activity (DLA) was quantified using gas chromatography and found to vary from 0.1 to 142.3 nM dimethyl sulphide (DMS) h -1 . Inside the bloom area the majority of DLA (> 74%) occurred in particles >10 µm, indicating that E. huxleyi (5 to 7 µm diameter) made only a minor contribution to total DLA. In surface waters, phototrophic dino- flagellates (>10 µm) made up a high proportion of the total phytoplankton biomass (~27%) towards the end of the coccolithophorid bloom and may have been the source of most of the DLA. This was also indicated by a significant correlation (p < 0.02) between DLA and the concentration of peridinin, a pigment used as a chemotaxonomic marker for dinophytes. The data presented here are the first field measurements of DLA in a coccolithophorid bloom and suggest that even a relatively low con- centration of photosynthetic dinoflagellates larger than 10 µm (during our study 18 to 105 cells ml -1 ) may contribute significantly to DMS production. Although dinoflagellates are recognised as an important source of particulate DMSP, recognition of their significance for DMS production in the field has previously been limited to a few observations in highly concentrated coastal and shelf blooms. Very little information exists on DLA in dinophytes and further investigations are warranted in order to improve our understanding of the biogeochemical and ecophysiological significance of DMSP lyases in this group of phytoplankton.

This paper presents an overview of dimethyl sulphide biogeochemistry within a coccolithophore bloom (DISCO), an integrated, multidisciplinary Lagrangian process study of the routes, rates and controls on the biogeochemical cycling of dimethyl sulphide (DMS) within a growing bloom of the coccolithophorid alga, Emiliania huxleyi . The Lagrangian study took place between 16 and 26 June 1999 in the northern North Sea. It was preceded by an 8-d survey of ∼52,000 km 2 of the region to locate an E. huxleyi bloom suitable for study. Although not originally planned, the survey was carried out because heavy cloud cover precluded use of remote sensing to locate a suitable bloom. E. huxleyi blooms, typically common in the region during mid-summer, were unusually sparse in the study area. The bloom chosen for the process study was initially centred ∼58°56′N 02°52′E, and a 40-km 2 patch of water was labelled for study with ∼30 g sulphur hexafluoride (SF 6 ) on 16 June. The original patch was reinfused with further SF 6 on 24 June. During the process study, the SF 6 -labelled patch moved in a south-easterly direction and the study ended when the patch subducted underneath less dense Norwegian coastal water. The process study comprised analyses of the time-varying biological, optical and physical properties of the patch as well as studies of DMS, dimethylsulphonioproprionate (DMSP), dimethylsulphoxide, nutrients, halocarbons, methylamines, carbon monoxide, dissolved organic carbon, and total dissolved nitrogen. The role of viruses, bacteria, phytoplankton, microzooplankton, and mesozooplankton, together with the dynamics of primary, new and bacterial production, plankton respiration, microzooplankton grazing, and sedimentation, were studied in relation to the biogeochemical cycling of DMS. Although the coccolithophore bloom water exhibited high optical backscatter, the algal community present was highly heterogeneous. Flagellates other than E. huxleyi were found to dominate the phytoplankton. A budget of the DMSP pools suggested that E. huxleyi accounted for only 13% of the stocks of particulate DMSP, showing that in this “ E. huxleyi bloom”, taxa other than E. huxleyi were important sources of DMSP. In this young bloom, particulate and dissolved DMSP and DMS concentrations averaged 1360, 155 and 60 μM m −2 , respectively, in the surface mixed layer. Surface-water particulate DMSP concentrations increased during the study at a net rate of 13% d −1 , as did concentrations of phytoplankton including E. huxleyi , confirming that the bloom was developing. Nutrient conditions were low in the mixed layer throughout the study, maintained by a strong pycnocline across which nitrate upflux was estimated to be ∼2 nM dm −3 d −1 . Primary production was fuelled by regenerated nutrients, although nitrification rates in surface waters were found to be significant. Microzooplankton grazing accounted for 91% of the particulate DMSP degradation and was considered to be a major control on the DMSP concentration. Vigorous microzooplankton grazing together with rapid uptake of dissolved DMSP by bacteria suggest that microzooplankton were the main route for the production of dissolved DMSP. The bacterial community was dominated by one taxon, an α proteobacteria related to Roseobacter that satisfied its entire sulphur demand by metabolising dissolved DMSP. Bacteriogenic DMS production amounted to 2 nM d −1 and was considered the main route for DMS production. In vitro DMSPlyase activity was very high, but there was little evidence for high in situ activity. Over the study period, DMS flux to the atmosphere was estimated to be 7 μM m −2 d −1 , equivalent to ∼1% of the DMSP sulphur produced in the surface mixed layer. A budget for DMS cycling in the upper mixed layer is presented based on the analytical and experimental measurements made in the DISCO study.

The structure and diversity of the microplankton, between 10 and 200 μm in size, were quantified during a Lagrangian SF 6 tracer experiment in the North Sea (59°N and 03°E) during June 1999. The experiment was conducted within a developing Emiliania huxleyi -rich phytoplankton bloom. Distinct thermoclines characterised the water column. The surface mixed layer extended to between 15 and 30 m, and the subsurface layer reached a depth of between 45 and 55 m. Despite the relatively high abundance of E. huxleyi ( −1 ), a diverse plankton community was observed throughout the study, with 59 autotrophic and 57 heterotrophic microplankton species being identified. Distinct spatial patterns were observed in the plankton, as smaller cells were common in the surface layer and larger species characterised the subsurface waters. The microplankton communities in the two layers, were found to be significantly different from each other. Prorocentrum minimum and P-Nitzschia ‘delicatissima ’ were the dominant microphytoplankton species in surface waters, while the larger diatom species P-Nitzschia ‘seriata’, Chaetoceros radicans , and Rhizosolenia stolterfothii were common in the subsurface layer. The microzooplankton were dominated by oligotrich ciliates and dinoflagellates, with smaller forms such as Strombidium spp., Diplopsalis sp., and Protoperidinium cf. cerasus characterising surface waters, and larger species, Strombidium ovale, Gyrodinium fusiforme , and Gyrodinium sp. dominating the subsurface layer. Significant temporal changes were observed in the composition of the surface-layer microplankton community, as well as in the structure of the subsurface microzooplankton. Measures of diversity indicated that there were significant differences in the microphytoplankton between surface and subsurface waters and that the temporal changes in the surface layer were also significant. These changes coincided with an increase in microzooplankton grazing pressure and a shift in the species composition of the mesozooplankton. Coupled with observations that regenerated nutrients provided an important nitrogen supply for phytoplankton growth during this study, our data suggest that ‘top-down’ control played an important role in enhancing diversity and determining the structure of the plankton in the stratified northern North Sea, during the summer. The significant changes observed in the microplankton during this study, occurred over a relatively short period of ca. 9 days and emphasise the importance of understanding short-term fluctuations in order to make accurate longer-term predictions.

Bacterioplankton-driven turnover of the algal osmolyte, dimethylsulphoniopropionate (DMSP), and its degradation product, dimethylsulphide (DMS) the major natural source of atmospheric sulphur, were studied during a Lagrangian SF6-tracer experiment in the North Sea (60°N, 3°E). The water mass sampled within the euphotic zone was characterised by a surface mixed layer (from 0 m to 13–30 m) and a subsurface layer (from 13–30 m to 45–58 m) separated by a 2°C thermocline spanning 2 m. The fluxes of dissolved DMSP (DMSPd) and DMS were determined using radioactive tracer techniques. Rates of the simultaneous incorporation of 14C-leucine and 3H-thymidine were measured to estimate bacterioplankton production. Flow cytometry was employed to discriminate subpopulations and to determine the numbers and biomass of bacterioplankton by staining for nucleic acids and proteins. Bacterioplankton subpopulations were separated by flow cytometric sorting and their composition determined using 16S ribosomal gene cloning/sequencing and fluorescence in situ hybridisation with designed group-specific oligonucleotide probes. A subpopulation, dominated by bacteria related to Roseobacter-(α-proteobacteria), constituted 26–33% of total bacterioplankton numbers and 45–48% of biomass in both surface and subsurface layers. The other abundant prokaryotes were a group within the SAR86 cluster of γ-proteobacteria and bacteria from the Cytophaga–Flavobacterium—cluster. Bacterial consumption of DMSPd was greater in the subsurface layer (41 nM d−1) than in the surface layer (20 nM d−1). Bacterioplankton tightly controlled the DMSPd pool, particularly in the subsurface layer, with a turnover time of 2 h, whereas the turnover time of DMSPd in the surface layer was 10 h. Consumed DMSP satisfied the majority of sulphur demands of bacterioplankton, even though bacterioplankton assimilated only about 2.5% and 6.0% of consumed DMSPd sulphur in the surface and subsurface layers, respectively. Bacterioplankton turnover of DMS was also faster in the subsurface layer (12 h) compared to the surface layer (24 h). However, absolute DMS consumption rates were higher in the surface layer, due to higher DMS concentrations in this layer. The majority of DMS was metabolised into dissolved non-volatile products, and bacteria could satisfy only 1–3% of their sulphur demands from DMS. Thus, structurally similar bacterioplankton communities exerted strong control over DMSPd and DMS concentrations both in the subsurface layer and surface mixed layer.

This study describes the routes and rates of transformation of dimethylsulphoniopropionate (DMSP) to dimethyl sulphide (DMS) in a phytoplankton bloom in the northern North Sea (59°N 2°E). A region on the edge of the high reflectance waters that characterised the bloom was labelled with SF6-tracer and tracked for 6 days. Within the framework of this Lagrangian experiment, it was possible to compile a comprehensive budget of the transformation of DMSP to DMS in the surface mixed layer of 15–30 m depth and a subsurface layer that extended to ⩾40 m. As a basis to the synthesis of the DMS(P) biogeochemistry, an attempt was made to accurately constrain DMS emissions from the Lagrangian water mass. Included in the study is a comparison of estimates of the transfer velocity and DMS sea to air flux derived from two well established and two recently introduced parameterisations. The two recent approaches encouragingly gave similar rates of total DMS sea to air flux of 1.60 mg S m−2 over the 6-day experimental period, and this value was intermediate to values derived from the two more routinely used approaches. The DMS sea to air flux was equivalent in terms of sulphur, to 10% of the DMS production and 1.3% of the particulate DMSP (DMSPp) production in the surface layer over the 6 days. Bacterial consumption accounted for the majority of DMS removal in both surface (62–82%) and subsurface layers (average 98%) and DMS concentrations decreased over the 6 days, despite increasing DMS production. Microzooplankton were the main agents of the transformation of DMSPp to the dissolved phase but little DMS appeared to be produced directly by the grazers. Instead, bacteria rapidly turned over the dissolved DMSP (DMSPd) made available by the grazers. An upper limit of 17% of the DMSPd consumed by bacteria in the surface layer was cleaved to DMS over the 6 days. Lower transformation efficiencies of ∼6% were estimated for the subsurface layer. An ecosystem model incorporating DMS(P) biogeochemistry was developed on the basis of the empirical data. Simple sensitivity analyses were employed to demonstrate the importance of both microzooplankton and bacterial metabolism in controlling the yield of DMS from DMSP production and hence, the DMS sea to air flux. Greater yields of DMS are likely to occur when DMSPp is transformed directly to DMS, in this case due to grazing, than when transformed to DMSPd and subsequently cleaved to DMS by bacterial activity. The point of DMS production in the DMSPp to DMS transformation pathway is likely to alter in relation to phytoplankton bloom progression and with season. The complex interaction amongst heterotrophic processes that control DMS concentrations suggests that attempts to predict global DMS flux may need to couple comprehensive ecosystem models of the type used in this study, to general circulation models.