The Agulhas Current plays a major role in heat and salt exchange between the Indian and Atlantic Oceans, yet little is known of its influence on ocean fertility. To investigate carbon production and export potential in the Agulhas Current system, we measured net primary production (NPP), nitrate and ammonium uptake, N-2 fixation, and nitrification along a transect of the current and adjacent subtropical subgyre (33.4 degrees S-35.7 degrees S) in winter when nutrient supply, and thus productivity, should be highest. Phytoplankton biomass was lowest in the current core, increasing into the subgyre as surface nitrate declined, and was dominated by nanoplankton (2.7-10 mu m; 62 +/- 5.1% of total biomass). NPP and nitrate uptake were generally high across the transect and increased from the current core into the subgyre; the rates were dominated by picoplankton (<2.7 mu m; 53-93%) in the current core and nanoplankton elsewhere (63-69%). On average, euphotic zone nitrification supplied 7.6 +/- 6.4% of the nitrate consumed by phytoplankton and N-2 fixation was also low (2.1 +/- 1.3% of new production); we thus consider nitrate uptake a reasonable proxy for new production, at least in winter. Nitrate uptake was highest at the southern edge of the current core, consistent with current-associated (sub)mesoscale mixing enhancing the upward nutrient supply. The fraction of NPP available for export (i.e., the f-ratio) was high across the transect, ranging from 0.44 to 0.69. Our data thus indicate that both total and new production are elevated across the Agulhas Current system in winter and suggest that the (sub)mesoscale dynamics associated with the current system may enhance carbon production and export in the otherwise oligotrophic southwest Indian Ocean.

This dataset contains environmental data (in situ temperature and chlorophyll a) and integrated biomass (mg C m-2) data for a number of copepod taxa, as well as total copepod biomass and abundance, on the Agulhas Bank, South Africa, as predicted by a Generalized Additive Model (GAM), during late austral spring (October-December) from 1988 to 2011. Mean environmental and copepod biomass parameters for each area and year are also provided. Relevant information on sampling and statistical analysis of spatial distributions has been extracted from the paper. Please see paper for full details and figures, including supplementary data; https://doi.org/10.1016/j.dsr2.2023.105265. Please see the Word document Huggett_et_al_2023_README.docx for a list of the data files and descriptions of the contents.

The well-established importance of Agulhas Current "leakage" for Indian-Atlantic Ocean exchange of heat and salt raises the question of the relevance of Agulhas eddies for carbon and nitrogen cycling. We measured net primary production (NPP), nitrate and ammonium uptake, and nitrification along a transect of the Cape Basin (CB) in winter 2017, including across an anticyclonic Agulhas eddy sampled at high-resolution. Euphotic zone-integrated rates of NPP and nitrogen uptake were more than three-fold lower at the eddy center than at its edges, with intermediate rates measured outside the eddy. Additionally, proportionally more nitrate was consumed at the eddy edges than at its center. Mixed-layer integrated nitrification rates were highest at the eddy-center, with lower rates at the edges and in the surrounding basin. Accounting for phytoplankton consumption of newly nitrified nitrate in the eddy decreased the carbon export potential inferred from nitrate uptake (i.e., the f-ratio) by 66% +/- 37% (vs. a 14% +/- 22% decrease across the rest of the transect). At most stations, NPP and total measured nitrogen uptake were decoupled relative to stoichiometric expectations, with roughly a third of NPP possibly supported by regenerated dissolved organic nitrogen (DON). Accounting for DON uptake decreased the average f-ratio from 0.28 +/- 0.26 to 0.22 +/- 0.21, with the highest f-ratio estimated for an eddy-edge station (0.64) and the lowest for its center (0.00). While the low productivity metrics measured in the eddy-center could imply that Agulhas leakage decreases carbon export potential in the CB, enhanced nutrient supply and consumption at eddy edges may (partly) offset this reduction.

The production and removal of ammonium (NH4+) are essential upper-ocean nitrogen cycle pathways, yet in the Southern Ocean where NH4+ has been observed to accumulate in surface waters, its mixed-layer cycling remains poorly understood. For surface seawater samples collected between Cape Town and the Marginal Ice Zone in winter 2017, we found that NH4+ concentrations were 5-fold higher than is typical for summer and lower north than south of the Subantarctic Front (0.01–0.26 µM versus 0.19–0.70 µM). Our observations confirm that NH4+ accumulates in the Southern Ocean's winter mixed layer, particularly in polar waters. NH4+ assimilation rates were highest near the Polar Front (12.9 ± 0.4 nM d−1) and in the Subantarctic Zone (10.0 ± 1.5 nM d−1), decreasing towards the Marginal Ice Zone (3.0 ± 0.8 nM d−1) despite the high ambient NH4+ concentrations in these southernmost waters, likely due to the low temperatures and limited light availability. By contrast, rates of NH4+ oxidation were higher south than north of the Polar Front (16.0 ± 0.8 versus 11.1 ± 0.5 nM d−1), perhaps due to the lower-light and higher-iron conditions characteristic of polar waters. NH4+ concentrations were also measured along five transects of the Southern Ocean (Subtropical Zone to Marginal Ice Zone) spanning the 2018/19 annual cycle. These measurements reveal that mixed-layer NH4+ accumulation south of the Subantarctic Front derives from sustained heterotrophic NH4+ production in late summer through winter that, in net, outpaces NH4+ removal by temperature-, light-, and iron-limited microorganisms. Our observations thus imply that the Southern Ocean becomes a biological source of CO2 to the atmosphere in autumn and winter not only because nitrate drawdown is weak but also because the ambient conditions favour net heterotrophy and NH4+ accumulation.

Nearshore water quality can be highly impacted by anthropogenic activities ongoing along the coast, the effects of which on natural environments can be permanent and irreversible, with consequences for ecosystem biodiversity and functioning, as well as for associated services. Benthic filter feeders (e.g., mussels) provide several services for coastal regions, including improving water quality by reducing eutrophication, being a major source of food for humans, and as a habitat-forming species. Here, we seek to understand the role of benthic filter feeders in enhancing water quality in an urban coastal system in order to assess their role as ecosystem service providers and how they should be included in ecosystem-based evaluations. Using as a model False Bay, South Africa's largest natural bay and a socio-economic hotspot, this multidisciplinary study was designed to identify possible pollution sources to a highly-urbanised coastal region, assess their effects on several biological and biogeochemical parameters, and evaluate the role of mussels in mitigating these anthropogenic inputs. We consider several sources of pollution, including nutrient loading from wastewater and river outflows, heavy metals, and aerosol deposition. We find that pollutant inputs are largely attenuated by the circulation of the bay and by the presence of filter feeders that bioaccumulate contaminants, thereby removing them from coastal waters. Our work thus emphasizes the potential for mussels and natural abiotic processes to ameliorate anthropogenic impacts, although these mitigation strategies are not without environmental risk. We recommend that such information should be included in national assessments used to develop appropriate strategies and policies for coastal environmental management and conservation.

Microalgae and bacteria living in Antarctic sea-ice play a fundamental role in polar-ocean biogeochemistry, accounting for -25% of primary production in sea-ice-covered regions of the Southern Ocean. However, a paucity of measurements in the marginal ice zone (MIZ), particularly during winter, means that sea-ice algal dynamics are poorly understood, resulting in uncertainties in the drivers of Antarctic food-web and carbon-cycle variability. We investigated the relationships among biogeochemical parameters and sea-ice algae in the MIZ of the Indian Southern Ocean in winter 2017 through measurements of sea-ice algae functional groups, chlorophyll, and nutrient concentrations, in the sea-ice and the underlying surface seawater. We observed an abundant, active algal community within the sea-ice, which we attribute to passing cyclones that kept the ice permeable, along with biogeochemical and irradiance conditions that allowed for algal growth. The sea-ice conditions facilitated the proliferation of sea-ice diatoms, to chlorophyll concentrations >5 mu g/L (versus 1.5 mu g/L in the underlying seawater). The dominant taxa were of the genera Fragilariopsis, Pseudo-nitzschia, Coscinodiscus, and Chaetoceros spp., similar to those observed previously near east Antarctica. While sea-ice algae growth should have caused significant nutrient drawdown, nitrate concentrations were higher in the ice than in the underlying seawater. Since silicate was strongly drawn down, presumably by sympagic diatoms, we attribute the elevated nitrate concentrations to an active microbial loop (i.e., regeneration and nitrification). Our study highlights the ecological significance of the Southern Ocean MIZ in winter, providing a vital environment for overwintering sea-ice algae and bacteria that remain active despite the harsh conditions.

1 Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, 6 Cape Town, South Africa 7 2 Southern Ocean Carbon and Climate Observatory (SOCCO), CSIR, Rosebank, Cape Town, 8 South Africa 9 3 Department of Conservation and Marine Sciences, Cape Peninsula University of 10 Technology, Cape Town, South Africa 11 4 Marine and Antarctic Research centre for Innovation and Sustainability (MARIS), 12 University of Cape Town, Cape Town, South Africa 13 14 * Corresponding author: smtsha023@myuct.ac.za 15 16

St Helena Bay (SHB), a retention zone in the southern Benguela upwelling system, supports 40–50% of the region's primary productivity. It also experiences harmful algal blooms (HABs) and hypoxic conditions that are difficult to predict given the high sub-seasonal variability. To better understand this variability, net primary production (NPP), nitrate and ammonium uptake, and phytoplankton community composition were monitored for ten days in SHB during summer, the season of enhanced upwelling. A period of active upwelling (days 1–5) was followed by one of relaxation (days 6–10). During upwelling, the mixed layer was deeper than the euphotic zone and phytoplankton were light-limited, evidenced by high ambient nitrate concentrations and low rates of NPP and nitrate uptake. During relaxation, stratification increased, restricting phytoplankton production to a shallow euphotic zone in which nitrate was exhausted after three days; the subsequent decline in NPP and nitrate uptake rates confirm that nutrient availability rapidly succeeded light as the dominant control on productivity. Nanophytoplankton (mainly the small diatoms, Chaetoceros spp. and Skeletonema costatum) dominated the biomass, NPP, and nitrate uptake throughout the experiment. We attribute this to their ability to respond quickly to newly-upwelled nitrate and to sustain elevated nitrate uptake rates for longer than pico- and microphytoplankton. They may also engage in luxury nitrate uptake, storing nitrate intracellularly under high-nutrient conditions and assimilating it later when nutrients are depleted. Additionally, Chaetoceros spp. and S. costatum are chain-formers, benefitting from the advantages of being small (i.e., as individual cells) and large (i.e., when aggregated into chains). A weakening of surface stratification late in the experiment may have prevented dinoflagellates, some of which are HAB species, from succeeding the diatoms. One implication of this is that understanding the rapid cycling between light and nutrient limitation of SHB phytoplankton, induced by an actively-upwelling versus stratified water column, may improve our capacity to anticipate HABs and the associated hypoxic events.

Spring and summer Southern Ocean phytoplankton communities have been well characterized, but winter communities are often overlooked. Diatoms are a major contributor to Southern Ocean particulate organic carbon (POC) production and export, and exert a strong control on Antarctic surface and Subantarctic ther-mocline nutrient concentrations, thus influencing the low-latitude nutrient supply. Understanding diatom distribution, seasonal community progression, and diatom-nutrient interactions is vital for improving biogeochemical models, particularly as climate change alters polar phytoplankton communities. We investigated the distribution of nanophytoplankton (>= 3 mu m) and their associated biogeochemical environments along 30 degrees E across the Indian Southern Ocean (Subtropical Zone; STZ to Antarctic Zone; AZ) in July 2017. Phytoplankton productivity inferred from chlorophyll-a was low compared to previously-published summertime values. Mixed layer nitrate (NO3-) and phosphate (PO43-) concentrations were similar to existing summer measurements, likely due to a combination of deep mixing and biological uptake, while silicic acid (Si(OH)(4)) was high relative to summer, particularly south of the Polar Front (PF). The PF emerged as an important biogeochemical boundary separating relatively high chlorophyll-a, flagellate-dominated northern waters from southern waters characterized by low chlorophyll-a and diatom-dominated communities. Mixed layer-integrated biogenic silica (bSi) decreased 12-fold from the southern AZ to the STZ, resulting in a strong south-north gradient in bSi-per-chl-a (from 3.6 to 0.1 mol/g) and bSi-per-POC (from 0.22 to 0.016 mol mol(-1)). We attribute this to a high abundance of heavily-silicified diatom species (e.g., Fragilariopsis spp., which dominated the AZ diatom community) and a limited contribution of other phytoplankton to chlorophyll-a and POC to the south - indeed, diatoms constituted 5-67% of the total POC south of the PF and only 3-7% to the north. While mixed-layer Si(OH)(4) concentrations decreased more than NO3- across the PF, likely due to preferential Si(OH)(4) consumption by iron limited diatoms, our data imply a lower ratio of Si(OH)(4) to NO3- uptake compared to summer. This suggests that iron limitation may be less severe in the AZ in winter, at least in the west Indian sector. We conclude that AZ diatoms impact the low-latitude nutrient supply and are potentially important for carbon export in winter, despite the lower productivity of the Southern Ocean during this season.

Mesoscale eddies are major features in ocean dynamics that significantly influence ocean production depending on their polarity, life-stage, location and time of formation. Eddies in the south-west Indian Ocean have been the focus of many studies, especially in the Mozambique Channel where southward-moving cyclonic and anticyclonic eddies feed into the Agulhas Current. However, eddies formed south of Madagascar that move westwards towards the coast of South Africa have been much less studied, despite their significant contribution to the Agulhas Current. This study aims to understand how the plankton communities (i.e. pico-, nano-, phyto-, microzoo- and mesozooplankton) are distributed in a cyclonic eddy generated off southern Madagascar. We also investigate a possible link between the Madagascar shelf and the eddy. Temperature, salinity, fluorescence and plankton abundance were measured at 25 stations spaced at 18.5 km intervals across a young (similar to 1 month old) cyclonic eddy, and at three locations on the Madagascar shelf, in July 2013. A patch of enhanced chlorophyll a concentration was found at four stations in the eddy, homogeneously distributed vertically in the upper mixed layer and characterised by a higher proportion of diatoms compared to the other stations. Abundances of picoplankton, nanoplankton, ciliates and dinoflagellates were very similar within and outside the eddy. Zooplankton followed a contrasting pattern with higher biomass at four stations just to the west of this high diatom patch. The mesozooplankton composition at these four stations revealed greater species similarities with the Madagascar shelf than with the other stations, suggesting a link between the shelf and the western side of the eddy. The periphery of the eddy seems to have been primarily influenced by a strong geostrophic current propagating from the southern shelf of Madagascar which then wrapped itself around the eddy. This filament may be the source of the high zooplankton biomass in the eddy periphery, which would account for the similarities in zooplankton community composition between the eddy periphery and the shelf.

The Department of Science and Technology's (DST's) 10-year Global Change Grand Challenge programme requires platforms to 'attract young researchers to the region and retain them by exciting their interest in aspects of global change; while developing their capacity and professional skills in the relevant fields of investigation' 1 .In addition, in July 2014, President Zuma officially launched Operation Phakisa and announced that a key target of this Oceans Economy initiative would be 'for the Department of Higher Education and Training to drive alignment between theoretical and workplace learning' 2 .SEAmester -South Africa's recently established Class Afloat -achieves just that.SEAmester introduces marine science as an applied and cross-disciplinary field to students who have shown an affinity for core science disciplines.It identifies with government's National Development Plan 3 on education, training and innovation -critical to South Africa's long-term

Most estuarine studies in South Africa have focused on larger estuaries, while numerous, smaller, temporarily open estuaries have received little research attention. This study examines the phytoplankton distribution in relation to physical and chemical variables through a quasi-seasonal cycle (from March to November 1997, including a flood event) in the Nyara, one of the smaller Eastern Cape estuaries. Vertical stratification of salinity was a persistent feature, indicating limited mixing of the water-column, except during flood events. Average chlorophyll-a concentrations for the estuary decreased from 1.3μg/l in March to 0.1μg/l in September, corresponding to the seasonal decline in rainfall and freshwater inflow. With the exception of the period during the November flood, the shallowness of the estuary as well as low chlorophyll-a concentrations always allowed an average of 30% of surface light to reach the sediment surface. Diatoms were most abundant in the water-column in March (mean = 105 cells/l), but numbers declined to below 104 cells/l in May and September. Picoplankton numbers increased over the study period, while nanociliates and dinoflagellates were fairly constant at 104 and 105 cells/l respectively. The most common group in terms of carbon, were the dinoflagellates. Freshwater, or possibly the input of nutrients associated with it, appears to be the most significant factor affecting phytoplankton biomass in the Nyara Estuary. When not receiving pulses of nutrients through freshwater inflow, the Nyara is best described as a predominantly low nutrient, low phytoplankton biomass, stratified system, dominated by the microbial food-web and possibly fed by detritus. This state can however, be altered rapidly by flood events that cause mouth opening.

PURPOSE: Disenrollment behavior has important implications for the cost, quality, and continuity of care under Medicare Managed Care (MMC). Although disenrollment data are sometimes used as a quality indicator, uncertainty about how personal and plan characteristics influence disenrollment decisions complicates their use as a valid quality measure. METHODS: To determine how personal characteristics influence MMC disenrollment decisions, we analyzed data from the Medicare Beneficiary File, the US Census, and MMC plans. The study population consisted of MBs enrolled in one of 9 MMC Risk plans during a 6 month period, January through June 1998. We estimated logistic regression models with disenrollment (i.e. leaving a plan during the study period) as the dependent variable, and individual MB characteristics (age, gender, race, education, economic indicators) as independent variables, adjusting for plan characteristics. For disenrolling MBs we also estimated models using rapid disenrollment and disenrollment to fee-for-service (FFS) as dependent variables. RESULTS: Of 84,443 Medicare Beneficiaries (MBs) enrolled in one of 9 MMC risk plans in Connecticut, 4,102 (4.9%) disenrolled at least once during the 6 month study period, January through June, 1998. Of 4,022 MBs who disenrolled only once, 2,661 (66.2%) joined another MMC plan, while 1,361 (33.8%) returned to fee-for-service (FFS). 948 (23.6%) disenrolled "rapidly" (<90 days after joining). MBs who switched to another MMC plan had been enrolled longer than those who went back to FFS (Median 303 vs. 120 days, p < .001). In logistic regression analysis, adjusting for plan characteristics, disenrollment was associated with non-White race/ethnicity, age <75, and residence in areas with higher poverty rates and lower education levels. Among disenrollees, rapid disenrollment and disenrollment to FFS were associated with female gender, age ≥75, and residence in areas with higher poverty rates. CONCLUSION: Higher disenrollment among poorer, minority MBs suggests that these groups may have more problematic experiences with MMC, or may be more severely affected by coverage limitations. The higher rate of rapid disenrollment and return to FFS among older, poor, female disenrollees may be related to inadequate plan information at enrollment. Further research is needed do determine how patient characteristics influence disenrollment decisions, and whether MMC plans can improve member retention by paying more attention to the medical needs of specific groups and the information provided to enrollees.

Seasonal surveys were carried out in the shallow, well-conserved temporarily open Nyara Estuary in the Eastern Cape, South Africa. Although temporarily open estuaries constitute over 70% of estuaries in South Africa, few data are available on the structure and functioning of these systems in the region. The main aim of the study was to test the hypothesis that owing to irregular nutrient input, temporarily open estuaries may exhibit poorly-developed pelagic food webs, with low phytoplankton and pelagic biomass in general. Results from the investigation indicate that phytoplankton biomass is generally low, and dominated by pico and nanophytoplankton, with almost total absence of diatoms. This may be explained in terms of the regenerated, rather than new, nutrient pool that is available to phytoplankton as a result of the semipermanently closed nature of the estuary. Chlorophyll a levels never exceeded 4·1mgm−3. However, low levels of phytoplankton biomass were in contrast to relatively large stocks of zooplankton, which attained maximum levels of about 2g (dry weight) m−3. Thus, there is an imbalance between the biomass of the primary producers and that of the consumers. In order to satisfy the zooplankton energy budget, either phytoplankton production rates are extremely high or a substantial proportion of their food demands must be met through utilization of alternative sources, such as detritus, protozoans and microphytobenthos. It is suggested that microphytobenthos in particular may play a major role in this regard because of the prevailing good conditions for its growth in this type of ecosystem. Microbenthic chlorophyll a concentrations in the Nyara Estuary are in the upper range of values measured in South African estuaries, with an average of ≅190mgm−2. Further studies are needed to investigate the ability of the dominant species of zooplankton to ingest and assimilate benthic microalgae, particularly during the day when most species remain in close association with the substratum.

Measurements of electron transport system (ETS) activity of microplankton and C-14 primary production were made during an Anchor Station in St Helena Bay in March 1987 and a cruise off Namibia in September 1988. ETS measurements for the Benguela are similar to those recorded in other upwelling regions. Activities depended on the phase of the phytoplankton bloom and hence on the phase of upwelling. When all three parameters are integrated above the 1% light level, there is a high correlation between ETS activity and both primary production and chlorophyll a concentration. The total respiratory demand during the Anchor Station, from microplankton, mesozooplankton and bacteria, ranges from 1,0 to 3,5 g C.m-2.day-1 and the R:P ratio from 0,24 to 0,75 with a mean value of 0,50.