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Seasonal phytoplankton blooms in the Antarctic Amundsen Sea Polynyas are thought to be supported by an external supply of iron (Fe) from circumpolar deep waters, benthic sediments, and/or ice shelf meltwaters. However, largely due to the limited amount of Fe data reported for the Amundsen Sea Polynyas, understanding of the sources and processes that affect the biogeochemistry of Fe in this region (notably within the ice shelf system) remains limited. Here, we present the first investigation of dissolved Fe isotope distributions (δ56Fe) along the conveyer belt of waters into and through the Amundsen Sea, via the Dotson Ice Shelf, from samples collected during austral summer (2017–2018). Our dataset allows us to characterize and compare the dissolved δ56Fe signatures of incoming modified Circumpolar Deep Water (mCDW) and of sedimentary sources on the continental shelf. The range in dissolved δ56Fe (–1 to +0.1 ‰) observed in the Amundsen Sea close to the seafloor, coupled with elevated dissolved Fe concentrations (up to 1.6 nmol/L), suggests that Fe is released from shelf sediments via a combination of reductive and non-reductive processes, with non-reductive dissolution input being relatively more important (20–56 %) than reductive dissolution (4–12 %). Near the Dotson Ice Shelf, the δ56Fe in the mCDW inflow (–0.70 ‰) was lower than the mCDW outflow (–0.23 ‰), whereas any change in dissolved Fe concentrations was negligible. We speculate that this shift in dissolved δ56Fe underneath the ice shelf is driven by a combination of enhanced preservation (and addition) of lithogenic colloidal Fe(III) and/or complexation with Fe-binding ligands, together with a differential loss of Fe2+. We also found distinct δ56Fe signatures in surface waters of the polynya, with apparent preferential uptake of isotopically light Fe in a bloom dominated by diatoms leading to a relatively heavy remnant dissolved δ56Fe signature of +1.06 ‰, compared to a bloom dominated by haptophytes where more modest and variable isotope fractionation was observed. The different isotopic composition between the two regions could be related to the dominance of different species, but this remains speculative. Despite prominent biological uptake, we suggest that other factors such as rapid recycling (e.g., adsorption and regeneration), bacterial regeneration, and complexation with organic ligands, together with the supply of lithogenic particles also play important roles in setting surface dissolved δ56Fe in the Amundsen Sea Polynyas. Overall, this study provides a further understanding of the external Fe sources and the biogeochemical processes in the Amundsen Sea and thus a baseline on how changing conditions in Antarctica can affect Fe cycling in the Southern Ocean and beyond. [ABSTRACT FROM AUTHOR]

Sedimentation rates in the northern Kara Sea between September 2019 and July 2020 were calculated from integrated monthly values for sedimentary matter, organic carbon, nitrogen, total silica, aluminum, fecal pellets, and phytoplankton using a multicup sediment trap moored at a depth of 110 m. The seasonal dynamics of the total flux and its components was characterized by three pronounced maxima, observed not only in the ice-free season in September, but also under ice in February and April. The flux of total matter and particulate organic carbon (POC) during these periods was 400–600 mg m–2 day–1 and 8–14 mgC m–2 d–1, respectively. The increase in the flux in winter and early spring was most likely due to the formation of polynyas. The contribution of the biogenic POC flux was 30% of the total POC flux in the winter months, 50% in the autumn, and up to 80% in May–June. The maximum flux of fecal pellets and larvacean "houses" (0.2 mgC m–2 d–1) was observed in autumn and did not exceed 0.02 mgC m–2 d–1 the rest of the year. The phytoplankton flux was about 0.05 mgC m–2 d–1 in autumn, and it decreased to <0.01 mgC m–2 d–1 in winter. An increase in the phytoplankton flux in May–June to its maximum values (0.2 mgC m–2 d–1) indicated the development of plankton algae long before ice melting. The calculated annual flux for total matter and POC (75 g m–2 yr–1 and 2 gC m–2 yr–1) was significantly lower than the published values for the Kara Sea shelf influenced by river runoff. [ABSTRACT FROM AUTHOR]

Polynyas are subject to variability in winds and ocean circulation and are important sites of ecological productivity. In February 2010, the B09B iceberg collided with the Mertz Glacier Tongue (MGT), calving a 78 × 40‐km giant iceberg which modified the icescape and primary productivity of the Mertz polynya. In this study, we use satellite ocean color and sea ice concentration to investigate the variability, trends, and drivers of phytoplankton blooms in the Mertz polynya since 1997. During the bloom, over 21 years, we found (i) a later ice retreat time, (ii) an increase in sea ice concentration, (iii) a decrease in open‐water period, (iv) a later bloom start, and (v) a decrease in bloom duration. Our results suggest that major postcalving changes in the physical characteristics of the polynya, mainly its icescape, are the primary drivers of phytoplankton phenology. More specifically, the MGT calving event resulted in significant seasonal and regional changes, with higher eastern chl‐a and mean summer chl‐a postcalving. While satellite data are useful to study long‐term variability in these inhospitable areas, they only focus on the ocean surface and are obscured by ice and clouds. Additional subsurface parameters from seal tags, gliders and moorings in the southernmost polar regions would strengthen our comprehension of phytoplankton and physical changes in ocean dynamics that may have far‐reaching consequences, from global circulation to carbon export. Plain Language Summary: Polynyas are the most productive areas in the Southern Ocean and play a role in the global oceanic circulation. Evaluating how unexpected and dramatic changes impact biology and chemical cycles in these areas is critical to understand the potential future impact such events may cause on a larger scale. In this paper, we investigated the impact of the calving of a major glacier tongue in the Mertz polynya on the phytoplankton bloom using satellite data. Significant changes happened after the calving event: (i) the bloom duration and open water period time decreased, (ii) the start of the bloom and the retreat of sea ice were delayed, and (iii) the chlorophyll‐a (the major phytoplankton pigment which indicates the phytoplankton biomass) and the sea ice concentration increased. These findings show that natural changes can impact the timing of phytoplankton growth that may have consequences for the rest of the ecosystem, from Antarctic krill to baleen whales. Key Points: The calving of the Mertz Glacier tongue led to changes in the phytoplankton bloom and sea ice phenology in the polynyaOur study highlights regional differences in the polynyaNatural changes in icescape like in the Mertz polynya will soon be accelerated, with consequences for ocean biogeochemistry and ecology [ABSTRACT FROM AUTHOR]

Previous studies showed that satellite‐derived estimates of chlorophyll a in coastal polynyas over the Antarctic continental shelf are correlated with the basal melt rate of adjacent ice shelves. A 5‐km resolution ocean/sea ice/ice shelf model of the Southern Ocean is used to examine mechanisms that supply the limiting micronutrient iron to Antarctic continental shelf surface waters. Four sources of dissolved iron are simulated with independent tracers, assumptions about the source iron concentration for each tracer, and an idealized summer biological uptake. Iron from ice shelf melt provides about 6% of the total dissolved iron in surface waters. The contribution from deep sources of iron on the shelf (sediments and Circumpolar Deep Water) is much larger at 71%. The relative contribution of dissolved iron supply from basal melt driven overturning circulation within ice shelf cavities is heterogeneous around Antarctica, but at some locations, such as the Amundsen Sea, it is the primary mechanism for transporting deep dissolved iron to the surface. Correlations between satellite chlorophyll a in coastal polynyas around Antarctica and simulated dissolved iron confirm the previous suggestion that productivity of the polynyas is linked to the basal melt of adjacent ice shelves. This correlation is the result of upward advection or mixing of iron‐rich deep waters due to circulation changes driven by ice shelf melt, rather than a direct influence of iron released from melting ice shelves. This dependence highlights the potential vulnerability of coastal Antarctic ecosystems to changes in ice shelf basal melt rates. Plain Language Summary Phytoplankton in Antarctic coastal waters grow more rapidly relative to waters farther offshore. This growth is limited by the availability of light for photosynthesis and the supply of the micronutrient dissolved iron. Earlier studies suggest that satellite‐based estimates of phytoplankton growth are related to the melting of nearby floating portions (called ice shelves) of the Antarctic ice sheet. In this study, a computer model, which includes melting of ice shelves, is used to examine the different sources of dissolved iron that supply the well‐lit summer surface waters around Antarctica. Dissolved iron is available in the floating ice shelves, and the direct supply of this iron to coastal waters by melting of the bottom of the ice shelf is important for enhancing biological production. However, melting creates less dense water at the ice shelf base that rises and brings deep waters that contain dissolved iron towards the surface in front of the ice shelf. The model shows that this input provides a larger source of dissolved iron to the open surface waters in many coastal regions than does direct supply from the ice shelf meltwater. This implies that phytoplankton growth may be vulnerable to changes in ice shelf basal melt. [ABSTRACT FROM AUTHOR]

The appearance of phytoplankton blooms within sea ice cover is of high importance considering the upper ocean primary production that controls the biological pump, with implications for atmospheric CO2 and global climate. Satellite-derived chlorophyll a concentration showed unprecedented phytoplankton blooms in the Maud Rise polynya, Southern Ocean, with chlorophyll a reaching up to 4.67 mg m -3 during 2017. Multi-satellite data indicated that the bloom appeared for the first time since the entire mission records started in 1978. An Argo float located in the polynya edge provided evidence of bloom conditions in austral spring 2017 (chlorophyll a up to 5.47 mg m -3) compared to the preceding years with prevailing low chlorophyll a. The occurrence of bloom was associated with the supply of nutrients into the upper ocean through Ekman upwelling (driven by wind stress curl and cyclonic ocean eddies) and improved light conditions of up to 61.9 einstein m -2 d -1. The net primary production from the Aqua Moderate Resolution Imaging Spectroradiometer chlorophyll-based algorithm showed that the Maud Rise polynya was as productive as the Antarctic coastal polynyas, with carbon fixation rates reaching up to 415.08 mg C m -2 d -1. The study demonstrates how the phytoplankton in the Southern Ocean (specifically over the shallow bathymetric region) would likely respond in the future under warming climate conditions and continued melting of Antarctic sea ice. [ABSTRACT FROM AUTHOR]

The importance of small phytoplankton in warming and stratified water conditions has been emphasized, but little information on the small phytoplankton contribution is currently available in the Southern Ocean. In this study, we analyzed temporal variations in the carbon uptake rates of large and small phytoplankton and the small phytoplankton contributions to the total primary production. The data were collected in the Amundsen Sea polynya region using three different field observations (early January, January–February, and February–March) from 2012 to 2016. The total primary production in the polynya region decreased from 0.84 ± 0.18 g C·m−2·day−1 in early January to 0.19 ± 0.04 g C·m−2·day−1 in February–March, which is typical of the seasonal variation in the primary production of phytoplankton in the Amundsen Sea. In contrast, the small phytoplankton contributions to the total carbon uptake rates increased considerably from early January (14.9 ± 8.4%) to February–March (50.8 ± 18.4%) in the polynya region. This substantial increase of small phytoplankton contribution was not caused by increasing biomass and the specific uptake rates of small phytoplankton, but rather by seasonally declining biomass and the specific uptake rates of large phytoplankton during the study period. The overall contribution (25.5 ± 15.7%) of small phytoplankton to the primary production in the polynya region based on the three different time observations is significantly higher than the previous result (14.9 ± 8.4%) based on one time observation. Therefore, a seasonal variation in the small phytoplankton contribution in the Amundsen Sea should be considered in further studies. Key Points: Small phytoplankton contributions to the total carbon uptake rates increased considerably in late austral summer seasonSeasonally increasing small phytoplankton contribution was by declining biomass and specific uptake rate of large phytoplanktonSmall phytoplankton contribution to the primary production in this study is significantly higher than the previous result [ABSTRACT FROM AUTHOR]

Data from a late spring survey of the northeast Chukchi Sea are used to investigate various aspects of newly ventilated winter water (NVWW). More than 96% of the water sampled on the shelf was NVWW, the saltiest (densest) of which tended to be in the main flow pathways on the shelf. Nearly all of the hydrographic profiles on the shelf displayed a two‐layer structure, with a surface mixed layer and bottom boundary layer separated by a weak density interface (on the order of 0.02 kg/m3). Using a polynya model to drive a one‐dimensional mixing model, it was demonstrated that, on average, the profiles would become completely homogenized within 14–25 hr when subjected to the March and April heat fluxes. A subset of the profiles would become homogenized when subjected to the May heat fluxes. Since the study domain contained numerous leads within the pack ice—many of them refreezing—and since some of the measured profiles were vertically uniform in density, this suggests that NVWW is formed throughout the Chukchi shelf via convection within small openings in the ice. This is consistent with the result that the salinity signals of the NVWW along the central shelf pathway cannot be explained solely by advection from Bering Strait or via modification within large polynyas. The local convection would be expected to stir nutrients into the water column from the sediments, which explains the high nitrate concentrations observed throughout the shelf. This provides a favorable initial condition for phytoplankton growth on the Chukchi shelf. Plain Language Summary: Shipboard data from a rare late spring cruise to the Chukchi Sea, in May–June 2014, are analyzed in order to understand the physical conditions of the waters on the shelf. More than 95% of the water on the shelf was very cold, high‐nutrient winter water. The vertical structure of the water column (in temperature, salinity, and density) consisted of two layers, a well‐mixed surface layer and a bottom boundary layer, separated by a weak density interface. When ice is formed at the surface, brine is rejected into the water column, densifying the surface water. This leads to convective overturning as dense water sinks and mixes with the subsurface water. If enough brine is rejected, the density interface between the mixed layers can be eroded and the water column mixes to the bottom, resulting in a uniform density profile. Nutrients are then stirred into the water column from the sediments, which preconditions the water for phytoplankton growth. Using realistic atmospheric conditions, together with a polynya model and ocean mixing model, it was demonstrated that this process was likely happening within small openings in the sea ice throughout the domain. This represents a new paradigm for forming winter water on the Chukchi shelf. Key Points: Newly ventilated Pacific‐origin winter water, with high nutrient content, is prevalent across the Chukchi Sea shelf in late spring.A polynya model, used with a 1D mixing model, demonstrates that brine rejection from refreezing leads quickly overturns the water column.Salinity signals and ice concentration data support the notion that winter water is modified within small leads throughout the shelf. [ABSTRACT FROM AUTHOR]

The Amundsen Sea Polynya (ASP) is distinguished by having the highest net primary production per unit area in the coastal Antarctic. Recent studies have related this high productivity to the presence of fast‐melting ice shelves, but the mechanisms involved are not well understood. In this study we describe the first numerical model of the ASP to represent explicitly the ocean‐ice interactions, nitrogen and iron cycles, and the coastal circulation at high resolution. The study focuses on the seasonal cycle of iron and carbon, and the results are broadly consistent with field observations collected during the summer of 2010–2011. The simulated biogeochemical cycle is strongly controlled by light availability(dictated by sea ice, phytoplankton self‐shading, and variable sunlight). The micronutrient iron exhibits strong seasonality, where scavenging by biogenic particles and remineralization play large compensating roles. Lateral fluxes of iron are also important to the iron budget, and our results confirm the key role played by inputs of dissolved iron from the buoyancy‐driven circulation of melting ice shelf cavities (the "meltwater pump"). The model suggests that westward flowing coastal circulation plays two important roles: it provides additional iron to the ASP and it collects particulate organic matter generated by the bloom and transports it to the west of the ASP. As a result, maps of vertical particulate organic matter fluxes show highest fluxes in shelf regions located west of the productive central ASP. Overall, these model results improve our mechanistic understanding of the ASP bloom, while suggesting testable hypotheses for future field efforts. Plain Language Summary: Winds in the coastal Antarctic regions tend to create openings in the sea ice cover called "coastal polynyas." Such polynyas are found in multiple locations around the Antarctic continent including in the Amundsen Sea. This particular embayment features the fastest‐melting ice shelves of the continent, but it is also distinguished by a large production of algae during the austral summer season. Recent studies have reported a statistical link between these two features but the mechanisms involved are not well understood. We use advanced computer simulations to gain insight into these mechanisms and use measurements collected in the Amundsen Sea to evaluate how realistic the simulations are. The results emphasize the two roles of the coastal circulation: it provides a necessary micronutrient (iron) to the algae and it also transports the algae away from where it grows during the summer. The computer simulations improve our understanding of why the Amundsen Sea Polynya is so productive, and they suggest new hypotheses that can be tested in future field efforts. Key Points: This is the first study that uses a 3‐D ice shelf‐sea ice‐ocean coupled model to represent the phytoplankton bloom of the Amundsen Sea PolynyaOceanic circulation plays two roles: provides dissolved iron to the polynya (meltwater pump) and also transports organic matter westwardModel results improve our mechanistic understanding of the ASP bloom while suggesting testable hypotheses for future field efforts [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) is the largest organic carbon pool in the ocean, and is the most active component in respect to the ocean carbon cycling. However, its study in Antarctica has been limited due to challenges associated with sample collection. In this study, we conduct an investigation on the chromophoric dissolved organic matter (CDOM) in a highly productive Amundsen Sea Polynya (ASP), where phytoplankton blooms occur annually during the austral summer, serving as the primary source of DOC and CDOM. The relative abundances of CDOM, as indicated by the absorption coefficient at 254 nm (a 254), exhibit significant variability, reaching up to 6.34 m−1. Four fluorescent components, including two humic-like components (C1 and C4) and two protein-like components (C2 and C3), are identified by excitation emission matrix coupled with parallel factor analysis (EEM-PARAFAC). Our findings suggest that heterotrophic metabolism primarily contributes to the formation of humic-like fluorescent components and DOC removal. Water mass, solar radiation, primary productivity as well as microbial degradation are identified as the main factors influencing CDOM dynamics in ASP. This study bears significant implications for advancing our understandings of the CDOM and DOC dynamics in the coastal polynyas of Antarctica, thus facilitating improved evaluation of carbon cycle in the Antarctica. • The relative abundances of CDOM as indicated by the absorption coefficient at 254 nm (a 254) are highly variable. • Two humic-like components (C1 and C4) and two protein-like components (C2 and C3) are identified by EEM-PARAFAC. • Heterotrophic metabolism primarily contributes to the formation of humic-like fluorescent components and the DOC removal. • The CDOM dynamics in the ASP are influenced by water mass, solar radiation, primary productivity, and microbial degradation. [ABSTRACT FROM AUTHOR]

Antarctic polynyas are among the most productive areas in the ocean. They are subjected to strong temporal variations, having profound effects on the local ecosystems. We tested the changes occurring in several biogeochemical features [Chlorophyll a (Chl a), phaeopigment, dissolved and particulate organic carbon (DOC and POC) concentrations] as well as in biological dynamics (prokaryotic and nanoplanktonic abundance, heterotrophic carbon production and degradative activities) at the transition between late spring and summer in surface waters (upper 125 m) in the Terra Nova Bay polynya, in the Ross Sea. Samplings were performed from January 18 to 21 and, after an interval of almost 5 days, from January 25 to 27. The first period was characterised by spring conditions still maintained by the presence in the area of two mega-icebergs (B15 and C19), which caused a delay in the ice melting that started during the second survey. Chlorophyll a concentrations were highest during the first period, whereas phaeopigments/Chl a ratio increased during the following sampling time suggesting variations in phytoplankton biomass/physiology and a change in organic matter (OM) quality. No significant differences in DOC and POC concentration were detected between the two periods together with unvaried abundances of prokaryotes and small protists. The changes in OM over time were evidenced also by changing exoenzymatic activity patterns. Notwithstanding the general variations occurring during the surveys, heterotrophic production and prokaryotic growth were not substantially modified, suggesting a rapid (timescales of days) acclimation to changing organic matter quality for the maintenance of prokaryotic metabolic requirements. [ABSTRACT FROM AUTHOR]

Marine ecological indicators can be used to assess the condition of the pelagic ecosystems. The bloom onset provides a warning bell for possible changes in trophic interactions and biogeochemical processes. However, depicting the phenology of phytoplankton blooms at high latitudes, where long-term observations are sparse or unavailable, is not a straightforward task. A data-interpolating empirical orthogonal function algorithm was applied to daily satellite-retrieved chlorophyll- a images to produce a long-term (1998-2014) and cloud-free data set over the North Water (NOW) polynya. The seasonal bloom was modeled using a multi-Gaussian approach from which a baseline of phenological characteristics was extracted. The correlation analysis highlights the influence of environmental factors, such as sea surface temperature, cloud fraction, wind stress, and sea-ice concentration, in modulating the bloom start date, its duration, and amplitude. The year-to-year variability in bloom onset appears to be controlled by a delicate balance between oceanographic and meteorological conditions. Blooms last longer during years characterized by a longer open-water period and are shorter during those characterized by greater sea-ice coverage. Noteworthy is the decrease in phytoplankton bloom amplitude over the 17 years examined. Collectively, these outcomes depict the NOW as a climate-sensitive region in which the pelagic marine ecosystem seems to be going toward a decline in chlorophyll- a concentrations. Satellite time series are still too short to differentiate between inter-annual variability, inter-decadal variability, and climate change signal. Should these changes persist; however, the NOW may no longer act as a productive regional oasis supporting thriving populations of zooplankton and top predators. [ABSTRACT FROM AUTHOR]

Climate change driven alterations to patterns of Arctic marine primary production, with increasing phytoplankton- and decreasing ice algal production, have the potential to change the resource utilisation and trophic structure of the benthic communities relying on the algae for food. To predict the benthic responses to dietary changes, we studied the macroinfaunal community compositions, and used the faunal δ13C and δ15N signatures to investigate their main food sources and trophic positions in North Water (NOW) and Lancaster Sound (LS) polynyas in the Canadian Arctic Archipelago. Macroinfaunal density (10 952 ind. m-2) and biomass (3190 mg C m-2) recorded in NOW were higher than previously found in the Arctic at depths >500m, and significantly higher than in LS (8355 ind. m-2 and 2110 mg C m-2). This was attributed to higher particulate organic matter fluxes to seafloor in NOW. Polychaetes were significant taxa at both sites in terms of density and biomass, and in addition crustacean density in NOW and bivalve density in LS were high. Facultative filter and surface deposit feeders were highly prevalent at both sites, suggesting feeding plasticity is a successful strategy for accessing different food sources. The macrofaunal δ13C signatures reflected the signatures of pelagic particulate organic matter at the sites, and an isotope mixing model confirmed phytoplankton as the main food source for most taxa and feeding guilds. The food web length in LS was longer than in NOW (3.2 vs. 2.8 trophic levels). This was attributed to a larger reliance on reworked organic matter by the benthic community in LS, whereas the high export fluxes at the highly productive NOW resulted in higher rates of selective consumption of fresh algal matter. Despite studies suggesting that loss of ice algae from consumer diets in the Arctic might have a negative impact on the benthos, this study suggests that Arctic macrobenthic communities thrive using phytoplankton as their main food source and should thus be able to cope or even benefit from predicted changes to patterns of primary production. [ABSTRACT FROM AUTHOR]

The 0–200 m surface layer of the Ross Sea was studied during summer 2014 to investigate the lability of the particulate organic matter (POM) in response to physical parameters. With the use of satellite information, we selected three zones, characterised by different physical setting: a northern offshore area, crossing the summer-polynya area of the Ross Sea (hereafter called ROME 1), a more coastal area next to the Terra Nova Bay polynya (ROME 2); a southern offshore area, towards the Ross Ice Shelf (ROME 3). Ice-maps showed that the seasonal ice retreat had already occurred in early December for most of the stations. Statistical analysis of the quantitative and qualitative characteristics of the POM pointed to significant differences between the stations, especially in the upper mixed layer (UML). A comparison with previous studies showed that the localised pulses of POM accumulation in the UML were similar to those recorded at the highly productive marginal ice zones, providing notable trophic support to the ecosystem. The UML, although rather thin and easily subjected to alterations, confirmed its pivotal role in the ecosystem dynamics. A POM quality favourable to consumers was highlighted at several stations in ROME 1 and ROME 3. Reduced trophic support was, instead, found in ROME 2. Limited POM consumption where deep-water formation takes place would increase the POM role in the transfer of C to the depths. [ABSTRACT FROM AUTHOR]

High-level primary production (PP) sustainability in the Antarctic coastal polynyas has not been adequately addressed. We investigated the relationship between the Antarctic coastal polynyas phytoplankton biomass (as reflected by Chl-a) and the cloud cover, which dominantly influences the light conditions. Phytoplankton biomass in Antarctic coastal polynyas showed a robust correlation with the photosynthetically active radiation (PAR) and the cloud fraction. However, a different relationship was observed between the phytoplankton biomass and the PAR. This relationship depended on both the ultraviolet (UV) radiation and the geographical location of the coastal polynyas. High intensity UV radiation is suspected to be an inhibitor of phytoplankton biomass. This indicates that the phytoplankton biomass is effectively limited by light intensity, which can be reduced by clouds, particularly mid- and high-level clouds, but it is not limited by light under high-level UV radiation exposure conditions. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editors discuss various reports within the issue on topics including the research project on the North Water (NOW), the big production of phytoplankton, and dynamic ecology.

Sea ice variability within the marginal ice zone (MIZ) and polynyas plays an important role for phytoplankton productivity and krill abundance. Therefore mapping their spatial extent, seasonal and interannual variability is essential for understanding how current and future changes in these biological active regions may impact the Antarctic marine ecosystem. Knowledge of the distribution of different ice types to the total Antarctic sea ice cover may also help to shed light on the factors contributing towards recent expansion of the Antarctic ice cover in some regions and contraction in others. The long-term passive microwave satellite data record provides the longest and most consistent data record for assessing different ice types. However, estimates of the amount of MIZ, consolidated pack ice and polynyas depends strongly on what sea ice algorithm is used. This study uses two popular passive microwave sea ice algorithms, the NASA Team and Bootstrap to evaluate the distribution and variability in the MIZ, the consolidated pack ice and coastal polynyas. Results reveal the NASA Team algorithm has on average twice the MIZ and half the consolidated pack ice area as the Bootstrap algorithm. Polynya area is also larger in the NASA Team algorithm, and the timing of maximum polynya area may differ by as much as 5 months between algorithms. These differences lead to different relationships between sea ice characteristics and biological processes, as illustrated here with the breeding success of an Antarctic seabird. [ABSTRACT FROM AUTHOR]

To evaluate the relationship between the phytoplankton composition and the CDOM production in the Amundsen Sea, we examined the taxonomic variability of phytoplankton, nutrients and chromophoric dissolved organic matter (CDOM) participating in two research cruises, ANA02C (February, 2012) and ANA04B (January, 2014). For both cruises, the peak concentrations of chlorophyll a (Chl-a) were measured in the Amundsen Sea Polynya (ASP) where the average Chl-a concentration was 2.31 μg L −1 (±1.01 μg L −1 ) in February 2012 and 3.92 μg L −1 (±3.14 μg L −1 ) in January 2014. The major phytoplankton groups were diatoms and prymnesiophytes, while the minor groups were cryptophytes, chlorophytes, dinophytes, chrysophytes and cyanophytes. The phytoplankton compositions in the sea ice zone (SIZ), the ASP and ice shelf (IS) was distinct. The predominance of Phaeocystis antarctica ( P. antarctica ) (70–90%) was observed at all stations, except in the SIZ in January 2014. While the CDOM concentrations ( a 355 ) ranged from 0.07 to 0.98 m −1 , the average values of a 355 of the euphotic layer in the ASP was 0.51 m −1 (±0.19 m −1 ), which was much higher than any other region of the Southern Ocean. CDOM showed strong correlation with Chl-a but no relationship with salinity, which implies that biological processes were the main source of CDOM. Among the major phytoplankton taxa, P. antarctica was confirmed as the most important contributor to the production of CDOM, according to the high correlation between marker pigment (hex-fuco) and CDOM. The unusually high CDOM in the ASP is expected to give as impact on the environment in various aspects. As a nature of CDOM, absorption of solar radiation at the surface can result in the increase of sea surface temperature (SSTs), which will accelerate the ice melting. Also the photo-oxidation of CDOM can serve as a feedback to increasing atmospheric CO 2 concentrations after peak bloom in austral summer. [ABSTRACT FROM AUTHOR]

The Amundsen Sea polynya is one of the largest and most productive polynyas in the Southern Ocean and has recently experienced a rapid change in sea ice coverage. However, very little is known about current physiological status of phytoplankton and its quality as food for pelagic herbivores and consequently higher trophic levels in the Amundsen Sea. Using a 13 C isotope tracer technique, macromolecular production measurements of phytoplankton at eleven stations were conducted at three light depths (100, 30, and 1%) onboard R/V ARAON in the Amundsen Sea, 2012. The concentrations of major inorganic nutrients were replete at all the productivity stations and no substantial difference in macromolecular production was found between polynya and non-polynya regions. Distinct vertical trends were not observed in low-molecular-weight metabolites (LMWM) and polysaccharide productions, but weak vertical patterns in lipid and protein productions were found during our cruise period. The vertical patterns of lipids slightly increased with depth whereas decreased for protein synthesis in this study, and these vertical trends were not consistent with the results reported previously in the Arctic Ocean. Overall, phytoplankton allocated more photosynthetic carbon into proteins (60.0%) than other macromolecules in the Amundsen Sea, which is markedly higher than those reported previously in the Antarctic Ocean, ranging from 7 to 23%. The high protein synthesis appears to be sustained by high concentrations of major nutrients, which might be a strong factor for general patterns of macromolecular productions of phytoplankton in polar oceans, even under potential iron limitation. [ABSTRACT FROM AUTHOR]

Dissolved Fe (DFe) was measured in the Ross Sea Polynya (RSP), Antarctica, during a GEOTRACES cruise between 20 December 2013 and 5 January 2014. DFe was measured over the full water column with special emphasis on samples near the seafloor. In the upper mixed layer, DFe was very low everywhere (< 0.10 nM). DFe increased with depth to values between 0.60 and 2.76 nM near the seafloor. The highest DFE concentrations were found at stations where a bottom nepheloid layer (BNL) was present (28 out of 32 stations). Deep DFe was lower (0.24–0.38 nM) at stations with no BNL. The main DFe supply to the upper mixed layer was vertical diffusive transport from the seafloor sediments, with a mean flux of 3.3 × 10 − 8 mol DFe m − 2 day − 1 . DFe fluxes showed large spatial variability of three orders of magnitude and were positively correlated to DFe concentrations near the sediment and vertical turbulent eddy diffusivity (K z ) and negatively correlated to water depth. The greatest fluxes were observed above the shallow banks such as Ross and Pennell Banks, and sediments with a BNL. We studied the horizontal diffusive transport from Franklin Island as an example of horizontal DFe transport from landmasses. No DFe transport was detected in the upper 100 m of the water column, probably due to uptake by phytoplankton. However, at 200 and 300 m depth, the DFe transport at distances between 50 and 100 km from Franklin Island was as large as the mean diffusive upward transport, indicating the potential importance of landmasses as a local source of DFe. Conversely, no horizontal transport of DFe from banks was detected. In addition, the Ross Ice Shelf (RIS) was a negligible source of DFe. Only the Ice Shelf Water (ISW), a water mass formed under the RIS, contained slightly elevated DFe (0.18–0.26 nM) compared to the surrounding waters. However, this elevated DFe did not reach into the RSP. Icebergs were not encountered and were not evaluated as a DFe source. Overall, we conclude that DFe from the seafloor and land mass sediments are the main DFe sources of DFe that support phytoplankton in the upper mixed layer of the Ross Sea Polynya in early summer. [ABSTRACT FROM AUTHOR]

Multiple year-round aerosol samplings were conducted from February 2005 to October 2008 at Zhongshan Station, a research base in East Antarctica, to study methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO42−). The concentrations of atmospheric sulfur species exhibited a seasonal cycle; the maximum and minimum concentrations occurred in austral summer and austral winter, respectively. Significant correlations between chlorophyll a (Chl a) in offshore polynyas and both MSA ( r = 0.726, n = 52, and p < 0.01) and nss-SO42− ( r = 0.724, n = 48, and p < 0.01) were found, indicating that the phytoplankton activity had a crucial effect on the sulfur aerosols. The sea ice dynamics in the polynyas and the variations in the polynya area may indirectly influence the sulfur aerosols in austral spring and summer. In austral winter, the sulfur compounds in the atmosphere are primarily originating in long-range transported by-products from remote regions because nearly no phytoplankton activity occurred in the offshore polynyas. [ABSTRACT FROM AUTHOR]

Rising temperatures and changing winds drive the expansion of the highly productive polynyas (open water areas surrounded by sea ice) abutting the Antarctic continent. Phytoplankton blooms in polynyas are often dominated by the haptophyte Phaeocystis antarctica, and they generate the organic carbon that enters the resident microbial food web. Yet, little is known about how Phaeocystis blooms shape bacterial community structures and carbon fluxes in these systems. We identified the bacterial communities that accompanied a Phaeocystis bloom in the Amundsen Sea polynya during the austral summers of 2007-2008 and 2010-2011. These communities are distinct from those determined for the Antarctic Circumpolar Current (ACC) and off the Palmer Peninsula. Diversity patterns for most microbial taxa in the Amundsen Sea depended on location (e.g., waters abutting the pack ice near the shelf break and at the edge of the Dotson glacier) and depth, reflecting different niche adaptations within the confines of this isolated ecosystem. Inside the polynya, P antarctica coexisted with the bacterial taxa Polaribacter sensu lato, a cryptic Oceanospirillum, SAR92 and Pelagibacter. These taxa were dominated by a single oligotype (genotypes partitioned by Shannon entropy analysis) and together contributed up to 73% of the bacterial community. Size fractionation of the bacterial community [<3 µm (free-living bacteria) vs. >3 µm (particle-associated bacteria)] identified several taxa (especially SAR92) that were preferentially associated with Phaeocystis colonies, indicative of a distinct role in Phaeocystis bloom ecology. In contrast, particle-associated bacteria at 250 m depth were enriched in Colwellia and members of the Cryomorphaceae suggesting that they play important roles in the decay of Phaeocystis blooms. [ABSTRACT FROM AUTHOR]

Abstract: Nutrient addition experiments were performed during the austral summer in the Amundsen Sea (Southern Ocean) to investigate the availability of organically bound iron (Fe) to the phytoplankton communities, as well as assess their response to Fe amendment. Changes in autotrophic biomass, pigment concentration, maximum photochemical efficiency of photosystem II, and nutrient concentration were recorded in response to the addition of dissolved free Fe (DFe) and Fe bound to different model ligands. Analysis of pigment concentrations indicated that the autotrophic community was dominated by the prymnesiophyte Phaeocystis antarctica throughout most of the Amundsen Sea, although diatoms dominated in two experiments conducted in the marginal ice zone. Few significant differences in bulk community biomass (particulate organic carbon, nitrogen, and chlorophyll a) were observed, relative to the controls, in treatments with Fe added alone or bound to the ligand phytic acid. In contrast, when Fe was bound to the ligand desferrioxamine B (DFB), decreases in the bulk biomass indices were observed. The concentration of the diatom accessory pigment fucoxanthin showed little response to Fe additions, while the concentration of the P. antarctica-specific pigment, 19′-hexanoyloxyfucoxanthin (19′-hex), decreased when Fe was added alone or bound to the model ligands. Lastly, differences in the nitrate:phosphate (NO3 −:PO4 3−) utilization ratio were observed between the Fe-amended treatments, with Fe bound to DFB resulting in the lowest NO3 −:PO4 3− uptake ratios (∼10) and the remaining Fe treatments having higher NO3 −:PO4 3− uptake ratios (∼17). The data are discussed with respect to glacial inputs of Fe in the Amundsen Sea and the bioavailability of Fe. We suggest that the previously observed high NO3 −:PO4 3− utilization ratio of P. antarctica is a consequence of its production of dissolved organic matter that acts as ligands and increases the bioavailability of Fe, thereby stimulating the uptake of NO3 −. [Copyright &y& Elsevier]

Abstract: The phytoplankton community composition and productivity in waters of the Amundsen Sea and surrounding sea ice zone were characterized with respect to iron (Fe) input from melting glaciers. High Fe input from glaciers such as the Pine Island Glacier, and the Dotson and Crosson ice shelves resulted in dense phytoplankton blooms in surface waters of Pine Island Bay, Pine Island Polynya, and Amundsen Polynya. Phytoplankton biomass distribution was the opposite of the distribution of dissolved Fe (DFe), confirming the uptake of glacial DFe in surface waters by phytoplankton. Phytoplankton biomass in the polynyas ranged from 0.6 to 14μgChl a L−1, with lower biomass at glacier sites where strong upwelling of Modified Circumpolar Deep Water from beneath glacier tongues was observed. Phytoplankton blooms in the polynyas were dominated by the haptophyte Phaeocystis antarctica, whereas the phytoplankton community in the sea ice zone was a mix of P. antarctica and diatoms, resembling the species distribution in the Ross Sea. Water column productivity based on photosynthesis versus irradiance characteristics averaged 3.00gCm−2 d−1 in polynya sites, which was approximately twice as high as in the sea ice zone. The highest water column productivity was observed in the Pine Island Polynya, where both thermally and salinity stratified waters resulted in a shallow surface mixed layer with high phytoplankton biomass. In contrast, new production based on NO3 uptake was similar between different polynya sites, where a deeper UML in the weakly, thermally stratified Pine Island Bay resulted in deeper NO3 removal, thereby offsetting the lower productivity at the surface. These are the first in situ observations that confirm satellite observations of high phytoplankton biomass and productivity in the Amundsen Sea. Moreover, the high phytoplankton productivity as a result of glacial input of DFe is the first evidence that melting glaciers have the potential to increase phytoplankton productivity and thereby CO2 uptake, resulting in a small negative feedback to anthropogenic CO2 emissions. [Copyright &y& Elsevier]

Abstract: Two coastal polynyas in the Amundsen Sea (Amundsen Polynya and Pine Island Polynya) were studied as part of the DynaLiFe project because of their unusually high rates of biological production and close association with melting ice shelves. In this component of the study, satellite imagery of ocean color (using Reprocessing R2009.1) and passive microwave sea ice distributions were used to characterize interannual changes in polynya dynamics and phytoplankton bloom development between 1997 and 2010. Interannual differences in sea ice cover were large in both polynyas, but especially so in the smaller Pine Island Polynya, and were related to synoptic scale wind patterns rather than climate state (ENSO or SAM). Open water season lasted 10 days longer in the Amundsen Polynya (132 days) than in the Pine Island Polynya (122 days). The phytoplankton bloom in the Amundsen Polynya lasted approximately two weeks longer and was 30% more productive per unit area than the bloom in the Pine Island Polynya. Because Pine Island Polynya was lower in both mean open water area and annual net primary production (NPP) per unit area, total annual NPP averaged over the 13-year time series was only 60% of that in the Amundsen Polynya (1.96 and 3.26TgCyr−1, respectively). Given the high interannual variability and the short time series duration, we did not observe a secular trend in primary production or any relationship with climate indices such as the Southern Annular Mode. However, with increased iron addition from melting ice shelves into the surface waters of the polynyas, we expect that primary production could be enhanced in the future. [Copyright &y& Elsevier]

Abstract: Primary productivity in the Amundsen Sea (Southern Ocean) is among the highest in Antarctica. The summer phytoplankton bloom in 2009 lasted for >70 days in both the Pine Island and Amundsen Polynyas. Such productive blooms require a large supply of nutrients, including the trace metal iron (Fe). The organic complexation of dissolved Fe was investigated in the Amundsen Sea during the spring of 2009 to better understand the potential role of ligands in enhancing the local stock of dissolved Fe. The main sources of Fe and ligands to the Amundsen Sea are thought to be melting sea-ice and the Circumpolar Deep Water (CDW), which is modified (MCDW) on the continental shelf and upwells beneath the coastal glaciers and ice-shelves. Upwelling of relatively warm MCDW is also responsible for the rapid melting of the Pine Island Glacier (PIG) and surrounding ice-shelves, resulting in additional release of Fe into surface waters. At upwelling stations near ice shelves, organic ligands were nearly saturated with Fe, thus enhancing the stock of Fe and its availability to the phytoplankton community. However, ligands had little capacity to buffer additional Fe input from glacial melt. In these coastal upwelling regions, much of the glacial Fe supply is lost due to vertical export of Fe via scavenging and precipitation. Conversely, within the phytoplankton bloom in the nearby coastal polynyas, the uptake of Fe combined with the production of organic matter enhanced the abundance of relatively unsaturated organic ligands capable of stabilizing additional Fe supplied from glacial melt. These unsaturated dissolved organic ligands, combined with the continuous input of Fe (dissolved and particulate) from glacial melt, appear to favor the solubilization of Fe, thus increasing the stock of bioavailable Fe and fueling the phytoplankton bloom. [Copyright &y& Elsevier]

Abstract: Surface water (<100m) samples were collected from the Terra Nova Bay polynya region of the Ross Sea (Antarctica) in January 2006, with the aim of evaluating the individual and combined effects of hydrographic structure, iron and copper concentration and availability on the phytoplankton growth. The measurements were conducted within the framework of the Climatic Long Term Interaction for the Mass-balance in Antarctica (CLIMA) Project of the Programma Nazionale di Ricerca in Antartide activities. Dissolved oxygen, nutrients, phytoplankton pigments and concentration and complexation of dissolved trace metals were determined. Experimental data were elaborated by Principal Component Analysis (PCA). As a result of solar heating and freshwater inputs from melting sea-ice, the water column was strongly stratified with an Upper Mixed Layer 4–16m deep. The integrated Chl a in the layer 0–100m ranged from 60mgm−2 to 235mgm−2, with a mean value of 138mgm−2. The pigment analysis showed that diatoms dominated the phytoplankton assemblage. Major nutrients were generally high, with the lowest concentration at the surface and they were never fully depleted. The Si:N drawdown ratio was close to the expected value of 1 for Fe-replete diatoms. We evaluated both the total and the labile dissolved fraction of Fe and Cu. The labile fraction was operationally defined by employing the chelating resin Chelex-100, which retains free and loosely bound trace metal species. The total dissolved Fe ranged from 0.48 to 3.02nM, while the total dissolved Cu from 3.68 to 6.84nM. The dissolved labile Fe ranged from below the detection limit (0.15nM) to 1.22nM, and the dissolved labile Cu from 0.31 to 1.59nM, respectively. The labile fractions measured at 20m were significantly lower than values in 40–100m samples. As two stations were re-sampled 5 days later, we evaluated the short-term variability of the physical and biogeochemical properties. In particular, in a re-sampled station at 20m, the total dissolved Fe increased and the total dissolved Cu decreased, while their labile fraction was relatively steady. As a result of the increase in total Fe, the percentage of the labile Fe decreased. An increase of the Si:N, Si:P and Si:FUCO ratios was measured also in the re-sampled station. On this basis, we speculated that a switch from a Fe-replete to a Fe-deplete condition was occurring. [Copyright &y& Elsevier]

The overwintering deployment of an icebreaker during the Canadian Flaw Lead study provided an opportunity to evaluate how protist communities (phytoplankton and other single-celled eukaryotes) respond to changing spring irradiance conditions in flaw lead polynyas, where open water persists between the central pack ice and land fast ice. We combined microscopic analysis of the protist communities (all cell sizes) with clone libraries of 18S rRNA genes and 18S rRNA (from RNA converted to cDNA) of size-fractionated seawater (0.2-3.0 μm) from 10 to 12 m depth in the surface mixed layer. The rRNA gene analysis provided information on the presence of organisms, while the rRNA analysis provided information on the most active members of the community. There was little overlap between the two types of clone libraries, and there were large community shifts over time. Heterotrophic dinoflagellates and ciliates were the most common sequences recovered. The relative proportion of photosynthetic protist sequences increased in March and April, and there was greater representation of Bacillariophyta, Prasinophyta, Haptophyta, and Cryptophyta in the rRNA compared to rRNA gene libraries. Microscopy indicated that large-celled diatoms dominated the community in May, when chlorophyll concentrations were greatest. However, the RNA sequencing showed that heterotrophic and putative parasitic protists were proportionately more active, and the concomitant decrease in nutrients suggested that the spring phytoplankton bloom had begun to decline by this time. These observations provide evidence of substantial changes in protist community structure and function during the spring transition. [ABSTRACT FROM AUTHOR]

During summer 2008, as part of the Circumpolar Flaw Lead system study, we measured phytoplankton photosynthetic parameters to understand regional patterns in primary productivity, including the degree and timescale of photoacclimation and how variability in environmental conditions influences this response. Photosynthesis-irradiance measurements were taken at 15 sites primarily from the depth of the subsurface chlorophyll a (Chl a) maximum (SCM) within the Beaufort Sea flaw lead polynya. The physiological response of phytoplankton to a range of light levels was used to assess maximum rates of carbon (C) fixation ( P), photosynthetic efficiency ( α), photoacclimation ( E), and photoinhibition ( β). SCM samples taken along a transect from under ice into open water exhibited a >3-fold increase in α and P, showing these parameters can vary substantially over relatively small spatial scales, primarily in response to changes in the ambient light field. Algae were able to maintain relatively high rates of C fixation despite low light at the SCM, particularly in the large (>5 μm) size fraction at open water sites. This may substantially impact biogenic C drawdown if species composition shifts in response to future climate change. Our results suggest that phytoplankton in this region are well acclimated to existing environmental conditions, including sea ice cover, low light, and nutrient pulses. Furthermore, this photoacclimatory response can be rapid and keep pace with a developing SCM, as phytoplankton maintain photosynthetic rates and efficiencies in a narrow 'shade-acclimated' range. [ABSTRACT FROM AUTHOR]

Abstract: We report results from two surveys of pCO2, biological O2 saturation (ΔO2/Ar) and dimethylsulfide (DMS) in surface waters of the Ross Sea polynya. Measurements were made during early spring (November 2006–December 2006) and mid-summer (December 2005–January 2006) using ship-board membrane inlet mass spectrometry (MIMS) for high spatial resolution (i.e. sub-km) analysis. During the early spring survey, the polynya was in the initial stages of development and exhibited a rapid increase in open water area and phytoplankton biomass over the course of our ∼3 week occupation. We observed a rapid transition from a net heterotrophic ice-covered system (supersaturated pCO2 and undersaturated O2) to a high productivity regime associated with a Phaeocystis-dominated phytoplankton bloom. The timing of the early spring phytoplankton bloom was closely tied to increasing sea surface temperature across the polynya, as well as reduced wind speeds and ice cover, leading to enhanced vertical stratification. There was a strong correlation between pCO2, ΔO2/Ar, DMS and chlorophyll a (Chl a) during the spring phytoplankton bloom, indicating a strong biological imprint on gas distributions. Box model calculations suggest that pCO2 drawdown was largely attributable to net community production, while gas exchange and shoaling mixed layers also exerted a strong control on the re-equilibration of mixed layer Ο2 with the overlying atmosphere. DMS concentrations were closely coupled to Phaeocystis biomass across the early spring polynya, with maximum concentrations exceeding 100nM. During the summer cruise, we sampled a large net autotrophic polynya, shortly after the seasonal peak in phytoplankton productivity. Both diatoms and Phaeocystis were abundant in the phytoplankton assemblages during this time. Minimum pCO2 was less than 100ppm, while ΔO2/Ar exceeded 30% in some regions. Mean DMS concentrations were ∼2-fold lower than during the spring, although the range of concentrations was similar between the two surveys. There was a significant correlation between pCO2, ΔO2/Ar and Chl a across the summer polynya, but the strength of these correlations and the slope of O2 vs. CO2 relationship were significantly lower than during the early spring. Summertime DMS concentrations were not significantly correlated to phytoplankton biomass (Chl a), pCO2 or ΔO2/Ar. In contrast to the early spring time, there were no clear temporal trends in summertime gas concentrations. Rather, small-scale spatial variability, likely resulting from mixing and localized sea-ice melt, was clearly evident in surface gas distributions across the polynya. Analysis of length-scale dependent variability demonstrated that much of the spatial variance in surface water gases occurred at scales of <20km, suggesting that high resolution analysis is needed to fully capture biogeochemical heterogeneity in this system. [Copyright &y& Elsevier]

Abstract: Phytoplankton assemblages of the Ross Sea are generally dominated by two functional groups: diatoms and haptophytes (Phaeocystis antarctica). Within this “normal” pattern of dominance, there is a substantial amount of temporal (over months, seasons and years) and spatial variability. Such variability has a significant impact on several biogeochemical cycles, such as the carbon and sulfur cycles, at the regional and global scales. We compiled all available accessory pigment data for the southern Ross Sea as a means to quantify the prevalence and dominance of each group, and generated a seasonal “climatology” of assemblage composition. The climatological pattern of phytoplankton pigments shows that haptophytes normally grow and accumulate early in the season, and largely in the southern Ross Sea polynya. Diatoms reach a biomass maximum later, and reach most extensive concentrations closer to the coast of Victoria Land. While the pattern of spring growth of P. antarctica followed by an increase in diatom abundance is found frequently, deviations from that pattern were observed. Two periods – November, 2006 and January, 2004 – illustrated that variations of up to one order of magnitude can occur relative to the climatology. These deviations may provide insights into the dominant control mechanisms of the two functional groups. [Copyright &y& Elsevier]

Our pigment analyses from a year-long study in the coastal Beaufort Sea in the western Canadian Arctic showed the continuous prevalence of eukaryotic picoplankton in the green algal class Prasinophyceae. Microscopic analyses revealed that the most abundant photosynthetic cell types were Micromonas-like picoprasinophytes that persisted throughout winter darkness and then maintained steady exponential growth from late winter to early summer. A Micromonas (CCMP2099) isolated from an Arctic polynya (North Water Polynya between Ellesmere Island and Greenland), an ice-free section, grew optimally at 6°C–8°C, with light saturation at or below 10 μmol photons·m−2·s−1 at 0°C. The 18S rDNA analyses of this isolate and environmental DNA clone libraries from diverse sites across the Arctic Basin indicate that this single psychrophilic Micromonas ecotype has a pan-Arctic distribution. The 18S rDNA from two other picoprasinophyte genera was also found in our pan-Arctic clone libraries: Bathycoccus and Mantoniella. The Arctic Micromonas differed from genotypes elsewhere in the World Ocean, implying that the Arctic Basin is a marine microbial province containing endemic species, consistent with the biogeography of its macroorganisms. The prevalence of obligate low-temperature, shade-adapted species in the phytoplankton indicates that the lower food web of the Arctic Ocean is vulnerable to ongoing climate change in the region. [ABSTRACT FROM AUTHOR]

Phytoplankton taxonomic pigments were measured by high-performance liquid chromatography (HPLC) during a 3-month survey (April–June 1998) in the North Water (NOW) Polynya (Canadian Arctic) to investigate changes in phytoplankton biomass and composition and the physical–chemical factors that influence these changes. A phytoplankton bloom with high chlorophyll a (Chl a) concentrations (up to 17.45 mg·m–3 at 15 m) occurred in mid-May along the Greenland coast in the southeastern part of the NOW Polynya. The initiation of the phytoplankton bloom was linked to shallow mixed-layer depths. The contribution of the different phytoplankton groups to Chl a inferred using a factorization program (CHEMTAX) indicated that the bloom was diatom-dominated (maximum 94% diatoms). The phytoplankton community structure was influenced by the water mass characteristics and the surface circulation pattern. Autotrophic flagellates dominated in April and May along the Canadian coast, where cold Arctic waters with relatively deep mixed layers were found. In contrast, diatoms dominated in May along the Greenland coast in warmer water masses of Atlantic origin and during June in the whole polynya, except in the southernmost part. [ABSTRACT FROM AUTHOR]

A satellite-based study was conducted to quantify the seasonal-to-interannual changes in phytoplankton production in the southwestern Ross Sea between 1997 and 2001. The size of the Ross Sea polynya from 1992 to 2001 was related to the climate state as expressed by the Multivariate ENSO Index (MEI). These data show that 76% of the variation in sea-ice extent between 1992 and 2000 can be explained by the MEI, with the El Niño year of 1997–1998 exhibiting the greatest sea-ice cover and the La Niña year of 1999–2000 the least. Whether the relationship between ice area and the MEI is caused by interannual changes in wind patterns or changes in the movement of circumpolar deep water onto the Ross Sea continental shelf has yet to be determined. The year 2000–2001 was excluded from the climate analysis, due to the effects of the enormous B-15 iceberg, which inhibited the normal advection pattern of sea-ice out of the Ross Sea, resulting in higher than normal sea-ice conditions. Phytoplankton blooms were markedly less extensive in the heavy sea-ice years of 1997–1998 and 2000–2001 and were significantly delayed in their development, with concentrations of Chl a peaking 2 months later (February) than in reduced sea-ice years (December). Annual production during the 1997–1998 El Niño (27.4 Tg C) was similar to that of the iceberg impacted year 2000–2001 (28.3 Tg C). These annual rates were approximately 40% below values estimated between November 1998 and March 2000 (47.5 Tg C) when open-water area during spring and summer was much greater. This decreased production during heavy sea-ice years is more a function of the delay in the start of the bloom than in the reduction of open water area. [Copyright &y& Elsevier]

Part II. Presents a study which investigated downward fluxes of siliceous phytoplankton, fecal pellets of several types and zooplankton in the polynya of Terra Nova Bay near Ross Sea in Antarctica. Description of the study area; Methodology; Results and discussion.

We investigated the abundance, body-size distribution, diet, and ingestion rates of appendicularian tunicates at 8 stations in the North Water polynya, northern Baffin Bay, during late June and July 1998. Abundance of appendicularians in the chlorophyll-rich surface layer (40-125 m) ranged from 38 to 11248 m[sup -2]. Body size of individuals ranged from 0.21 to 4.8 mm. Gut chlorophyll content varied from 0 to 84 ng ind[sup -1] and increased with increasing body size. Gut passage time varied from 42 to 104 min (mean ± SD: 58 ± 18 min). Phytoplankton ingestion rates ranged from 0.007 to 2.083 mg chlorophyll m[sup -2] d[sup -1], with a median of 0.49 mg chlorophyll m[sup -2] d[sup -1]. The median daily grazing impact of the oikopleurid populations was 0.42% and 5.4% of total phytoplankton biomass and primary production, respectively. Since this represents ingested phytoplankton and does not include cells trapped within the mucous houses, the contribution of appendicularian populations to phytoplankton mortality could be 2-fold higher (i.e. ca. 10% of primary production). The faecal pellets of animals incubated onboard in water from the subsurface chlorophyll maximum contained primarily small diatoms at the northern stations, and a mixture of diatoms, dinoflagellates, flagellates, and ciliates at southern stations. The median, daily flow of biogenic carbon from phytoplankton to appendicularian faecal pellets was 8 mg C m[sup -2], which represents 4% of biogenic carbon export during the month of July. Because the phytoplankton was dominated by small Chaetoceros cells, appendicularian ingestion rates were not inhibited by the clogging of the filtration mechanism at high Chl a concentrations. Thus, the impact of appendicularians on daily primary production during this season was a simple function of appendicularian abundance, population size structure, and gut throughput rate. [ABSTRACT FROM AUTHOR]

During the 1999 expedition of the International North Water Polynya Study in northern Baffin Bay, the photosynthetic characteristics of phytoplankton were measured using pulse-amplitude-modulated (PAM) fluorescence methods. We focused mainly on the functioning of the diadinoxanthin cycle, which was expected to be an effective protection system of photosystems from high irradiance under low temperature. In spite of the low temperature (ca. 0°C), the diadinoxanthin cycle was very active, as monitored by the change of non-photochemical quenching of fluorescence (NPQ). The extent of NPQ became larger when phytoplankton were exposed to higher irradiance, and seemed to be in good correlation with the total amount of diadinoxanthin plus diatoxanthin, which are responsible for the diadinoxanthin cycle. The NPQ was more active in the surface phytoplankton community than in the phytoplankton living at the bottom of the euphotic zone, which had less xanthophyll pigments. These results indicate that the diadinoxanthin cycle actively drains out excess light energy as non-radiative heat and protects photosystems under high irradiance. The active NPQ reduced the effective irradiance; the effective quantum yield of photosystem II became smaller under higher irradiance. Reflecting these features, the resulting rate of photosynthesis in surface phytoplankton tended to be smaller than that of phytoplankton at the bottom of the euphotic zone. The irradiance at the onset of light saturation (I[sub k]) was generally higher than the reported values for polar phytoplankton and ice algae, probably because of the presence of a water-water cycle and/or other metabolic pathways. Our work also shows the usefulness of PAM measurements for ecological and biochemical studies of psychrophilic phytoplankton in polar regions. [ABSTRACT FROM AUTHOR]

Specific taxa and community structure of the phytoplankton and other protists in the North Water, a productive polynya ecosystem between Ellesmere Island (Canada) and Greenland, were determined from April to July 1998. Distinct temporal and spatial trends were observed within different geographic regions, influenced by water masses determined by temperature and salinity (TS) characteristics. In April, the northernmost communities were primarily flagellates and dinoflagellates, resembling those of the Arctic Basin, while large centric diatoms dominated in May and persisted through July. Throughout the rest of the polynya in April, centric diatoms and alveolates (ciliates and dinoflagellates) dominated. In the eastern part of the polynya in May and June, centric diatoms, especially Thalassiosira spp., reached very high levels, comprising 50-80% of biomass carbon there, while in July the eastern community was primarily Chaetoceros spp. and large alveolates. The community in the western region in April was more mixed, with Actinocyclus spp./Coscinodiscus spp. and alveolates as the dominant groups. In May and June, pennate-ribbon diatoms were mixed with Thalassiosira and Chaetoceros spp., while in July western biomass was primarily alveolates and Chaetoceros spp. Throughout the polynya and the 4-month sampling period, the community of phytoplankton and other protists was dominated by microplankton (20-200 µm). Records of this size class from the current study were comparable to historical records collected since 1876. The planktonic communities along the southern border of the North Water, which was ice-covered until mid-June and had TS characteristics of the West Greenland Current (WGC), differed from those in the rest of the polynya. They contained few planktonic diatoms and had a higher proportion of nanoplankton size (2-20 µm) flagellates and gymnodinoid dinoflagellates. The WGC microplankton were dominated by thecate dinoflagellates such as Ceratium... [ABSTRACT FROM AUTHOR]

The North Water, a large polynya in northern Baffin Bay, has an extensive period of continuous primary production lasting up to 6 months. The well-known richness of this polynya in higher trophic levels, including numerous marine mammals and birds, must be based on the magnitude and duration of that production. As part of the International North Water Polynya Study, we observed that two groups of phytoplankton were responsible for most of the primary production in 1997-1999: the large centric diatoms, Thalassiosira spp., and the smaller, colonial centric diatom, Chaetoceros socialis Lauder. We studied the phytoplankton community assemblage in the North Water in August 1997, April-July 1998, and August-September 1999 using epifluorescence microscopy on samples fixed and filtered at sea, with unpreserved cells observed in July 1998. Blooms of C. socialis occurred at southern stations in June and throughout the polynya from July to September. Maximum concentrations reached 30,100 cells ml[sup -1] (496 µg Cl[sup -1]) in the western area in early July. Blooms of C. socialis directly followed those of Thalassiosira spp., which were more intense but also more transient. C. socialis appeared to maintain its population in the euphoric layer for up to 3 months by surviving at low nutrient levels, sinking to depths where dynamic physical regimes brought nutrient replenishment, and producing resting spores (up to 233 µg Cl[sup -1]). The resting spores may survive low-nutrient periods and be introduced, some from fecal pellets, to waters of more favorable growth conditions; in July, 86% of fecal pellets and 76% of aggregates observed in shallow, floating sediment traps contained resting spores of C. socialis. Colonies of C. socialis were a potential mid-water food source and major contributor to the benthos: C. socialis contributed up to 91% of total phytoplankton cells (49% phytoplankton carbon) identified in moored sediment traps. Mats of polymer gels (mucous)... [ABSTRACT FROM AUTHOR]

The seasonal patterns of phytoplankton biomass and production were determined in the North Water, located between Greenland and Ellesmere Island (Canadian Arctic), in August 1997, April-July 1998, and August-September 1999. The patterns differed among the four defined regions of this large polynya, i.e. North (> 77.5°N), East (> 75°W), West (<75°W), and South (<76°N). Phytoplankton biomass and production were low during April throughout the North Water. Biomass first increased in the East during April. From there, the biomass spread north- and westwards during May-June, when the bloom culminated (chlorophyll a concentrations up to 19.8 mg m[sup -3]). The large-sized (> 5 µm) fraction dominated the biomass and production during the bloom. During July, August, and September, biomass and production decreased over the whole region, with the highest biomass, dominated by large cells, occurring in the North. The annual particulate and dissolved phytoplankton production were the highest ever reported for the high Arctic, reaching maximum values of 254 and 123 g C m[sup -2] yr[sup -1], respectively, in the East. Rates in the North and West were considerably lower than in the East (ca. two- and three-fold, respectively). The f-ratios (i.e. ratio of new to total production), derived from the size structure of phytoplankton, were high north of 76°N (0.4-0.7). Regionally, this indicated a high potential export of particulate organic carbon (E[sub POC]) from the phytoplankton community to other trophic compartments and/or downwards in the East (155 g C m[sup -2]yr[sup -1]), with lower values in the North and West (i.e. 77 and 42 gCm[sup -2]yr[sup -1], respectively). The seasonal and spatial patterns of E[sub POC] were consistent with independent estimates of potential carbon export. Phytoplankton biomass and production were generally dominated by the large size fraction, whereas Eeoc seemed to be dominated by the large size fraction early in the... [ABSTRACT FROM AUTHOR]

A 4-month multidisciplinary expedition, beginning at the end of winter to track the spring phytoplankton bloom to its termination in summer, was conducted from April to July 1998. The aim of the expedition was to investigate possible mechanisms responsible for the high biological productivity of the North Water, the most productive and largest polynya in the Northern Hemisphere. The aims of the present study were to investigate: (1) the effects of the physical forces, driving the formation of the polynya, on the dynamics of the phytoplankton stock in the polynya over the spring-summer period; and (2) the factors that limit the maximum biomass of phytoplankton. Contrasting physical characteristics, including ice concentration, surface mixed-layer depth (MLD), salinity and temperature in the surface mixed layer (SML), were observed between the east and west sides of the polynya. The Greenland (eastern) side of the polynya was characterized by a shallow SML, warm temperature, and high salinity relative to the Ellesmere Island, Canada (western) side. Chlorophyll a (Chl) > 1 mg m[sup -3] was observed in late April on the eastern side, and in late May on the western side. The peak phytoplankton bloom occurred in the southeastern part of the polynya, with average Chl of 15 mg m[sup -3] (240-300 mg m[sup -2]) in the euphotic zone during the end of May and beginning of June. The increased phytoplankton biomass was associated with higher salinity and warmer temperature on the eastern side of the polynya. Low temperature in April and May decoupled the increase of Chl biomass from the shallowed SML, as predicted by Sverdrup's model. As Chl in the euphotic zone increased to 5 mg m[sup -3], the proportion of light absorption by phytoplankton could not increase further with Chl biomass, which might have limited the increase of primary production in the water column. Although the initial nutrient inventories largely determined the maximum biomass of phytoplankton, self-shading... [ABSTRACT FROM AUTHOR]

Phytoplankton carbon fluxes were studied in the Northeast Water (NEW) Polynya, off the eastern coast of Greenland (79° to 81°N, 6° to 17°W), during summer 1993. The downward flux of organic particles was determined during 54 days using a sediment trap moored at a fixed location, below the pycnocline (130 m). The hypothesis of the present study is that wind events were ultimately responsible for the events of diatoms downward flux recorded in the trap.Wind conditions can influence the vertical transport of phytoplankton by affecting (1) the environmental conditions (e.g. hydrostatic pressure, nutrient concentrations, and irradiance) encountered by phytoplankton during their vertical excursion, and (2) the aggregation and disaggregation of phytoplankton flocs. The first mechanism affects the physiological regulation of buoyancy, whereas the second one affects the size and shape of settling particles.Using field data (wind velocity, density profiles and phytoplankton abundance), we assessed the potential aggregation and the vertical excursion of phytoplankton in surface waters. The results show that, upstream from the trap, wind and hydrodynamic conditions were sometimes favourable to the downward export of phytoplankton. Lag-correlation between time series of wind and phytoplankton downward flux shows that flux events lagged wind events by ca. 16 days. Given that the average current velocity in the top 100 m was ca. 10 cm s−1, a lag of 16 days corresponded to a lateral transport of ca. 130 km, upstream from the sediment trap, where phytoplankton production was lower than at the location of the trap. According to that scenario, 21% to 60% of primary production was exported to depth during wind events. If we had assumed instead a tight spatial coupling between the material collected in the trap and the relatively high phytoplankton production at the location of the trap, we would have concluded that <7% of primary production was exported to depth. The difference between the two scenarios has great implications for the fate of phytoplankton. Our results stress the importance of investigating the spatial coupling between surface and trap data before assessing the pathways of phytoplankton carbon cycling. [Copyright &y& Elsevier]

Presents a study which examined the variations in phytoplankton biomass and composition within the Ross Sea polynya in Antarctica to test its biogeochemical characteristics. Materials and methods; Results; Discussion.

Presents information on a study which determined microplankton abundances and phytoplankton mortality rates at six stations during four cruises spanning three seasons in the Ross Sea polynya, Antarctica from 1996 to 1997. Materials and methods; Results; Discussion.

Explores the phytoplankton and microzooplankton community structure in the Ross Sea polynya from November to December 1994. Elements that may develop during the senescence of Phaeocystis blooms; Concentration of aliquots used for enumerating autotrophic and heterotrophic nanoplankton; Percentage of the total nano- and microplankton biomass found in the heterotropic biomass.

Focuses on the bacterial biomass dynamics found during the phytoplankton bloom in the Ross Sea polynya from November to December 1994. Possible factors responsible to the delay in the response time following the phytoplankton bloom; Average bacterial abundance; Euphoric zone productivity.

Describes the preliminary findings from dilution experiments conducted on the phytoplankton blooms in the Ross Sea polynya during the spring cruise from November to December 1994. Element that plays a major role in transforming phytoplankton biomass into small particles; Percentage of light depth used for the experiments; Increase of chlorophyll in the southern transect; Estimated growth rate of Phaeocystis based on changes in cell number in the dilution experiments.

Describes the preliminary findings of a study involving phytoplankton biomass and productivity in the Ross Sea polynya from November to December 1994. Maximum concentrations of chlorophyll concentrations; Mean particulate organic carbon concentrations at the surface; Average increase in productivity between the two transects.