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Sea spray aerosol (SSA) is one of the largest global sources of atmospheric aerosol, but little is known about SSA generated in coastal regions with salinity gradients near estuaries and river outflows. SSA particles are chemically complex with substantial particle-to-particle variability due to changes in water temperature, salinity, and biological activity. In previous studies, the ability to resolve the aerosol composition to the level of individual particles has proven necessary for the accurate parameterization of the direct and indirect aerosol effects; therefore, measurements of individual SSA particles are needed for the characterization of this large source of atmospheric aerosol. An integrated analytical measurement approach is required to probe the chemical composition of individual SSA particles. By combining complementary vibrational microspectroscopic (Raman and optical photothermal infrared, O-PTIR) measurements with elemental information from computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM-EDX), we gained unique insights into the individual particle chemical composition and morphology. Herein, we analyzed particles from four experiments on laboratory-based SSA production using coastal seawater collected in January 2018 from the Gulf of Maine. Individual salt particles were enriched in organics compared to that in natural seawater, both with and without added microalgal filtrate, with greater enrichment observed for smaller particle sizes, as evidenced by higher carbon/sodium ratios. Functional group analysis was carried out using the Raman and infrared spectra collected from individual SSA particles. Additionally, the Raman spectra were compared with a library of Raman spectra consisting of marine-derived organic compounds. Saccharides, followed by fatty acids, were the dominant components of the organic coatings surrounding the salt cores of these particles. This combined Raman, infrared, and X-ray spectroscopic approach will enable further understanding of the factors determining the individual particle composition, which is important for understanding the impacts of SSA produced within estuaries and river outflows, as well as areas of snow and ice melt.

Marine polymer gels play a critical role in regulating ocean basin scale biogeochemical dynamics. This brief review introduces the crucial role of marine gels as a source of aerosol particles and cloud condensation nuclei (CCN) in cloud formation processes, emphasizing Arctic marine microgels. We review the gel’s composition and relation to aerosols, their emergent properties, and physico-chemical processes that explain their change in size spectra, specifically in relation to aerosols and CCN. Understanding organic aerosols and CCN in this context provides clear benefits to quantifying the role of marine nanogel/microgel in microphysical processes leading to cloud formation. This review emphasizes the DOC-marine gel/aerosolized gel-cloud link, critical to developing accurate climate models.

Microplastic fibers (MPF) are a ubiquitous marine contaminant, making up to 90% of global microplastic concentrations. Imaging flow cytometry was used to measure uptake and ingestion rates of MPF by blue mussels (Mytilus edulis). Mussels were fed a diet of Rhodomonas salina and MPF concentrations up to 30 MPF mL−1, or 0.374% of available seston. Filtration rates were greatly reduced in mussels exposed to MPF. Uptake of MPF followed a Holling's Type II functional response with 95% of the maximum rate (5227 MPF h−1) occurring at 13 MPF mL−1. An average of 39 MPF (SE ± 15, n = 4) was found in feces (maximum of 70 MPF). Most MPF (71%) were quickly rejected as pseudofeces, with approximately 9% ingested and

The growth of phytoplankton at high latitudes was generally thought to begin in open waters of the marginal ice zone once the highly reflective sea ice retreats in spring, solar elevation increases, and surface waters become stratified by the addition of sea-ice melt water. In fact, virtually all recent large-scale estimates of primary production in the Arctic Ocean (AO) assume that phytoplankton production in the water column under sea ice is negligible. However, over the past two decades, an emerging literature showing significant under-ice phytoplankton production on a pan-Arctic scale has challenged our paradigms of Arctic phytoplankton ecology and phenology. This evidence, which builds on previous, but scarce reports, requires the Arctic scientific community to change its perception of traditional AO phenology and urgently revise it. In particular, it is essential to better comprehend, on small and large scales, the changing and variable icescapes, the under-ice light field and biogeochemical cycles during the transition from sea-ice covered to ice-free Arctic waters. Here, we provide a baseline of our current knowledge of under-ice blooms (UIBs), by defining their ecology and their environmental setting, but also their regional peculiarities (in terms of occurrence, magnitude, and assemblages), which is shaped by a complex AO. To this end, a multidisciplinary approach, i.e., combining expeditions and modern autonomous technologies, satellite, and modeling analyses, has been used to provide an overview of this pan-Arctic phenological feature, which will become increasingly important in future marine Arctic biogeochemical cycles.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. " The Arctic Ocean is overwhelmingly forced by its lateral boundaries, and interacts with, the global system. For the development of nested conceptual models of the Arctic Ocean ecosystem we here choose the full pan-Arctic as our focal scale. Understanding the pan-Arctic scale, however, requires that we look at the underlying scales of its major components, by considering regionality, connectivity and seasonality. Six regions are identified on the basis of hydro-morphological characteristics, which subsequently reflect ecological function and traits. Regions are static, tied to geography, but are linked by contiguous domains of shared function that facilitate material transports and share key ecological features. The pan-Arctic scale also requires attention to forcing by the seasonal light intensity, wherein the maximum length of a single day varies from near 24 h at the Arctic Circle to about 4400 h (183 days) at the North Pole. The light climate forces a strong phenology in the Arctic, as reflected in the periodic life cycle events of organisms. In addition to light climate, Arctic Ocean ecosystems are dominated by three fundamental variables: ice cover, nutrient/food availability and advection. The conditions under which each of these variables play out in the course of a year are set by the regions and contiguous domains within which they operate and interact. Together, the defined regions and their seasonality, the contiguous domains and their connectivity, and the three fundamental variables allow unambiguous application of scale-nested, parsimonious and adaptive, conceptual models, from which to 1) create testable hypotheses, 2) plan and then modify field campaigns, and 3) communicate essential results to managers and the general public. The development of these nested conceptual pan-Arctic scale models creates a vital step into the future of unifying, integrative oceanographic and ecological work.

The Microbiological-Ocean-Cloud Coupling in the High Arctic (MOCCHA) campaign was conducted during August-September 2018 to examine the coupling between the changing High Arctic surface and atmospheric composition, including the production of sea spray aerosols (SSA) from sea ice leads. Sentinel-1 satellite SAR data are used for observations of sea ice leads and open water where SSA may originate due to wind forcing. Results from selected Sentinel-1 SAR scenes show that sea ice leads and open water areas can be identified with co-polarized backscatter at larger incidence angles while water misclassification occurs at smaller incidence due to stronger surface scattering. In contrast, the cross-polarized backscatter remains low at low and high incidence angles. Melt on sea ice during the MOCCHA summertime can confound backscatter signatures that misidentify open sea water. Future analyses can be improved using SARs in synergy with other satellite sensors.

Accurate estimation of the climate sensitivity requires a better understanding of the nexus between polar marine ecosystem responses to warming, changes in sea ice extent, and emissions of marine biogenic aerosol (MBA). Sea ice brine channels contain very high concentrations of MBA precursors that, once ventilated, have the potential to alter cloud microphysical properties, such as cloud droplet number, and the regional radiative energy balance. In contrast to temperate latitudes, where the pelagic phytoplankton are major sources of MBAs, the seasonal sea ice dynamic plays a key role in determining MBA concentration in both the Arctic and Antarctic. We review the current knowledge of MBA sources and the link between ice melt and emissions of aerosol precursors in the polar oceans. We illustrate the processes by examining decadal-scale time series in various satellite-derived parameters such as aerosol optical depth (AOD), sea ice extent, and phytoplankton biomass in the sea ice zones of both hemispheres. The sharpest gradients in aerosol indicators occur during the spring period of ice melt. In sea ice–covered waters, the peak in AOD occurs well before the annual maximum in biomass in both hemispheres. The results provide strong evidence that suggests seasonal changes in sea ice and ocean biology are key drivers of the polar aerosol cycle. The positive trend in annual-mean Antarctic sea ice extent is now almost one-third of the magnitude of the annual-mean decrease in Arctic sea ice, suggesting the potential for different patterns of aerosol emissions in the future.

Greenland extends from 60° to 83° N, with 80% of its land mass covered by the Greenland Ice Sheet (GrIS). This extensive latitudinal gradient is associated with concomitant environmental gradients that impact the biogeochemical properties of its coastal waters. Although the Greenland fjords have been the subject of intense study, less is known of the productivity in the continental shelves, regions that support local fisheries and influence fjord oceanography. This study provides a large-scale overview of annual net primary production rates (NPP) and their spatial variability in 7 regional shelves around Greenland, over the last decade (2008–2017), with special emphasis on spring bloom initiation. NPP is estimated by two independent approaches already established as best for estimating Arctic productivity: a physically-biologically coupled, regional 3D ocean model (SINMOD) and a spectrally-resolved, light-photosynthesis model of primary production (UQUAR-Takuvik model) that is applied to satellite observations of phytoplankton chlorophyll a, which is derived from ocean color remote sensing (OCRS). Both OCRS and SINMOD provide similar estimates of the timing and rates of productivity in Greenlandic waters, when compared with scarce field estimates. Bloom initiation shows a strong south-north gradient, beginning in April in the southern regions and late June in the Arctic Ocean. OCRS-modeled NPP highlights the effect of sea ice presence on bloom initiation; this method depicts the start of the bloom consistently later, by up to 13 days on average, than SINMOD-modeled NPP estimates. In contrast, numerical modeling is able to detect early phytoplankton growth in Greenland shelves, particularly underneath seasonal sea ice. Rates of annual NPP show a strong south-north gradient, with higher NPP rates observed in the North Atlantic water-influenced southern regions, with up to a factor of 3 decrease in NPP towards the north, in the western Eurasian Basin of the Arctic Ocean. Annual NPP varies from 78.3 ± 12.3 g C m−2 yr−1 and 80.3 ± 18.7 g C m−2 yr−1 in the southern regions to 24.7 ± 3.9 g C m−2 yr−1 in the Arctic Ocean. In each region, sea ice distribution and timing of formation and retreat affect location and timing of seasonal productivity with earlier and higher NPP offshore, moving inshore towards the summer. An average 55% to 75% of the annual production is estimated to be exported to depth, higher in Arctic and sub-Arctic regions, suggesting Greenland shelf waters have a potential for high carbon export to depth and relatively less carbon associated with the microbial food web.

The main environmental factors driving spatial patterns, variability and change in primary production (PP) in the Arctic Ocean are reviewed. While instantaneous PP rates are predominantly influenced by the local factors affecting light penetration through clouds, sea ice and water, net PP (NPP) at the annual scale is conditioned by a hierarchy of remote and local processes that affect nutrient supply and light availability in general. Nutrient supply sets spatial differences in realized or potential trophic status (i.e. oligotrophic or eutrophic), whereas light availability modulates PP within each regime. Horizontal nutrient supply through Atlantic and Pacific ocean gateways differ markedly, which is explained by their position at opposite ends of the global meridional overturning circulation and imbalanced nitrogen (N) cycling in the Pacific sector. Nutrient supply by rivers is locally important, but does not appear to sustain a major portion of overall pan-Arctic NPP so far. Horizontal nutrient inputs to the surface Arctic Ocean are eventually transferred to the halocline through winter convection and the decomposition of settling organic matter. The subsequent re-injection of these nutrients to the euphotic zone varies by two orders of magnitude across sectors, depending on the strength and persistence of the vertical stratification. Such differences in nutrient delivery are commensurate with those of PP and NPP rates. Widespread N deficiency in surface waters fosters the occurrence and seasonal persistence of subsurface layers of maximum chlorophyll a (SCM) and phytoplankton carbon biomass in several sectors. The contribution of these layers to NPP is possibly higher in the Arctic than in thermally-stratified waters of the subtropical gyres due to a combination of extreme acclimation to low light and a shallow nitracline in the former. The overall impacts of SCM layers on biogeochemical fluxes remain to be quantified directly, both regionally and at the pan-Arctic scale. While CO2 intake by the Arctic Ocean should respond positively to reduced sea-ice extent, which facilitates air-sea exchange, the negative influence of rising temperatures and runoff on CO2 solubility might counteract the positive effect of modest PP increases in seasonally open waters. Overall, this review shows that local changes in light availability resulting from reduced sea-ice is only one factor in the intricate web of local and remote drivers of PP and CO2 drawdown in the Arctic Ocean. Understanding and predicting change requires an integrated biogeochemical approach that connects the small Arctic Ocean to adjacent ones and adequately resolves vertical nutrient supply processes at regional and local scales.

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.

We investigate the relationship between satellite-derived time series for microalgal biomass, measured using remotely sensed chlorophyll- a (CHL: mg m −3 ), aerosol optical depth (AOD) and sea ice cover (ICE) in the Greenland Sea (10° W–10° E, 65–80° N) over the decadal period 2003–2012. Zonal averages for all variables were computed in 5-degree latitude bands. Unlike other regions of the Arctic Ocean, the marginal ice zone in the Greenland Sea is confined to north of 75° N. The CHL time series is characterized by high interannual variability, especially in the northern marginal sea ice zone (MIZ) where variability in sea ice extent is a likely factor. The ten-year climatology shows that CHL increases from March, reaching a seasonal peak in May in the southern sector and in June in the northern sectors. The climatological peak of AOD is achieved in April in all latitude bands, about a month before the peak in CHL. This suggests that the Arctic aerosol burden is strongly affected by continental sources in early spring. Interestingly, a summer increase in AOD (which succeeds the CHL maximum) is seen in some years. Sea ice extent in early spring is less than 40% in the northern sector in all years. There is considerable interannual variability in both the onset of melt and the extent of sea ice loss during summer, with the minimum summer sea ice extent decreasing to almost zero in five of the years. Cross-correlation analyses of the three times series identify a statistically significant relation between CHL and AOD in six of the years in the southern sector, but no correlation between CHL and ICE was found in the northern sector. There is, however, a significant correlation between AOD and ICE time series in the northern sector in four years. High AOD values registered in early spring are most likely of anthropogenic origin, however, peaks later in summer coincident with the phytoplankton bloom and high emissions of biogenic aerosol precursors such as dimethylsulfide and other primary aerosols of marine origin, suggest the summer atmospheric aerosol burden is likely influenced by biogenic emissions.

[1] Dimethylsulfide (DMS) is a biogenic gas with potential climatic effects, and its marine emission depends on the interplay between microbial activity and physical forcing in the oceanic upper mixed layer. We investigated the diel cycling patterns of DMS and its precursor dimethylsulfoniopropionate (DMSP) in four experiments (28 to 48 h long) performed in mesotrophic to ultraoligotrophic Mediterranean and Sargasso Sea waters. Samples taken every 4 or 6 h were analyzed for dimethylated sulfur pools and incubated to measure DMS and DMSP cycling rates, as well as primary and bacterial production. In all four experiments, DMS budgets showed pronounced day versus night variability. In the three summer experiments, gross community DMS production (GPDMS) increased by twofold to threefold from nighttime to daytime, peaking 0–4 h after solar noon. This excess GPDMS was balanced by higher photochemical and microbial sinks during the day, effectively buffering DMS concentrations. In the only winter experiment, GPDMS exhibited opposed temporal dynamics and peaked at nighttime in parallel to total DMSP consumption. Community DMSP to DMS conversion yields were generally 15% (even >50%) during the day in summer, presumably due to phytoplankton radiative stress. Our data suggest that (1) diel variability should be taken into account in process studies, diagnostic, and prognostic models of DMS cycling and (2) the community DMS yield is a key variable that defines characteristic DMS cycling regimes.

Recent warming of surface waters, accompanied by reduced ice thickness and extent may have significant consequences for climate-driven changes of primary production (PP) in the Arctic Ocean (AO). However, it has been difficult to obtain a robust benchmark estimate of pan-Arctic PP necessary for evaluating change. This paper provides an estimate of pan-Arctic PP prior to significant warming from a synthetic analysis of the ARCSS-PP database of in situ measurements collected from 1954 to 2007 and estimates derived from satellite-based observations from 1998 to 2007. Vertical profiles of in situ chlorophyll a (Chl a ) and PP revealed persistent subsurface peaks in biomass and PP throughout the AO during most of the summer period. This was contradictory with the commonly assumed exponential decrease in PP with depth on which prior satellite-derived estimates were based. As remotely sensed Chl a was not a good predictor of integrated water column Chl a , accurate satellite-based modeling of vertically integrated primary production (IPP sat ), requires knowledge of the subsurface distribution of phytoplankton, coincident with the remotely sensed ocean color measurements. We developed an alternative approach to modeling PP from satellite observations by incorporating climatological information on the depths of the euphotic zone and the mixed layer that control the distribution of phytoplankton that significantly improved the fidelity of satellite derived PP to in situ observations. The annual IPP of the Arctic Ocean combining both in situ and satellite based estimates was calculated here to be a minimum of 466 ± 94 Tg C yr −1 and a maximum of 993 ± 94 Tg C yr −1 , when corrected for subsurface production. Inflow shelf seas account for 75% of annual IPP, while the central basin and Beaufort northern sea were the regions with the lowest annual integrated productivity, due to persistently stratified, oligotrophic and ice-covered conditions. Although the expansion of summertime ice retreat should stimulate photosynthesis by exposing more of the AO to solar irradiance, total PP is ultimately limited by nutrient availability. Therefore, changes in AO PP will be forced by the balance between stratification and mixing, the effects of which are not yet quantified.

The spatial and seasonal magnitude and variability of primary production in the Arctic Ocean (AO) is quantified with a pan-arctic approach. We synthesize estimates of primary production (PP), focusing on surface waters (0–5 m), using complementary methods that emphasize different spatial and temporal scales. These methods include (1) in situ observations of 14 C uptake mostly and possibly some O 2 production reported in units of carbon (in situ PP), (2) remotely sensed primary production (sat-PP), and (3) an empirical algorithm giving net PP as a function of in situ chlorophyll a (in situ Chl-PP). The work presented herein examines historical data for PP collected in surface waters only, as they form the majority of the values of a larger ensemble of PP data collected over >50 years (ARCSS-PP) by many national and international efforts. This extended set of surface and vertically-resolved data will provide pan-Arctic validation of remotely sensed chlorophyll a and PP, an extremely valuable tool in this environment which is so difficult to sample. To this day, PP data in the AO are scarce and have uneven temporal and spatial coverage which, when added to the AO’s regional heterogeneity, its strong seasonal changes, and limited access, have made and continue to make obtaining a comprehensive picture of PP in the AO difficult. Daily surface in situ PP averaged 70 and 21 mg C m −3 d −1 for spring and summer, respectively, for the ca. 50 year period across the AO. Average daily estimates of in situ PP in surface waters on a pan-Arctic basis were several fold higher with respect to remotely sensed PP (sat-PP) and in situ chlorophyll-derived PP (Chl-PP) in the spring period, likely due to differences in data availability and coverage. Summer daily averages for surface in situ PP and sat-PP were similar and twice as high as in situ Chl-PP. Differences among annual estimates of surface in situ PP, in situ Chl-PP and sat-PP across the Arctic Ocean are presented and discussed. The mode of all three differences was 0 mg C m −3 y −1 and the median difference was within ±400 mg C m −3 y −1 . Agreement between annual surface sat-PP and in situ PP was within ±1000 mg C m −3 y −1 for half of the data. We hope that the entire ARCSS-PP data set, especially if combined with any archiving by the latest IPY effort, will be employed to develop an improved, panarctic-specific PP remote sensing algorithm and to test hypotheses regarding the controls of PP in the AO. Furthermore, the ARCSS-PP is openly available to be used by AO modelers to calibrate, assimilate, validate and compare community ecosystem and biogeochemical numerical models and, when combined with field work, will allow for improved understanding, detection and prediction of long-term biological patterns.

Combining nitrate, nitrite and phosphate data from several sources with additional quality control produced a database that eliminates many questionable values. This database, in turn, facilitated estimation of net community production (NCP) in the Arctic Marine System (AMS). In some regions, the new database enabled quantitative calculation of NCP over the vegetative season from changes in nutrient concentrations. In others, useful inferences were possible based on nutrient concentration patterns. This analysis demonstrates that it is possible to estimate NCP from seasonal changes in nutrients in many parts of the Arctic, however, the data were so sparse that most of our estimates for 14 sub-regions of the AMS are attended by uncertainties >50%. Nevertheless, the wide regional variation of NCP within the AMS (∼two orders of magnitude) may make the results useful. NCP for the entire AMS was estimated as 345 ± 72 Tg C over the vegetative season. Converting this value to annual primary production (PP) as determined by typical 14C incubations suggests an annual primary production rate for the AMS of ∼1000 Tg C. We divided the AMS and its marginal seas into the same 13 sub-regions employed in the companion studies of Matrai et al. (2012) and Hill et al. (2013) and estimated NCP for each. We also made separate estimates for the Eurasian and Amerasian portions of the Arctic Basin. Significant findings include: 1. NCP in the Arctic Basin is low, but there are regional variations in the controls and in rates. In the Amerasian Basin (particularly in the Canada sub-basin), nitrate concentrations from 0 to ∼50 m are very low (∼0 μM) even in winter. Thus, nutrient limitation suppresses NCP in this region. In the Eurasian Basin, light or grazing or both may be important limiting factors since significant surface layer nutrient concentrations persist during summer. 2. Low wintertime nitrate concentrations in the upper layers of the Amerasian Basin and Northern Beaufort Sea suggest that NCP in these sub-regions may be insensitive to changes in the ice and light regimes. 3. Although tentative because of limitations in the data, we group NCP in the 14 sub-regions as follows: a. Very high NCP (∼70–100 g C m−2) in the Bering and Southern Chukchi sub-regions. b. High NCP (∼30–40 C m−2) in the Nordic and Barents seas and the Canadian Archipelago. c. Moderate NCP (>10 to ∼15 g C m−2) in the Eurasian Basin, Southern Beaufort, Southern East Siberian Sea + Laptev, Kara Sea and Greenland Shelf sub-regions. d. Low (NCP ∼ 10 g C m−2) in Northern East Siberian Sea + Laptev and Northern Chukchi sub-regions. e. Extremely low NCP (∼1–5 g C m−2) in the Northern Beaufort and Amerasian Basin sub-regions.

Despite the global importance of dimethylsulfoniopropionate (DMSP)/dimethyl sulfide (DMS) and their role in climate regulation, little is known about the mechanisms of their production and storage in Phaeocystis sp., a major contributor of DMS in polar areas. Phaeocystis secretes polymer microgels, by regulated exocytosis, remaining in condensed phase while stored in secretory vesicles (Chin et al. 2004). In secretory cells, vesicles also store small molecules, which are released during exocytosis. Here, we demonstrated that DMSP and DMS were stored in the secretory vesicles of Phaeocystis antarctica G. Karst. They were trapped within a polyanionic gel matrix, which prevented an accurate measurement of their concentration in the absence of a chelating agent such as EDTA. Understanding the production and the export mechanisms of DMSP and DMS into seawater is important because of the impact the cellular and extracellular pools of these highly relevant biogeochemical metabolites have on the environment. The pool of total DMSP in the presence of Phaeocystis may be underestimated by as much as half. Obtaining accurate budget measurements is the first step toward gaining a better understanding of key issues related to the DMS ocean-air interaction and the effect of phytoplankton DMS production on climate change.

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.

Marine microgels play an important role in regulating ocean basin-scale biogeochemical dynamics. In this paper, we demonstrate that, in the high Arctic, marine gels with unique physicochemical characteristics originate in the organic material produced by ice algae and/or phytoplankton in the surface water. The polymers in this dissolved organic pool assembled faster and with higher microgel yields than at other latitudes. The reversible phase transitions shown by these Arctic marine gels, as a function of pH, dimethylsulfide, and dimethylsulfoniopropionate concentrations, stimulate the gels to attain sizes below 1 μm in diameter. These marine gels were identified with an antibody probe specific toward material from the surface waters, sized, and quantified in airborne aerosol, fog, and cloud water, strongly suggesting that they dominate the available cloud condensation nuclei number population in the high Arctic (north of 80°N) during the summer season. Knowledge about emergent properties of marine gels provides important new insights into the processes controlling cloud formation and radiative forcing, and links the biology at the ocean surface with cloud properties and climate over the central Arctic Ocean and, probably, all oceans.

Dimethyl sulphide (DMS) is an important precursor of cloud condensation nuclei (CCN), particularly in the remote marine atmosphere. The SE Pacific is consistently covered with a persistent stratocumulus layer that increases the albedo over this large area. It is not certain whether the source of CCN to these clouds is natural and oceanic or anthropogenic and terrestrial. This unknown currently limits our ability to reliably model either the cloud behaviour or the oceanic heat budget of the region. In order to better constrain the marine source of CCN, it is necessary to have an improved understanding of the sea-air flux of DMS. Of the factors that govern the magnitude of this flux, the greatest unknown is the surface seawater DMS concentration. In the study area, there is a paucity of such data, although previous measurements suggest that the concentration can be substantially variable. In order to overcome such data scarcity, a number of climatologies and algorithms have been devised in the last decade to predict seawater DMS. Here we test some of these in the SE Pacific by comparing predictions with measurements of surface seawater made during the Vamos Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in October and November of 2008. We conclude that none of the existing algorithms reproduce local variability in seawater DMS in this region very well. From these findings, we recommend the best algorithm choice for the SE Pacific and suggest lines of investigation for future work.

Despite the global importance of dimethylsulfoniopropionate (DMSP)/dimethyl sulfide (DMS) and their role in climate regulation, little is known about the mechanisms of their production and storage in Phaeocystis sp., a major contributor of DMS in polar areas. Phaeocystis secretes polymer microgels, by regulated exocytosis, remaining in condensed phase while stored in secretory vesicles (Chin et al. 2004). In secretory cells, vesicles also store small molecules, which are released during exocytosis. Here, we demonstrated that DMSP and DMS were stored in the secretory vesicles of Phaeocystis antarctica G. Karst. They were trapped within a polyanionic gel matrix, which prevented an accurate measurement of their concentration in the absence of a chelating agent such as EDTA. Understanding the production and the export mechanisms of DMSP and DMS into seawater is important because of the impact the cellular and extracellular pools of these highly relevant biogeochemical metabolites have on the environment. The pool of total DMSP in the presence of Phaeocystis may be underestimated by as much as half. Obtaining accurate budget measurements is the first step toward gaining a better understanding of key issues related to the DMS ocean-air interaction and the effect of phytoplankton DMS production on climate change.

Marine primary productivity studies pursued in the Canadian Arctic in 1961–1963, using standard techniques (oxygen and carbon-14) of the time, showed that maximum production developed rapidly in July at 5-m depth under very low light intensities and under intact but melting sea ice. The time of maximum production was correlated with depths of snow in spring. Low production levels were found in August in those years and estimated in 1956. Nitrate exhaustion occurred before maximum production and was followed by rapid sinking of chlorophyll and productivity and reduction of assimilation numbers. Gross production values greatly exceeded net production values in 1961–1963, and gross production in 2 years was about double that of a third year, which may be due to different initial conditions of nutrient concentrations in the euphotic zone. Interannual variability in primary production may be the result of changes in water masses in the surface layer, as driven by atmospheric pressure patterns.

A buoy based instrument platform (the "O-buoy") was designed, constructed, and field tested for year-round measurement of ozone, bromine monoxide, carbon dioxide, and meteorological variables over Arctic sea ice. The O-buoy operated in an autonomous manner with daily, bi-directional data transmissions using Iridium satellite communication. The O-buoy was equipped with three power sources: primary lithium-ion battery packs, rechargeable lead acid packs, and solar panels that recharge the lead acid packs, and can fully power the O-buoy during summer operation. This system was designed to operate under the harsh conditions present in the Arctic, with minimal direct human interaction, to aid in our understanding of the atmospheric chemistry that occurs in this remote region of the world. The current design requires approximately yearly maintenance limited by the lifetime of the primary power supply. The O-buoy system was field tested in Elson Lagoon, Barrow, Alaska from February to May 2009, and deployed in the Beaufort Sea in October 2009. Here, we describe the design and present preliminary data.

14 pages,11 figures The major source of reduced sulfur in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world’s oceans and released through food web interactions. Relevant fluxes and concentrations of DMS, its phytoplankton-produced precursor, dimethylsulfoniopropionate (DMSP) and related parameters were measured during an intensive Lagrangian field study in two mesoscale eddies in the Sargasso Sea during July–August 2004, a period characterized by high mixed-layer DMS and low chlorophyll—the so-called ‘DMS summer paradox’. We used a 1-D vertically variable DMS production model forced with output from a 1-D vertical mixing model to evaluate the extent to which the simulated vertical structure in DMS and DMSP was consistent with changes expected from field-determined rate measurements of individual processes, such as photolysis, microbial DMS and dissolved DMSP turnover, and air–sea gas exchange. Model numerical experiments and related parametric sensitivity analyses suggested that the vertical structure of the DMS profile in the upper 60 m was determined mainly by the interplay of the two depth variable processes—vertical mixing and photolysis—and less by biological consumption of DMS. A key finding from the model calibration was the need to increase the DMS(P) algal exudation rate constant, which includes the effects of cell rupture due to grazing and cell lysis, to significantly higher values than previously used in other regions. This was consistent with the small algal cell size and therefore high surface area-to-volume ratio of the dominant DMSP-producing group—the picoeukaryotes. We gratefully acknowledge the financial assistance provided through NSF Biocomplexity funding (OPP-0083078) and an Australian Research Council Discovery Grant. We are grateful to the comments by D.J. Kieber. We recognize the participation and help of K. Bailey, J. Bisgrove, B. Blomquist, I. Forn, H. Harada, B. Huebert, D. Jones, L. Maroney, A. Neely, S. Riseman, C. Smith, J. Stefels, K. Tinklepaugh, M. Vila-Costa, G. Westby, H. Zemmelink and the R/V Seward Johnson crew. DiTullio et al., 2001; Simo ́ and Dachs, 2002; Simon and Azam, 1989; Zemmelink et al., 2005

Lagrangian time series of dimethylsulfide (DMS) concentrations from a cyclonic and an anticyclonic eddy in the Sargasso Sea were used in conjunction with measured DMS loss rates and a model of vertical mixing to estimate gross DMS production in the upper 60 m during summer 2004. Loss terms included biological consumption, photolysis, and ventilation to the atmosphere. The time- and depth (0–60 m)-averaged gross DMS production was estimated to be 0.73±0.09 nM d −1 in the cyclonic eddy and 0.90±0.15 nM d −1 in the anticyclonic eddy, with respective DMS replacement times of 5±1 and 6±1 d. The higher estimated rate of gross production and lower measured loss rate constants in the anticyclonic eddy were equally responsible for this eddy's 50% higher DMS inventory (0–60 m). When normalized to chlorophyll and total dimethylsulfoniopropionate (DMSP), estimated gross production in the anticyclonic eddy was about twice that in the cyclonic eddy, consistent with the greater fraction of phytoplankton that were DMSP producers in the anticyclonic eddy. Higher rates of gross production were estimated below the mixed layer, contributing to the subsurface DMS maximum found in both eddies. In both eddies, gas exchange, microbial consumption, and photolysis were roughly equal DMS loss terms in the surface mixed layer (0.2–0.4 nM d −1 ). Vertical mixing was a substantial source of DMS to the surface mixed layer in both eddies (0.2–0.3 nM d −1 ) owing to the relatively high DMS concentrations below the mixed layer. Estimated net biological DMS production rates (gross production minus microbial consumption) in the mixed layer were substantially lower (by almost a factor of 3) than those estimated in a previous study of the Sargasso Sea, which may explain the relatively low mixed-layer DMS concentrations found here during July 2004 (∼3 nM) compared to previous summers (∼4–6 nM).

Particulate organic matter, including microorganisms, small water-insoluble particles and microaggregates, can form a substantial part of the summer aerosol over the open leads of the central Arctic Ocean. The increased presence of leads during the summer melt increases the biological activity of the region possibly resulting in accumulation of organic material, especially in the surface microlayer. Preferential microlayer enrichment of particulate and dissolved organic matter and gases might be reflected in derived aerosols. The subsurface seawater and microlayer concentrations were determined for dimethyl sulfide (DMS) and its biogenic precursor dimethylsulfoniopropionate (DMSP), dissolved combined amino acids (DCAA) and individual aminoacids, proteins, chlorophyll a (chl a) and bacterial cells as well as bacterial production. Enrichment factors and surface excess concentrations in the surface microlayer were calculated. Concentrations of particulate and dissolved DMSP, chl a-containing material, and bacterial cells were consistently enriched in arctic lead microlayers at 89°N in August 2001. DMS, protein and DCAA concentrations, however, were not in excess in surface microlayers, although proteins were occasionally enriched. The average mole % DCAA composition was similar in subsurface and microlayer water. Enrichment spikes and excess concentrations followed freezing events and did not co-occur with enhanced bacterial production.

Dimethylsulfoniopropionate (DMSP), produced by many marine phytoplankton, is the main precursor of the climate relevant gas dimethylsulfide (DMS). Currently, it is generally accepted that the relationship between DMSP and phytoplankton biomass (as chlorophyll a), while not representative of the absolute magnitude of the DMSP pool, is a good indicator of ecosystem structure. In this study we test the strength of the relationships between DMSP and various phytoplankton parameters in Arctic shelf waters of the Barents Sea. Our objective is to assess the predictive value that traditional phytoplankton carbon parameters have on DMSP. We discuss C:DMSP-S variability as a function of seasonality, water masses, grazing and nutrient limitation. For this purpose we analyze data from 5 cruises including winter, spring and summer conditions and across the seasonal ice zone at the time of the study. Highest phytoplankton DMSP concentration was usually measured at the ice edge. Marked seasonal variability was observed in phytoplankton carbon biomass and production but not necessarily in the particulate fraction of DMSP (DMSPp), resulting in seasonally varying C:DMSP-S. High winter DMSPp concentrations, when chlorophyll a and primary production were lowest and flagellates dominant, suggest a heterotrophic source. The production of extracellular carbon and the pool of dissolved DMSP (DMSPd) followed similar seasonal trends, with enhanced concentrations in spring, and we suggest that high dissolved primary production induced by nutrient limiting conditions resulted in high DMSPd concentrations. Mesoscale changes in total DMSP (particulate+dissolved) may be modeled from basin-wide total phytoplankton primary production (rather than from phytoplankton biomass) at seasonal and interannual scales. We conclude there is predictive power of DMSP concentrations in the Barents Sea based on seasonality, the position of the ice edge and the distribution of phytoplankton variables.

We estimated primary and bacterial production, mineral nutrients, suspended chlorophyll a (Chl), particulate organic carbon (POC) and nitrogen (PON), abundance of planktonic organisms, mesozooplankton fecal pellet production, and the vertical flux of organic particles of the central Arctic Ocean (Amundsen basin, 89–88° N) during a 3 week quasi-Lagrangian ice drift experiment at the peak of the productive season (August 2001). A visual estimate of ≈15% ice-free surface, plus numerous melt ponds on ice sheets, supported a planktonic particulate primary production of 50–150 mg C m −2 d −1 (mean 93 mg C m −2 d −1 , n = 7), mostly confined to the upper 10 m of the nutrient replete water column. The surface mixed layer was separated from the rest of the water column by a strong halocline at 20 m depth. Phototrophic biomass was low, generally 0.03–0.3 mg Chl m −3 in the upper 20 m and −3 below, dominated by various flagellates, dinoflagellates and diatoms. Bacterial abundance (typically 3.7–5.3 × 10 5 , mean 4.1 × 10 5 cells ml −1 in the upper 20 m and 1.3–3.7 × 10 5 , mean 1.9 × 10 5 cells ml −1 below) and Chl concentrations were closely correlated ( r = 0.75). Mineral nutrients (3 μmol NO 3 l −1 , 0.45 μmol PO 4 l −1 , 4–5 μmol SiO 4 l −1 ) were probably not limiting the primary production in the upper layer. Suspended POC concentration was ∼30–105 (mean 53) mg C m −3 and PON ∼5.4–14.9 (mean 8.2) mg N m −3 with no clear vertical trend. The vertical flux of POC in the upper 30–100 m water column was ∼37–92 (mean 55) mg C m −2 d −1 without clear decrease with depth, and was quite similar at the six investigated stations. The mesozooplankton biomass (≈2 g DW m −2 , mostly in the upper 50 m water column) was dominated by adult females of the large calanoid copepods Calanus hyperboreus and Calanus glacialis (≈1.6 g DW m −2 ). The grazing of these copepods (estimated via fecal pellet production rates) was ≈15 mg C m −2 d −1 , being on the order of 3% and 20% of the expected food-saturated ingestion rates of C. hyperboreus and C. glacialis , respectively. The stage structure of these copepods, dominated by adult females, and their unsatisfied grazing capacity during peak productive period suggest allochthonous origin of these species from productive shelf areas, supported by their long life span and the prevailing surface currents in the Arctic Ocean. We propose that the grazing capacity of the expatriated mesozooplankton population would match the potential seasonal increase of primary production in the future decreased ice perspective, diminishing the likelihood of algal blooms.

We investigated 32 net primary productivity (NPP) models by assessing skills to reproduce integrated NPP in the Arctic Ocean. The models were provided with two sources each of surface chlorophyll‐a concentration (chlorophyll), photosynthetically available radiation (PAR), sea surface temperature (SST), and mixed‐layer depth (MLD). The models were most sensitive to uncertainties in surface chlorophyll, generally performing better with in situ chlorophyll than with satellite‐derived values. They were much less sensitive to uncertainties in PAR, SST, and MLD, possibly due to relatively narrow ranges of input data and/or relatively little difference between input data sources. Regardless of type or complexity, most of the models were not able to fully reproduce the variability of in situ NPP, whereas some of them exhibited almost no bias (i.e., reproduced the mean of in situ NPP). The models performed relatively well in low‐productivity seasons as well as in sea ice‐covered/deep‐water regions. Depth‐resolved models correlated more with in situ NPP than other model types, but had a greater tendency to overestimate mean NPP whereas absorption‐based models exhibited the lowest bias associated with weaker correlation. The models performed better when a subsurface chlorophyll‐a maximum (SCM) was absent. As a group, the models overestimated mean NPP, however this was partly offset by some models underestimating NPP when a SCM was present. Our study suggests that NPP models need to be carefully tuned for the Arctic Ocean because most of the models performing relatively well were those that used Arctic‐relevant parameters., Key Points The models reproduced primary productivity better using in situ chlorophyll‐a than satellite valuesThe models performed well in low‐productivity seasons and in sea ice‐covered/deep‐water regionsNet primary productivity models need to be carefully tuned for the Arctic Ocean

The flux of cells from germinated cysts is critical in the population dynamics of many dinoflagellates. Here, data from a large-scale cyst survey are combined with surveys in other years to yield an Alexandrium fundyense cyst distribution map for the Gulf of Maine that is massive in geographic extent and cyst abundance. The benthic cyst population extends nearly 500 km alongshore. Embedded within it are several distinct accumulation zones or “seedbeds,” each 3000–5000 km 2 in area. Maximal cyst abundances range from 2–20×10 6 cysts m −2 . Cysts are equally or more abundant in deeper sediment layers; nearshore, cysts are fewer by a factor of 10 or more. This cyst distribution reflects sedimentary dynamics and the location of blooms in overlying surface waters. The flux of germinated cells from sediments was estimated using a combination of laboratory measurements of cyst germination and autofluorescence and observations of cyst autofluorescence in the field. These measurements constrained a germination function that, when applied to the cyst distribution map, provided an estimate of the germination inoculum for a physical/biological numerical model. In the laboratory studies, virtually all cysts incubated at different temperatures and light regimes became autofluorescent, but the rate of development was slower at lower temperatures, with no difference between light and dark incubations. Germination rates were highest at elevated temperatures, and were 2-fold greater in the light than in the dark. Laboratory and field fluorescence measurements suggest that>70% of the cysts in the top cm of sediment would germinate over a 60–90 day period in offshore waters. The combination of laboratory germination experiments and numerical modeling predicts nearly 100% germination of cysts in the top cm of sediment and resulting early season cell concentrations that are comparable in magnitude to observed cell distributions. It cannot account for late-season peaks in cell abundance that are heavily influenced by vegetative growth. Cyst germination flux from deep-water (>50 m) cyst seedbeds is 14X the flux in shallow waters. A conceptual model is proposed that is consistent with observed and modeled A. fundyense cyst and motile cell distributions and dynamics in the Gulf of Maine. Cysts germinate within the Bay of Fundy seedbed, causing localized, recurrent blooms that are self-seeding and “propagatory” in nature, supplying cells that populate the eastern segment of the Maine Coastal Current (MCC) and eventually deposit cysts offshore of Penobscot and Casco Bays. These cysts serve as a seed population for western Maine blooms that are transported to the west by the western segment of the MCC, where cells are removed either by mortality or advection from the region. Without the localized, “incubator” characteristic of the Bay of Fundy bloom zone, A. fundyense populations in the Gulf of Maine should diminish through time. Their persistence over many decades highlights the effectiveness of the mechanisms described here.

Species of the marine dinoflagellate Alexandrium, present in most of the Gulf of Maine (GOM), Bay of Fundy and Gulf of St. Lawrence as well as in many other areas of the world, are known to cause toxicity to marine organisms and humans alike. Excystment of Alexandrium fundyense from the eastern region of the GOM (Penobscot Bay to Bay of Fundy) was followed through four germination cycles (4 years). An annual, free-running oscillation in germination was observed under constant environmental conditions, indicating control by an endogenous clock for these eastern cysts, as shown earlier for cysts from the western region of the GOM. This circannual endogenous clock had an average period of 11 months. The phase of germination remained constant for cysts from all three stations sampled. Cysts did not germinate, despite favorable growth conditions, in summer-to-fall and this timing was consistent among cysts from all stations. The timing of cyst germination is highly relevant to modeling of Alexandrium sp. bloom initiation and depletion, as there are cyst “seed beds” near shore and offshore in the eastern and western regions of the GOM.

Dimethylsulfoniopropionate (DMSP) and glycine betaine (GBT) are organic osmolytes abundant in many marine phytoplankton. Herein, we field tested for the first time the hypothesis that GBT production might be favored over DMSP in natural phytoplankton populations growing in high-N environments or when N is added to a system. Concentrations of particulate DMSP (DMSPp; 15–45 nmol·L–1) were equal to, or greater than, concentrations of particulate GBT (GBTp; 0–15 nmol·L–1) in the upper water column. Homarine, another N-containing osmolyte, was detected at lower levels in all samples. These are the first reported values of GBTp, and homarine in seawater. During N enrichment experiments, no consistent pattern of response in the DMSP pool resulted. Under N stress, nitrate addition either caused DMSP to be released but without an equivalent increase in GBT or DMSP dynamics were not affected but GBT increased. In populations under less N stress, GBT levels were similar to those of DMSPpthroughout the experiments. Homarine levels remained low at all times. We conclude that no simple switch between DMSP and GBT occurs as a function of N availability in natural populations. Variable responses to N supply probably resulted from differences in species composition and physiological state of the populations present.

The climate in the Arctic is changing faster than anywhere else on earth. Poorly understood feedback processes relating to Arctic clouds and aerosol–cloud interactions contribute to a poor understanding of the present changes in the Arctic climate system, and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Improved formulations in climate models require such observations, which can only come from measurements in situ in this difficult-to-reach region with logistically demanding environmental conditions. The Arctic Summer Cloud Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year (IPY) 2007–2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed from Longyearbyen on Svalbard on 2 August and returned on 9 September 2008. In transit into and out of the pack ice, four short research stations were undertaken in the Fram Strait: two in open water and two in the marginal ice zone. After traversing the pack ice northward, an ice camp was set up on 12 August at 87°21' N, 01°29' W and remained in operation through 1 September, drifting with the ice. During this time, extensive measurements were taken of atmospheric gas and particle chemistry and physics, mesoscale and boundary-layer meteorology, marine biology and chemistry, and upper ocean physics. ASCOS provides a unique interdisciplinary data set for development and testing of new hypotheses on cloud processes, their interactions with the sea ice and ocean and associated physical, chemical, and biological processes and interactions. For example, the first-ever quantitative observation of bubbles in Arctic leads, combined with the unique discovery of marine organic material, polymer gels with an origin in the ocean, inside cloud droplets suggests the possibility of primary marine organically derived cloud condensation nuclei in Arctic stratocumulus clouds. Direct observations of surface fluxes of aerosols could, however, not explain observed variability in aerosol concentrations, and the balance between local and remote aerosols sources remains open. Lack of cloud condensation nuclei (CCN) was at times a controlling factor in low-level cloud formation, and hence for the impact of clouds on the surface energy budget. ASCOS provided detailed measurements of the surface energy balance from late summer melt into the initial autumn freeze-up, and documented the effects of clouds and storms on the surface energy balance during this transition. In addition to such process-level studies, the unique, independent ASCOS data set can and is being used for validation of satellite retrievals, operational models, and reanalysis data sets., Atmospheric Chemistry and Physics, 14 (6), ISSN:1680-7375, ISSN:1680-7367

An important focus of climate-change research is the understanding of the role of ecosystems in shaping climate. Central to this aim is the identification of any feedbacks by which ecosystems may moderate anthropogenic forcing of climate. One possible ecosystem feedback involves the marine food-web and the biogenic sulfur compound dimethylsulfide (DMS). DMS is produced by algae containing the precursor compound dimethylsulfoniopropionate (DMSP), and once ventilated to the atmosphere can be transformed to sulfate aerosols and global climate. It was hypothesized that an increase in biogenically produced sulfate aerosols leading to formation of more cloud condensation nuclei (CCN), and brighter clouds, could stabilize the climate against perturbations due to greenhouse warming. Although a large database of DMS seawater measurements exist, attempts to statistically correlate DMS concentrations with other biological parameters, such as chlorophyll a or nutrients, have failed. This underscores the complex and dynamic nature of the DMS cycle, and means that simple regression-type predictive models are unlikely to be useful, except at local scales. Regional-scale simulations of the DMS cycle have involved multi-parameter, deterministic formulations based on ecological food-web approaches but with the added challenge of properly simulating the behavior of coupled sulfur and nitrogen (or carbon) cycles. Here we review the current DMS modeling approaches, outline the parameterization of key processes, and identify areas where our knowledge is poor and improvements should be made. Model skill can only be assessed against detailed regional and global data sets, however data have not always been collected in a form suitable for model parameter estimation or model calibration/validation. DMS time series, which are essential for calibration of seasonal or multi-annual simulations, are rare. We discuss the minimum requirements for a successful future integration of observational and theoretical efforts.

The nitrogenous organic osmolyte glycine betaine (GBT) and its sulfur analog dimethylsulfoniopropionate (DMSP) are quantitatively significant solutes in many marine algae. Although an inverse relationship has been suggested between these two compounds in marine phytoplankton that may be regulated by nitrogen availability, our results in Part I of this work (same issue) showed no such relationship in batch cultures of six species. In experiments conducted in August 1994, we reexamined this relationship in three axenic strains of phytoplankton [Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, Emiliania huxleyi Hay et Mohler, and Amphidinium carterae Hulbert] in chemostat cultures at three N-limited growth rates. Levels of DMSP, GBT, and homarine, another nitrogenous osmolyte, were inversely related in T. pseudonana, with GBT and homarine preferentially produced at the highest growth rate when cells were N-replete. DMSP concentrations did not change in E. huxleyi, although GBT and homarine levels did increase at the highest growth rate. In A. carterae, neither DMSP nor GBT varied in any systematic way. In all three algae, additions of nitrogen to N-limited cultures resulted in short-term increases in GBT production. Levels of cellular DMSP remained fairly constant in E. huxleyi and A. carterae and were much greater than levels of comparable nitrogenous osmolytes like GBT and homarine. The dominance of DMSP makes a reciprocal relationship with the nitrogenous osmolytes unlikely in most species. Phytoplankton appear to be capable of directly assimilating extracellular GBT, although it is not known if they eliminate equivalent amounts of DMSP in the process.

The quantitative significance of the nitrogenous compound glycine betaine (GBT) and its sulfur analog dimethylsulfoniopropionate (DMSP) to intracellular pools in marine phytoplankton is not well known. In a series of experiments conducted in August 1993, we measured these compounds, as well as total organic sulfur, carbon, and nitrogen, over the growth cycle in six isolates of marine phytoplankton, Amphidinium carterae Hulburt, Chrysochromulina sp. Lackey, Emiliania huxleyi Hay et Mohler, Prorocentrum minimum (Pavillard) Schiller, Skeletonema costatum (Greville) Cleve, and Tetraselmis sp. At the same time, we measured cellular concentrations of protein, amino acids, chlorophyll, and inorganic nutrients. All six species produced DMSP, while three produced GBT at lesser levels. In the Chrysochromulina sp. isolate, levels of GBT were greater than DMSP during the exponential phase of growth, but declined sharply as the culture approached stationary phase. This change appeared to coincide with the onset of nitrogen limitation. Other nitrogenous osmolytes were produced in five of the six species but in much smaller quantities. DMSP contributed significantly to cellular sulfur throughout the growth cycle although, in some algae, the proportion of dissolved DMSP increased substantially during stationary growth. When present, GBT formed a sizeable fraction of the cellular nitrogen only during exponential growth. A significant percentage (ca. 50%) of the organic nitrogen could not be accounted for even when cellular pools of protein, amino acids, inorganic nitrogen, and nitrogenous osmolytes were combined. Based on these experiments, there does not appear to be a reciprocal relationship between DMSP and GBT production, although GBT production does appear to be correlated with nitrogen availability.

In the marine environment, phytoplankton are the fundamental producers of dimethylsulfoniopropionate (DMSP), the precursor of the climatically active gas dimethylsulfide (DMS). DMSP is released by exudation, cell autolysis, and zooplankton grazing during phytoplankton blooms. In this study, we developed a model of phytoplankton DMSP and DMS production allowing quantification of the exudation rates of these compounds during different growth phases. The model was tested on published data from axenic cultures of Prorocentrum minimum and Phaeocystis sp.; DMSP exudation rates vary considerably between the 2 species. Model results show that P. minimum exudes around 1% d(-1) of its DMSP quota during the latent, exponential and senescent phases. This is comparable to the average exudation rate estimated from previous laboratory experiments. However, Phaeocystis sp. exudes from 3 to 11% d(-1) of its DMSP quota. For this species, DMSP exudation rates apparently show an inverse relationship with the population growth rate. The maximum DMSP exudation rate in Phaeocystis sp. is 10 times higher than previously reported DMSP or DMS exudation rates. Our results suggest that exudation may be as important as cell autolysis in the release of DMSP during Phaeocystis sp. blooms. We conclude that exudation should be incorporated in models of DMS cycling in the marine environment. Moreover, our results for Phaeocystis sp. suggest that a low and constant exudation rate, as sometimes used in models, is not suitable for all conditions.

Large phytoplankton biomass accumulates during ice-edge blooms in Arctic waters, where taxa such as diatoms or the prymnesiophyte Phaeocystis pouchetii usually dominate. Based on characteristics from temperate phytoplankton, we hypothesized that in Barents Sea waters, a larger fraction of primary production would be extracellular (for synthesis of colonial mucilage) during periods of dominance by P. pouchetii as opposed to periods when diatoms dominated. This alternation of P. pouchetii and diatoms would affect the relationship between the particulate and dissolved carbon pools in the upper water column of the marginal ice zone (MIZ). Results presented in this paper do not support this hypothesis. Although P. pouchetii contributed strongly to the extracellular carbon pool (mucilage and dissolved organic carbon, DOC) during an ice-edge bloom in May 1993, arctic diatoms contributed an equal amount of exuded carbon. Three process stations visited along a north-south transect in the MIZ in the Barents Sea, presented between 36% and 55% of the primary production as extracellular carbon, defined as labeled organic matter which passes through a Whatman GF/C filter. No difference in the carbon allocation between diatom- and P. pouchetii-rich phytoplankton was observed in these stations. In contrast, the station located in ice-free waters had 18% of primary production in the extracellular fraction. These results (1) highlight similar carbon allocation for diatom- and P. pouchetii-dominated phytoplankton in surface waters of the Barents Sea during the spring and/or ice-edge bloom at the MIZ and (2) suggest that polar phytoplankton may be stronger producers of extracellular carbon, and possibly DOC, than previously thought.

Methyl bromide production was followed in cultures of a selection of marine phytoplankton species throughout their growth cycle. The cultures were species common to coastal and(or) open ocean areas, as well as originating from various latitudinal bands. Methyl bromide was produced by 13 out of 19 species tested, with production rates ranging from 1.7 to 30.1 pg CH3Br µg−1 Chl a d−1. Species that are strictly coastal or more abundant in coastal areas appeared to be stronger producers of CH,Br than species that are more abundant in open ocean areas. The prymnesiophyte Phaeocystis sp., a bloom-forming, ubiquitous species, showed the highest daily production rate normalized to Chl a of 30.1 pg CH3Br µg−1 Chl a d−1. The range of methyl bromide production by the phytoplankton cultures tested suggests that the input of methyl bromide by marine phytoplankton can be significant in coastal oceans.

Phytoplankton is known to be a key element in the production and eventual oceanic efflux of dimethyl sulfide (DMS) to the atmosphere. We hypothesized that the alternation of Phaeocystis pouchetii and diatoms, the two major algal components of the spring bloom, would modulate the input of particulate organic sulfur (POS), dimethylsulfoniopropionate (DMSP), and DMS into the mixed layer of the marginal ice zone. A bloom of diatoms is expected to present similar pathways but to have very different rates of POS/DMSP/DMS production and POS/DMSP sinking and no or low DMS flux to the atmosphere as contrasted to the cycling occurring during the P. pouchetii phase of the bloom. Our initial hypothesis cannot be accepted based on our observations in the Barents Sea during the spring of 1993. The contribution of diatoms to the water column budgets of DMSP and DMS was significant and cannot be overlooked. We suggest that the physiological stage of the bloom is perhaps more important to biogeochemical cycling than its phytoplankton species composition in controlling DMSP and DMS fluxes in Arctic waters. Loss of paniculate DMSP in the mixed layer was mainly by release into the dissolved pool and by sedimentation father than by grazing, except in ice-free waters. Cycling of DMS in the mixed layer was predominantly biological in ice-free waters, while in Polar Front waters, ventilation was proportionally more important due to depressed microbiology.

Aqueous and atmospheric dimethyl sulfide (DMS) concentrations (0.4–4 nM, 5–350 ppt) were determined across a Gulf Stream meander in April 1989. Physical processes along and across the front were expected to result in biological changes also reflected in DMS levels. Highest concentrations of aqueous DMS (3–4 nM) were observed in surface waters coinciding with Slope waters at, or inshore of, the Gulf Stream north wall. This water also contained the highest chlorophyll levels (up to 2.6 μg chl/l) which co-varied with the sea surface DMS concentrations. Concentrations of atmospheric DMS were lower and more constant (21 ± 9 ppt) outside the frontal area, either in Slope or Stream waters. At the northern wall of the Gulf Stream, a strikingly sharp increase in atmospheric DMS (up to 350 ppt) was observed.