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The response of phytoplankton to different temperatures has been studied for decades, and although the general relationship of growth to temperature is known to be exponential, the responses at very low temperatures (from −2 to +2°C) are poorly constrained. To address this uncertainty, we conducted a series of growth experiments with eight species of polar phytoplankton; each was grown in batch culture at temperatures ranging from −1.5 to 4°C, and growth rates determined from changes in cell abundance and chlorophyll. We found that different species responded differently to temperature, although the general response was an increase in growth rate at higher temperatures. The mean response from all taxa was less (ca. 35% less) than predicted from the classic [Eppley, R. W. (1972) Temperature and phytoplankton growth in the sea. Fish. Bull., 7, 1063–1085.] formulation. Maximum growth rates also were most reliable when determined from changes in cell abundance. If the variability in maximum growth rates among taxa is reflective of in situ responses to polar temperature increases, it will be challenging to predict the assemblage response to elevated in situ temperatures in future years.

The database on ocean primary productivity comprises over two decades (1985–2008) of data that the authors have participated in collecting, using the assimilation of inorganic 14C through photosynthesis, in incubations carried out in situ. The dataset is perhaps unique in that it uses, overwhelmingly, consistent methodology while covering a wide geographic range. Ancillary data are included. Using the database, it is hoped that investigators can test for the relationships among the environmental drivers for ocean productivity, the meaning of the 14C method in terms of phytoplankton physiology and the dynamics in the water column, and as a resource for further development of productivity algorithms using satellite ocean color imagery.

In recent decades the development of desalination plants (DPs) for desalination of seawater has increased dramatically, while little attention has been paid to the effects of this activity on the accumulation of heavy metals (HMs) in the sediments of affected ecosystems. The present study was implemented to evaluate (1) heavy metal accumulation in sediments impacted by DPs discharges, (2) spatial and temporal changes of HMs and the contamination degree by different types of pollution indexes (single and integrated indices), and (3) ecological risk assessment of cadmium (Cd), lead (Pb), zinc (Zn) and copper (Cu) in sediments affected by DPs discharges. A total of 288 sediment samples were collected seasonally at 24 stations from November 2019 to October 2020. Analysis of HMs concentrations in sediments near the desalination plant discharge provided evidence of local contamination. Maximum concentration of Cu and Pb elements were found in sediments near the desalination plant discharge point. Hierarchical cluster analysis revealed clear segregation of stations impacted by desalination plant discharges and away from discharges. The values of PLI index in sediments of all sampling stations were 1, indicating that there was no metal pollution by this index. The potential ecological risk index (PERI) ranged from 5.33 ± 0.51 to 11.81 ± 4.98 in sampling sediments and were classified as "low potential ecological risk". These results demonstrate that the DPs discharge increased HMs concentrations in the sediments in close proximity to outlets. The necessary and practical regulations and policies regarding the rejection of the DPs discharge and disposal of chemical compounds must be implemented and enforced.

Diapausing populations of Calanus finmarchicus at depth in the Lofoten Basin (LB) return to the continental shelf and slope off the Lofoten-Vesterålen Islands during the phytoplankton spring bloom to feed and spawn, forming surface swarms with a great abundance. To study how overwintering populations of C. finmarchicus move with the deep currents and return to the shelf, Lagrangian transport characteristics of particles in deep water between 2008 and 2019 were analyzed using Global Ocean Reanalysis and Simulation re-analysis data and Lagrangian Coherent Structures (LCSs). Our analyses revealed that persistent eastward transport of diapausing C. finmarchicus between LB and continental slope occurred mainly between 600 and 1,100 m in the Arctic Intermediate Water. The consistency of the vertical distributions of C. finmarchicus abundance and salinity further suggests that physical factors control the horizontal distribution of the species. Hovmöller diagrams of kinetic energy indicate that there is an eastward advection of mean current at depth. The co-occurrence between the eastward transport of LCSs and the eastward advection of the mean current provides direct evidence that the life history of C. finmarchicus is subjected to physical control in the Norwegian Sea.

The cosmopolitan algae Phaeocystis globosa forms harmful algal blooms frequently in a number of tropical and subtropical coastal regions in the past two decades. During the bloom, the giant colony, which is formed by P. globosa, is the dominant morphotype. However, the microenvironment and the microbial composition in the intracolonial fluid are poorly understood. Here, we used high-throughput 16S rRNA amplicon sequencing to examine the bacterial composition and predicted functions in intracolonial fluid. Compared with the bacterial consortia in ambient seawater, intracolonial fluids possessed the lower levels of microbial richness and diversity, implying selectivity of bacteria by the unique intracolonial microenvironment enclosed within the P. globosa polysaccharide envelope. The bacterial consortia in intracolonial fluid were dominated by Balneola (48.6% of total abundance) and Labrezia (28.5%). The bacteria and microbial function enriched in intracolonial fluid were involved in aromatic benzenoid compounds degradation, DMSP and DMS production and consumption, and antibacterial compounds synthesis. We suggest that the P. globosa colonial envelope allows for the formation of a specific microenvironment; thus, the unique microbial consortia inhabiting intracolonial fluid has close interaction with P. globosa cells, which may benefit colony development.

Due to extremely high rates of evaporation and low precipitation in the Persian Gulf, discharges from desalination plants (DPs) can lead to ecological stresses by increasing water temperatures, salinities, and heavy metal concentrations, as well as decreasing dissolved oxygen levels. We discuss the potential ecological impacts of DPs on marine organisms and propose mitigating measures to reduce the problems induced by DPs discharges. The daily capacity of DPs in the Persian Gulf exceeds 11 million m3 per day, which is approximately half of global daily freshwater production; multistage flash distillation (MSF) is the dominant desalination process. Results from field and laboratory studies indicate that there are potentially serious and chronic threats to marine communities following exposure to DP discharges, especially within the zoobenthos, echinodermata, seagrasses, and coral reefs. DP discharges can lead to decreases in sensitive species, plankton abundance, hard substrate epifauna, and growth rates of seagrasses. However, the broad applicability of any one of these impacts is currently hard to scale because of the limited number of studies that have been conducted to assess the ecological impacts of DP discharge on Persian Gulf organisms. Even so, available data suggest that appropriately sited, designed, and operated DPs combined with current developments in impingement and entrainment reduction technology can mitigate many of the negative environmental impacts of DPs.

The distributions of nutrients, chlorophyll (chl), particulate organic carbon (POC), particulate organic nitrogen (PON), and biogenic silica (BSi) were compiled from 42 cruises to the Ross Sea continental shelf to generate a monthly climatology. Nutrients were elevated in spring and removal began in early spring (November) and continued through December, at which time nitrate and phosphate removal largely ceased. Silicic acid concentrations also began to decrease in November, but continued throughout the summer. Chlorophyll concentrations increased simultaneously with nitrate removal and became maximal in December, after which time they decreased. Concentrations of chlorophyll remained relatively low and constant through February, although smaller regions remained elevated and contributed to the expected substantial spatial variability. Particulate organic carbon and nitrogen concentrations paralleled chlorophyll concentrations through the spring maximum, and also decreased in early summer. However, a notable increase in particulate organic carbon occurred later in summer that was largely independent of chlorophyll changes, resulting in a dramatic rise of POC:chl ratios. Biogenic silica concentrations were low through December but continued to increase throughout summer. We suggest that the spring pigment and particulate matter concentrations resulted from phytoplankton blooms dominated by the haptophyte Phaeocystis antarctica, consistent with repeated observations in the region. We further suggest that the late summer POC increase was due to growth of diatoms that exhibited enhanced POC:chl ratios due to iron limitation. POC concentrations in late summer were similar to those in spring, indicating that the summer bloom was substantial. Estimates of productivity from a bio-optical model using variable POC:chl ratios suggest that the summer blooms may be a significant contributor to seasonal productivity, and that estimates of productivity based on satellite pigments underestimate annual production by at least 70%. Satellite productivity estimates have substantial uncertainties in the Ross Sea and need to be constrained using in situ constituent ratios.

Seasonal resource pulses can have enormous impacts on species interactions. In marine ecosystems, air-breathing predators often drive their prey to deeper waters. However, it is unclear how ephemeral resource pulses such as near-surface phytoplankton blooms alter the vertical trade-off between predation avoidance and resource availability in consumers, and how these changes cascade to the diving behaviour of top predators. We integrated data on Weddell seal diving behaviour, diet stable isotopes, feeding success and mass gain to examine shifts in vertical foraging throughout ice break-out and the resulting phytoplankton bloom each year. We also tested hypotheses about the likely location of phytoplankton bloom origination (advected or producedin situwhere seals foraged) based on sea ice break-out phenology and advection rates from several locations within 150 km of the seal colony. In early summer, seals foraged at deeper depths resulting in lower feeding rates and mass gain. As sea ice extent decreased throughout the summer, seals foraged at shallower depths and benefited from more efficient energy intake. Changes in diving depth were not due to seasonal shifts in seal diets or horizontal space use and instead may reflect a change in the vertical distribution of prey. Correspondence between the timing of seal shallowing and the resource pulse was variable from year to year and could not be readily explained by our existing understanding of the ocean and ice dynamics. Phytoplankton advection occurred faster than ice break-out, and seal dive shallowing occurred substantially earlier than local break-out. While there remains much to be learned about the marine ecosystem, it appears that an increase in prey abundance and accessibility via shallower distributions during the resource pulse could synchronize life-history phenology across trophic levels in this high-latitude ecosystem.

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(2), (2021): e2020JC016856, https://doi.org/10.1029/2020JC016856.

The Arctic Ocean is more susceptible to ocean acidification than other marine environments due to its weaker buffering capacity, while its cold surface water with relatively low salinity promotes atmospheric CO2 uptake. We studied how sea-ice microbial communities in the central Arctic Ocean may be affected by changes in the carbonate system expected as a consequence of ocean acidification. In a series of four experiments during late summer 2018 aboard the icebreaker Oden, we addressed microbial growth, production of dissolved organic carbon (DOC) and extracellular polymeric substances (EPS), photosynthetic activity, and bacterial assemblage structure as sea-ice microbial communities were exposed to elevated partial pressures of CO2 (pCO(2)). We incubated intact, bottom ice-core sections and dislodged, under-ice algal aggregates (dominated by Melosira arctica) in separate experiments under approximately 400, 650, 1000, and 2000 mu atm pCO(2) for 10 d under different nutrient regimes. The results indicate that the growth of sea-ice algae and bacteria was unaffected by these higher pCO(2) levels, and concentrations of DOC and EPS were unaffected by a shifted inorganic C/N balance, resulting from the CO2 enrichment. These central Arctic sea-ice microbial communities thus appear to be largely insensitive to short-term pCO(2) perturbations. Given the natural, seasonally driven fluctuations in the carbonate system of sea ice, its resident microorganisms may be sufficiently tolerant of large variations in pCO(2) and thus less vulnerable than pelagic communities to the impacts of ocean acidification, increasing the ecological importance of sea-ice microorganisms even as the loss of Arctic sea ice continues.

Large aggregations of the copepod Calanus finmarchicus occur each spring in the shelf-slope-oceanic regions off the Lofoten-Vesterålen Islands where productive fisheries have traditionally supported local economies. The retention and off-shelf transport of populations of C. finmarchicus populations were studied by analyzing ocean color remote sensing, satellite altimetry data, and Lagrangian Coherent Structures (LCS) between 2010 and 2019. Our analysis revealed the existence of a transport barrier reoccurring at the shelf break that retains C. finmarchicus on the shelf for 30–70 days in the spring when C. finmarchicus were seasonally ascending to the surface layer. The analysis of baroclinic and barotropic energy conversions indicated that the topographically steered Norwegian Atlantic Current is the primary mechanism in the formation of the transport barrier, which restricts exchanges of C. finmarchicus populations between shelf and oceanic waters. In the mid- to late April, an increase in baroclinicity leads to an increase in mesoscale eddies generated on the shelf break near Lofoten-Vesterålen Islands, breaking down transport barriers and causing off-shelf transport of C. finmarchicus. The transport barrier predictably reoccurs in early spring which supports the entrapment of C. finmarchicus in the shelf region.

Phytoplankton biomass in estuarine and continental shelf regions are regulated and modified by physical processes, but these interactions have mostly been investigated at a scale of tens of kilometers, and the role of meso- to sub-mesoscale dynamical processes of freshwater plumes in regulating the spatial and temporal variations of algal biomass is largely unknown. To assess the importance of features at these scales, high-resolution (horizontal spacing smaller than 1 km) cross-sectional profiles of hydrographic and biogeochemical variables were collected in the lower Changjiang Estuary and adjacent continental shelf with a towed, undulating vehicle equipped with sensors measuring fluorescence, turbidity and irradiance. Discrete stations were also occupied to allow for the characterization of nutrients. Multiple physical features at different scales regulated the spatial variation of phytoplankton biomass. Phytoplankton biomass was initialized by an improved irradiance field driven by reduced turbidity together with a rapid development of subsurface stratification at the main plume front (isohaline of 23) downstream from the turbidity maximum zone. Phytoplankton blooms did not occur until outcrops located within the main front that were characterized by surface convergence and downwelling, which contributed to large algal biomass by mass trapping and enhanced light penetration. Wave-like features were detected seaward of the main front, coinciding with deacceleration of currents, indicating that they are front-released internal waves that increase algal retention time. This study revealed the critical role of small-scale processes near the plume front in triggering phytoplankton blooms under the large-scale context of improved light conditions, coastal upwelling and nutrient additions from intruding oceanic waters.

Autonomous vehicles are able to sample continuously and over different spatial and temporal scales. They routinely determine biological properties, such as phytoplankton fluorescence and optical backscatter. Using appropriate calibration samples taken in close proximity to the vehicles, these properties can be converted to absolute quantities of chlorophyll and particulate organic carbon (POC), respectively. By sampling through time in a relatively uniform and restricted area, the vertical distributions of particulate organic carbon can be determined, and when coupled to an estimate of surface phytoplankton production, an estimate of the vertical flux of carbon can be made from the changes in POC through time. These flux estimates in turn can be used to determine the extent of remineralization within the water column. Such remineralization rates are difficult to determine using alternative sampling techniques, and using autonomous vehicles in this manner can greatly improve our understanding of the rates of vertical flux of organic matter from the upper layers of the ocean to the deep sea, as well as the time varying rates of remineralization. These data can then provide critical inputs into global carbon cycle estimates and the role of the ocean in sequestering carbon.

International audience; A one-dimensional numerical model that includes the complex life cycle of Phaeocystis antarctica, diatom growth, dissolved iron (dFe) and irradiance controls, and the taxa's response to changes in these variables is used to evaluate the role of different iron sources in supporting phytoplankton blooms in the Ross Sea. Simulations indicate that sea ice melt accounts for 20% of total dFe inputs during low light conditions early in the growing season (late November-early December), which enhances blooms of P. antarctica in early spring. Advective inputs of dFe (60% of total inputs) maintain the P. antarctica bloom through early January and support a diatom bloom later in the growing season (early to mid-January). In localized regions near banks shallower than 450 m, suspension of iron-rich sediments and entrainment into the upper layers contributes dFe that supports blooms. Seasonal dFe budgets constructed from the simulations show that diatom-associated dFe accounts for the largest biological reservoir of dFe. Sensitivity studies show that surface input of dFe from sea ice melt, a transient event early in the growing season, sets up the phytoplankton sequencing and bloom magnitude, suggesting that the productivity of the Ross Sea system is vulnerable to changes in the extent and magnitude of sea ice.

The export of particulate organic carbon (POC) from the sea surface is an essential part of the biological pump. Export fluxes are the result of what is produced in surface water and how much is consumed during particle sinking in the water column. In the Ross Sea, fluxes of POC and total mass are well correlated implying that particle fluxes are dominated by biogenic debris.Here, we report new and reference data of vertical particle fluxes to below the productive layer obtained on decadal time scales (1990-2017) by automatic sediment traps tethered to moorings in the western Ross Sea (Antarctica). Compilation of all data available in the Ross Sea (23 sites, >1000 samples) shows that annual POC fluxes to below 200 m average 4.4±3.3 g C m-2 y-1. Particle fluxes are relatively low when primary production is high (spring-summer) followed by enhanced sedimentation in late summer-fall. The high degree of decoupling between production and sedimentation is unusual compared to records of Antarctic Peninsula and may represent low grazing rates. Furthermore, data exhibit a large interannual variability and a decreasing trend over time, with a clear shift after 2000. Do the reduced export fluxes depend on lower biological production, enhanced OM consumption, or other processes (e.g., lateral transfer of biogenic particles outside the study area)?Satellite observations allow us to reconstruct the seasonal and interannual change of chlorophyll biomass, and sea ice extent and duration. Water temperature recorded at mid-depth is used to monitor the different intrusion over time of CDW, the main driver of temporal variability of Fe supply for the Ross Sea. Time series of particle fluxes, chlorophyll, sea ice cover and mid-depth temperature will be compared in order to test if the recent reduction of downward particle fluxes depend on primary production changes.

A 5-km horizontal resolution regional ocean–sea ice–ice shelf model of the Ross Sea is used to examine the effects of changes in wind strength, air temperature, and increased meltwater input on the formation of high-salinity shelf water (HSSW), on-shelf transport and vertical mixing of Circumpolar Deep Water (CDW) and its transformation into modified CDW (MCDW), and basal melt of the Ross Ice Shelf (RIS). A 20% increase in wind speed, with no other atmospheric changes, reduced summer sea ice minimum area by 20%, opposite the observed trend of the past three decades. Increased winds with spatially uniform, reduced atmospheric temperatures increased summer sea ice concentrations, on-shelf transport of CDW, vertical mixing of MCDW, HSSW volume, and (albeit small) RIS basal melt. Winds and atmospheric temperatures from the SRES A1B scenario forcing of the MPI ECHAM5 model decreased on-shelf transport of CDW and vertical mixing of MCDW for 2046–61 and 2085–2100 relative to the end of the twentieth century. The RIS basal melt increased slightly by 2046–61 (9%) and 2085–2100 (13%). Advection of lower-salinity water onto the continental shelf did not significantly affect sea ice extent for the 2046–61 or 2085–2100 simulations. However, freshening reduces on-shelf transport of CDW, vertical mixing of MCDW, and the volume of HSSW produced. The reduced vertical mixing of MCDW, while partially balanced by the reduced on-shelf transport of CDW, enhances the RIS basal melt rate relative to the twentieth-century simulation for 2046–61 (13%) and 2085–2100 (17%).

The Ross Sea is one of the most well studied marginal seas in Antarctica. However, information of phytoplankton and in particular dinoflagellate diversity is limited. In this present study, samples at the surface and depth of the chlorophyll maximum at 40 stations during summer (2011–2012) were investigated for Protoperidinium species. A new species has described: Protoperidinium smithii H. Doan-Nhu, L. Phan-Tan et L. Nguyen-Ngoc sp. nov. This species belongs to subgenus Minusculum and have all of the features of this group. The new species is similar to Protoperidinium bipes (Paulsen) Balech 1974 and Protoperidinium defectum (Balech) Balech 1974. However, it is clearly differentiated from both species by its cell body shape, very short right antapical spine, a short apical horn, and the largest 1′′′ plate among species in subgenus Minusculum. Protoperidinium defectum is also amended by several characteristics including detail plates tabulation since the first description in 1965.

The Ross Sea is a region characterized by high primary productivity in comparison to other Antarctic coastal regions, and its productivity is marked by considerable variability both spatially (1–50 km) and temporally (days to weeks). This variability presents a challenge for inferring phytoplankton dynamics from observations that are limited in time or space, which is often the case due to logistical limitations of sampling. To better understand the spatiotemporal variability in Ross Sea phytoplankton dynamics and to determine how restricted sampling may skew dynamical interpretations, high-resolution bio-optical glider measurements were assimilated into a one-dimensional biogeochemical model adapted for the Ross Sea. The assimilation of data from the entire glider track using the micro-genetic and local search algorithms in the Marine Model Optimization Testbed improves the model–data fit by ∼ 50 %, generating rates of integrated primary production of 104 g C m−2 yr−1 and export at 200 m of 27 g C m−2 yr−1. Assimilating glider data from three different latitudinal bands and three different longitudinal bands results in minimal changes to the simulations, improves the model–data fit with respect to unassimilated data by ∼ 35 %, and confirms that analyzing these glider observations as a time series via a one-dimensional model is reasonable on these scales. Whereas assimilating the full glider data set produces well-constrained simulations, assimilating subsampled glider data at a frequency consistent with cruise-based sampling results in a wide range of primary production and export estimates. These estimates depend strongly on the timing of the assimilated observations, due to the presence of high mesoscale variability in this region. Assimilating surface glider data subsampled at a frequency consistent with available satellite-derived data results in 40 % lower carbon export, primarily resulting from optimized rates generating more slowly sinking diatoms. This analysis highlights the need for the strategic consideration of the impacts of data frequency, duration, and coverage when combining observations with biogeochemical modeling in regions with strong mesoscale variability.

Zooplankton (meso- and macrozooplankton) distributions and biomass are poorly known in the Ross Sea despite their importance in energy transfer within food webs and biogeochemical cycles. Mesozooplankton abundance and biomass on the continental shelf are spatially variable and span two orders of magnitude during austral summer. Selected sub-regions (near the shelf break or ice shelf) show similar variability, suggesting that other processes, either oceanographic or biological, influence zooplankton on smaller scales. Biomass at one location (76.5° S, 172° E) was consistently elevated throughout January, although the causes of this “hotspot” were unclear. At a station near the ice shelf, abundance and biomass of the pteropod Limacina antarctica was very high. Zooplankton biomass at this location was sevenfold greater than any other station, and while the high biomass was driven by pteropod contributions, copepods were also abundant. Copepods dominated the mesozooplankton composition at all other stations, comprising 90 and 78% on average of the total abundance and biomass. Zooplankton biomass comprised on average 3.96% of the total particulate carbon (0–200 m) and was weakly correlated with chlorophyll and biogenic silica. We suggest that summer zooplankton growth and biomass, while linked to organic matter concentrations, are regulated by other factors (e.g., predation by crystal krill and Antarctic silverfish), as both grazers may be responsible for significant losses. Our data indicate that, contrary to other suggestions, summer zooplankton biomass and abundance in the Ross Sea are similar to those in other Antarctic coastal regions.

The Ross Sea, a highly productive region of the Southern Ocean, is expected to experience warming during the next century along with reduced summer sea ice concentrations and shallower mixed layers. This study investigates how these climatic changes may alter phytoplankton assemblage composition, primary productivity and export. Glider measurements are used to force a one-dimensional biogeochemical model, which includes diatoms and both solitary and colonial forms of Phaeocystis antarctica. Model performance is evaluated with glider observations, and experiments are conducted using projections of physical drivers for mid- and late-21st century. These scenarios reveal a 5% increase in primary productivity by mid-century and 14% by late-century and proportional increase in carbon export, which remains approximately 18% of primary production. In addition, scenario results indicate diatom biomass increases while P. antarctica biomass decreases in the first half of the 21st century. In the second half of the century, diatom biomass remains relatively constant and P. antarctica biomass increases. Additional scenarios examining the independent contributions of expected future changes (temperature, mixed layer depth, irradiance, and surface iron inputs from melting ice) demonstrate that earlier availability of low light due to reduction of sea ice early in the growing season is the primary driver of productivity increases over the next century; shallower mixed layer depths additionally contribute to changes of assemblage composition and export. This study further demonstrates how glider data can be effectively used to facilitate model development and simulation, and inform interpretation of biogeochemical observations in the context of climate change. This article is protected by copyright. All rights reserved.

The Ross Sea continental shelf supports very high productivity and phytoplankton biomass. Conventional methods, including ship-based sampling, instrumented moorings, satellite imagery, and modelling, have illustrated the typical patterns of seasonal progression of the phytoplankton blooms. While variability in the Ross Sea is substantial over relatively large scales, our understanding of smaller scales of variability (on the order of a few hours or several kilometers) is limited. Utilizing data from an autonomous glider, we examined the mechanisms driving both the transitions between stages of the phytoplankton bloom and the short-term perturbations in chlorophyll concentrations. Three phases within the bloom were defined based on chlorophyll changes: an accumulation phase, a dissipation period, and a post-dissipation phase. Short-term perturbations in chlorophyll were repeatedly observed and correlated with wind speed. These chlorophyll perturbations were strongly influenced by the degree of temporal coupling between wind events and the depth of mixing, which varied among phases. Delays of 12–24 h between wind events and chlorophyll changes were observed during the accumulation phase, but shortened to 2–12 h in the dissipation phase. Furthermore, while physical factors contributed to the observed short-term reductions in biomass and the appearance of chlorophyll at depth throughout the study, we hypothesize that turbulence during the period of maximum biomass induced aggregate formation and led to rapid vertical flux. These results suggest that the small-scale, short-term physical perturbations may induce substantial vertical redistribution of biogenic material, which in turn can have significant biogeochemical impacts.

Phaeocystis, a genus with a cosmopolitan distribution and a polymorphic life cycle, was observed during summer in the Ross Sea, Antarctica, where large blooms of this haptophyte regularly occur. The mesoscale vertical and horizontal distributions of colonies of Phaeocystis antarctica were assessed using a towed Video Plankton Recorder (VPR). The mean size of colonies was 1.20 mm, and mean abundances within the three VPR surveys were 4.86, 1.96, and 11.5 mL− 1. In addition to the typical spherical, transparent colonies, the VPR quantified an optically dissimilar form of colony that had a distinctive translucent appearance. It also measured the abundance of collapsed colonies, similar to those observed previously from cultures and mesocosms, which we called “ghost colonies”. The translucent colonial form had a different distribution than the more common colonial form, and at times was more abundant. Relative to intact colonies, the ghost colonies occurred less frequently, with mean abundances in the three surveys being 0.01, 0.08, and 0.0004 mL− 1. Ghost colonies generally were found below the euphotic zone, where they often were in greater abundance than intact colonies. However, the relationship of ghost colonies to intact P. antarctica colonies was not direct or consistent, suggesting that the formation of ghost colonies from living colonies and their appearance within the water column were not tightly coupled. Given their relative scarcity and low carbon content, it is unlikely that ghost colonies contribute substantially to vertical flux; however, it is possible that we did not sample periods of major flux events, and as a result minimized the importance of ghost colonies to vertical flux. They do, however, represent a poorly documented feature of polar haptophyte life cycles.

The vertical distribution and temporal changes in aggregate abundance and sizes were measured in the Ross Sea, Antarctica between 2002 and 2005 to acquire a more complete understanding of the mechanisms and rates of carbon export from the euphotic layer. Aggregate abundance was determined by photographic techniques, and water column parameters (temperature, salinity, fluorescence, transmissometry) were assessed from CTD profiles. During the first three years the numbers of aggregates increased seasonally, being much more abundant within the upper 200 m in late summer than in early summer from 50 to 100 m (12.5 L–1 in early summer vs. 42.9 L–1 in late summer). In Year 4 aggregate numbers were substantially greater than in other years, and average aggregate abundance was maximal in early rather than late summer (177 vs. 84.5 L–1), which we attributed to the maximum biomass and aggregate formation being reached earlier than in other years. The contribution of aggregate particulate organic carbon to the total particulate carbon pool was estimated to be 20%. Ghost colonies, collapsed colonies of the haptophyte Phaeocystis antarctica, were observed during late summer in Year 4, with maximum numbers in the upper 100 m of ca. 40 L–1. Aggregate abundance, particulate organic carbon and ghost colonies all decreased exponentially with depth, and the rate of ghost colony disappearance suggested that their contribution to sedimentary input was small at the time of sampling. Bottom nepheloid layers were commonly observed in late summer in both transmissometer and aggregate data. Late summer nepheloid layers had fluorescent material within them, suggesting that the particles were likely generated during the same growing season. Longer studies encompassing the entire production season would be useful in further elucidating the role of these aggregates in the carbon cycle of these regions.

The Ross Sea is one of the most productive regions in the Southern Ocean, with a significant role in carbon cycling as well as the massive abundance of higher trophic levels. The seasonal cycle is well established with an early summer Phaeocystis antarctica bloom that declines followed by a diatom bloom in late summer. This seasonal progression of the phytoplankton has been linked to the availability of iron in the mixed layer. Investigating the temporal progression of iron limitation is often limited by both the decreased sampling resolution from traditional platforms, such as ships, and the lack of regular deployments of specific sensors that can measure phytoplankton physiology. Through the use of a novel technique that uses the degree of quenching (NPQGlider), determined from a standard fluorometer deployed on a buoyancy glider, a proxy for iron limitation, αNPQ, was calculated for a glider time series in the Ross Sea from December 2011 to February 2012. Surface chlorophyll concentrations indicated that there were four stages: the first being a pre-bloom phase, the second in which phytoplankton growth was rapid, resulting in the accumulation of biomass; the third in which biomass in the surface layer decreased, and the fourth in which chlorophyll concentrations remained low but the POC:Chl ratio increased. The levels of NPQGlider in this region were much higher compared to other Southern Ocean regions, with the highest levels in the third phase. Similarly, αNPQ remained low throughout most of the time series except for the transition from the second to third phase when the surface biomass decreases. The increase in POC:Chl ratios in the final phase combined with the low values of αNPQ suggest the switch from a Phaeocystis antarctica bloom to a potentially non-iron limited diatom bloom. These results confirm that the application of novel methodologies to proven and reliable sensors will provide a greater understanding of biogeochemical cycles and their controls throughout the ocean.

The Ross Sea is a highly productive region of the Southern Ocean, and net phytoplankton growth varies seasonally, ranging from zero to near the temperature-limited maximum. Given that variations in growth can result from a number of factors (such as irradiance and iron concentrations), variability in net growth rate was investigated using structural equation modeling (SEM) and data collected during a January–February 2012 cruise to the Ross Sea. Structural equation modeling indicated that summer growth rates were significantly affected by iron concentrations and particulate organic carbon (POC) levels, the latter which most likely contributed to the seasonal depletion of iron by phytoplankton. Conversely, growth rates did not strongly vary with mixed layer depth (and hence irradiance). SEM indicated that if iron concentrations were increased by 1.0 standard deviation (ca. 0.12 nM), summer growth rates would increase by 0.5 standard deviation (ca. 0.07 day−1). Similarly, if POC (a measure of phytoplankton biomass in this region) was increased by 1.0 standard deviation (from 23.0 to 39.4 µmol L−1), growth rates would decrease by 0.31 standard deviations (~0.04 day−1), which we speculate is likely due to heightened iron limitation via increased total iron uptake. This modeling exercise confirms the dominant role of iron in regulating summer phytoplankton rates over the continental shelf of the Ross Sea.