Annual Net Community Production Research Articles

One-third of Southern Ocean productivity is supported by dust deposition.

Natural iron fertilization of the Southern Ocean by windblown dust has been suggested to enhance biological productivity and modulate the climate1-3. Yet, this process has never been quantified across the Southern Ocean and at annual timescales4,5. Here we combined 11 years of nitrate observations from autonomous biogeochemical ocean profiling floats with a Southern Hemisphere dust simulation to empirically derive the relationship between dust-iron deposition and annual net community production (ANCP) in the iron-limited Southern Ocean. Using this relationship, we determined the biological response to dust-iron in the pelagic perennially ice-free Southern Ocean at present and during the last glacial maximum (LGM). We estimate that dust-iron now supports 33% ± 15% of Southern Ocean ANCP. During the LGM, when dust deposition was 5-40-fold higher than today, the contribution of dust to Southern Ocean ANCP was much greater, estimated at 64% ± 13%. We provide quantitative evidence of basin-wide dust-iron fertilization of the Southern Ocean and the potential magnitude of its impact on glacial-interglacial timescales, supporting the idea of the important role of dust in the global carbon cycle and climate6-8.

Crucial Role of Bacterial Processes in the Net Community Production of the Amundsen Sea Polynya Disclosed by a Modeling Study

AbstractWe investigated seasonal net community production (NCP) variations in the productive Amundsen Sea Polynya, integrating observational data and ecosystem modeling. NCP estimates (NCPO2/Ar) from in situ O2/Ar data during the austral summer (January‐March) from 2011 to 2018 were compared with those from a one‐dimensional ecosystem model. Early January saw the highest NCPO2/Ar values ranging from 115 to 139 mmol O2 m−2 d−1 among observations. Over the summer, NCPO2/Ar gradually decreased, reaching 40 mmol O2 m−2 d−1 by late February. Late summer values, though one‐third of early January, remained notably positive, indicating net autotrophy. This persisted despite sea surface temperature dropping from >−0.4°C in January to −1.33°C in late February. Refining NCPO2/Ar, we modified bacterial dynamics in our ecosystem model. Significantly improved model performance resulted from two key modifications. First, we introduced bacterial uptake dependency on Phaeocystis primary production. Second, we heightened temperature‐dependent bacterial respiration and production approximately fifteenfold. These changes revealed NCP's remarkable sensitivity to minor temperature fluctuations (<1°C). Furthermore, modified bacterial dynamics delayed the net primary production peak by 2 weeks, underlining the importance of phytoplankton‐bacteria interaction in the ocean carbon cycle. Model results estimated annual NCP in the Amundsen Sea Polynya at 4.04 mol C m−2, aligning with summer NCP estimates (0.2–5.9 mol C m−2) in observational study. Our study advances NCP understanding in polar regions, emphasizing comprehensive observations, including bacterial processes, for understanding intricate biotic interactions. These findings align with past observations on bacterial metabolism and Phaeocystis ecological properties in the Antarctic oceans.

Quantifying Net Community Production and Calcification at Station ALOHA Near Hawai'i: Insights and Limitations From a Dual Tracer Carbon Budget Approach

AbstractA budget approach is used to disentangle drivers of the seasonal mixed layer carbon cycle at Station ALOHA (A Long‐term Oligotrophic Habitat Assessment) in the North Pacific Subtropical Gyre (NPSG). The budget utilizes data from the WHOTS (Woods Hole—Hawaii Ocean Time‐series Site) mooring, and the ship‐based Hawai'i Ocean Time‐series (HOT) in the NPSG, a region of significant oceanic carbon uptake. Parsing the carbon variations into process components allows an assessment of both the proportional contributions of mixed layer carbon drivers and the seasonal interplay of drawdown and supply from different processes. Annual net community production reported here is at the lower end of previously published data, while net community calcification estimates are 4‐ to 7‐fold higher than available sediment trap data, the only other estimate of calcium carbonate export at this location. Although the observed seasonal cycle in dissolved inorganic carbon in the NPSG has a relatively small amplitude, larger fluxes offset each other over an average year. Major supply comes from physical transport, especially lateral eddy transport throughout the year and entrainment in the winter, offset by biological carbon uptake in the spring. Gas exchange plays a smaller role, supplying carbon to the surface ocean between Dec‐May and outgassing in Jul‐Oct. Evaporation‐precipitation (E‐P) is variable with precipitation prevailing in the first half and evaporation in the second half of the year. The observed total alkalinity signal is largely governed by E‐P with a somewhat stronger net calcification signal in the wintertime.

Regionally Variable Contribution of Dissolved Organic Phosphorus to Marine Annual Net Community Production

AbstractMarine dissolved organic phosphorus (DOP) serves as an organic nutrient to marine autotrophs, sustaining a portion of annual net community production (ANCP). Numerical models of ocean circulation and biogeochemistry have diagnosed the magnitude of this process at regional to global scales but have thus far been validated against DOP observations concentrated within the Atlantic basin. Here we assimilate a new marine DOP data set with global coverage to optimize an inverse model of the ocean phosphorus cycle to investigate the regionally variable role of marine DOP utilization by autotrophs contributing to ANCP. We find ∼25% of ANCP accumulates as DOP with a regionally variable pattern ranging from 8% to 50% across nine biomes investigated. Estimated mean surface ocean DOP lifetimes of ∼0.5–2 years allow for transport of DOP from regions of net production to net consumption in subtropical gyres. Globally, DOP utilization by autotrophs sustains ∼14% (0.9 Pg C yr−1) of ANCP with regional contributions as large as ∼75% within the oligotrophic North Atlantic and North Pacific. Shallow export and remineralization of DOP within the ocean subtropics contributes ∼30%–80% of phosphate regeneration within the upper thermocline (<300 m). These shallow isopycnals beneath the subtropical gyres harboring the preponderance of remineralized DOP outcrop near the poleward edge of each gyre, which when combined with subsequent lateral transport equatorward by Ekman convergence, provide a shallow overturning loop retaining phosphorus within the subtropical biome, likely helping to sustain gyre ANCP over multiannual to decadal timescales.

New Estimates of Southern Ocean Annual Net Community Production Revealed by BGC‐Argo Floats

AbstractUsing an expanding Southern Ocean fleet of biogeochemical Argo (BGC‐Argo) floats, we developed a novel approach to estimate annual net community production (ANCP) by integrating subsurface oxygen drawdown from all available BGC‐Argo oxygen profiles. Our results suggest that, on average, 14% of remineralization occurs between 500 and 1,000 m and 15% occurs between the euphotic depth and 100 m. Using the improved methodology, we estimated total basin‐integrated ANCP in the Southern Ocean to be 3.89 GT C year−1 suggesting a more important role for the Southern Ocean in regulating oceanic carbon storage, atmospheric CO2 exchange and climate than previously assumed.

Exploring Five Methods for Estimating Net Community Production on the Siberian Continental Shelf and Slope of the Arctic Ocean

The loss of sea ice and changes to vertical stratification in the Arctic Ocean are altering the availability of light and nutrients, with significant consequences for net community production (NCP) and carbon export. However, a general lack of quality data, particular during winter months, inhibits our ability to quantify such change. As a result, two parameters necessary for calculating annual NCP, integration depth (Zint) and pre-bloom nitrate concentration (Npre), are often either assigned or estimated from summer measurements. Vertical profiles of temperature, salinity, nitrate, and dissolved oxygen were collected during three cruises conducted between August and October of 2013, 2015, and 2018 in a data-sparse region of the Arctic Ocean along the Siberian continental slope. Estimates of NCP were calculated from these data using five different methods that either assigned constant values for Zint and/or Npre or estimated these parameters from summer observations. The five methods returned similar mean values of Zint (44–54 m), Npre (5.4–5.7 mmol m–3), and NCP (12–16 g C m–2) across the study region; however, there was considerable variability among stations/profiles. It was determined that the NCP calculations were particularly sensitive to Npre. Despite this sensitivity, mean NCP estimates calculated along four transects re-occupied during the three cruises generally agreed across the five methods with two important exceptions. First, methods with pre-assigned Zint and/or Npre underestimated the NCP when the nitracline shoaled in the Laptev Sea and when high-nutrient shelf waters were advected northward from the East Siberian Sea shelf in 2015. In contrast, the methods that directly estimated both Zint and Npre did not suffer from this bias. These results suggest that assignment of Npre and/or Zint provides reasonable estimates of NCP, particularly averaged over larger spatial scales and/or longer time scales, but these approaches are not suitable for evaluating interannual variability in NCP, particularly in dynamic regions. Combining all methods across the three cruise years indicates NCP in the Laptev Sea and Lomonosov Ridge areas (10–11 g C m–2) was slightly lower than that north of Severnaya Zemlya (13 g C m–2) and in the East Siberian Sea (16 g C m–2).

Skin Temperature Correction for Calculations of Air‐Sea Oxygen Flux and Annual Net Community Production

AbstractWe evaluated the influence of skin temperature correction on the calculations of air‐sea O2 flux and annual net community production (ANCP). The skin temperature correction term (ΔT) was derived from the fifth generation European Center for Medium‐Range Weather Forecasts Reanalysis, which has large spatial and temporal variations that are consistent with independent measurements on three ocean moorings from areas with very different air‐sea heat flux and ΔT. The result revealed that ΔT is dominated by the cool skin effect (which leads to the increase in air‐to‐sea O2 flux and decrease in ANCP), except for the equatorial region or summertime when the warm layer effect is significant. Using data from three Argo profiling floats in the subarctic, subtropical, and tropical Pacific as examples, the calculations indicated that the correction could lead to −1%–+36% difference in annual air‐to‐sea O2 flux and −33%–+5% difference in ANCP.

Euphotic Zone Metabolism in the North Pacific Subtropical Gyre Based on Oxygen Dynamics

AbstractWe report in situ rates of gross oxygen production (GOP), community respiration (R), and net community production (NCP) in the North Pacific Subtropical Gyre derived from mixed layer O2/Ar measurements. The measurements were conducted between November 2013 and January 2019 at the site of the Hawaii Ocean Time‐series program. Biological O2 concentration anomalies in the mixed layer showed a consistent diel variation, with values increasing during daytime due to net community production and decreasing during nighttime due to respiration. In situ mixed layer GOP and R, determined from these variations, covaried but showed no clear seasonal pattern, averaging 0.9 and 0.8 mmol O2 m−3 d−1, respectively. In situ rates of NCP determined from mixed layer O2/Ar ranged between −1.2 and 16.6 mmol O2 m−2 d−1. Our analyses indicate that at certain times of the year the diapycnal flux of O2 across the base of the mixed layer may be non‐negligible and, therefore, a fraction of O2/Ar‐derived NCP may form below the mixed layer. The seasonal climatology of NCP below the mixed layer (down to 150 m) was also estimated using near‐monthly changes in dissolved O2 concentrations. These calculations allowed us to estimate NCP for the entire euphotic zone (0–150 m), which shows pronounced seasonality, with a maximum in April‐May and a minimum in December, when the ecosystem becomes temporarily net heterotrophic. Annual NCP was estimated to be 2.1 ± 0.6 mol O2 m−2 yr−1, approximately 1.7 times the export of C through sinking particles captured in sediment traps at 150 m.

Coastal urbanization alters carbon cycling in Tokyo Bay

The carbon budget of Tokyo Bay, a highly urbanized coastal basin, was estimated using a box model that incorporated inorganic and organic carbon data over an annual cycle (2011–2012). The surface water represented net autotrophic system in which the annual net community production (NCP) was 19 × 1010 gC year−1. The annual loading of dissolved inorganic carbon and total organic carbon (TOC) from freshwater inputs was 11.2 × 1010 and 4.9 × 1010 gC year−1, respectively. The annual TOC sedimentation rate was 3.1 × 1010 gC year−1, similar to the annual air–sea CO2 uptake (5.0 × 1010 gC year−1). Although the NCP and TOC loading from freshwater inputs were respectively 3.0 and 2.7 times lower than those in the 1970s, the TOC sedimentation rate was similar. Therefore, a relatively high carbon efflux from Tokyo Bay likely occurred in the 1970s, including CO2 efflux to the atmosphere and/or export of labile organic carbon to the open ocean. The changes in carbon flow between the 1970s and 2011–2012 resulted from improved water quality due to increased sewage treatment facilities and improved sewage treatment efficiency in the catchment, which decreased the amount of labile organic carbon flowing into the bay.

Annual Net Community Production of Particulate and Dissolved Organic Carbon From a Decade of Biogeochemical Profiling Float Observations in the Northeast Pacific

AbstractCarbon export out of the surface ocean via the biological pump is a critical sink for atmospheric carbon dioxide. This process transports organic carbon to the deep ocean through sinking particulate organic carbon (POC) and the downward transport of suspended POC and dissolved organic carbon (DOC). Changes in the relative contribution of each pathway can significantly affect the magnitude and efficiency of carbon export to depth. Net community production (NCP), an analog of carbon export under steady state assumptions, is typically estimated using budgets of biologically important chemical tracers in the upper ocean constrained by ship‐board or autonomous platform observations. In this study, we use measurements from biogeochemical profiling floats, the Ocean Station Papa mooring, and recently developed algorithms for carbonate system parameters to constrain budgets for three tracers (nitrate, dissolved inorganic carbon, and total alkalinity) and estimate NCP in the Northeast Pacific from 2009 to 2017. Using our multiple‐tracer approach, and constraining end‐member nutrient ratios of the POC and DOC produced, we not only calculate regional NCP throughout the annual cycle and across multiple depth horizons, but also partition this quantity into particulate and dissolved portions. We also use a particle backscatter‐based approach to estimate POC attenuation with depth and present a new method to constrain particle export across deeper horizons and estimate in situ export efficiency. Our results agree well with previously published estimates of regional carbon export annually and suggest that the approaches presented here could be used to assess the magnitude and efficiency of carbon export in other regions of the world's oceans.

Weddell Sea Phytoplankton Blooms Modulated by Sea Ice Variability and Polynya Formation

AbstractSeasonal sea ice retreat is known to stimulate Southern Ocean phytoplankton blooms, but depth‐resolved observations of their evolution are scarce. Autonomous float measurements collected from 2015–2019 in the eastern Weddell Sea show that spring bloom initiation is closely linked to sea ice retreat timing. The appearance and persistence of a rare open‐ocean polynya over the Maud Rise seamount in 2017 led to an early bloom and high annual net community production. Widespread early ice retreat north of Maud Rise in 2017, however, had a similar effect, suggesting that the polynya most impacted the timing of bloom initiation. Still, higher productivity rates at Maud Rise relative to the surrounding region are observed in all years, likely supported by flow‐topography interactions. The longer growing season in 2017–2018 also allowed for separation of distinct spring and fall bloom signals, the latter of which was primarily subsurface and associated with mixed‐layer deepening.

Nutrient Controls on Export Production in the Southern Ocean

We use observations from novel biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling program to estimate annual net community production (ANCP; associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (∼3 mol C m−2 y−1), compared to lower rates in the subtropical zone (≤ 1 mol C m−2 y−1) and the seasonal ice zone (<2 mol C m−2 y−1). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of particulate organic carbon and overall ANCP in this region. A model‐based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.

Seasonal dynamics of the marine CO2 system in Adventfjorden, a west Spitsbergen fjord

Time series of the marine CO2 system and related parameters at the IsA Station, by Adventfjorden, Svalbard, were investigated between March 2015 and November 2017. The physical and biogeochemical processes that govern changes in total alkalinity (TA), total dissolved inorganic carbon (DIC) and the saturation state of the calcium carbonate mineral aragonite (ΩAr) were assessed on a monthly timescale. The major driver for TA and DIC was changes in salinity, caused by river runoff, mixing and advection. This accounted for 77 and 45%, respectively, of the overall variability. It contributed minimally to the variability in ΩAr (5%); instead, biological activity was responsible for 60% of the monthly variations. For DIC, the biological activity was also important, contributing 44%. The monthly effect of air–sea CO2 fluxes accounted for 11 and 15% of the total changes in DIC and ΩAr, respectively. Net community production (NCP) during the productive season ranged between 65 and 85 g C m−2, depending on the year and the presence of either Arctic water or transformed Atlantic water (TAW). The annual NCP as estimated from DIC consumption was 34 g C m−2 yr−1 in 2016, which was opposite in direction but similar in magnitude to the integrated annual air–sea CO2 flux (i.e., uptake of carbon from the atmosphere) of −29 g C m−2 yr−1 for the same year. The results showed that increased intrusions of TAW into Adventfjorden in the future could possibly lower the NCP, with the potential to reduce the CO2buffer capacity and ΩAr over the summer season.

The effect of the 2013–2016 high temperature anomaly in the subarctic Northeast Pacific (the “Blob”) on net community production

Abstract. A large anomalously warm water patch (the “Blob”) appeared in the NE Pacific Ocean in the winter of 2013–2014 and persisted through 2016 causing strong positive upper ocean temperature anomalies at Ocean Station Papa (OSP, 50∘ N, 145∘ W). The effect of the temperature anomalies on annual net community production (ANCP) was determined by upper ocean chemical mass balances of O2 and dissolved inorganic carbon (DIC) using data from a profiling float and a surface mooring. Year-round oxygen mass balance in the upper ocean (0 to 91–111 m) indicates that ANCP decreased after the first year when warmer water invaded this area and then returned to the “pre-Blob” value (2.4, 0.8, 2.1, and 1.6 mol C m−2 yr−1 from 2012 to 2016, with a mean value of 1.7±0.7 mol C m−2 yr−1). ANCP determined from the DIC mass balance has a mean value that is similar within the errors as that from the O2 mass balance but without a significant trend (2.0, 2.1, 2.6, and 3.0 mol C m−2 yr−1 with a mean value of 2.4±0.6 mol C m−2 yr−1). This is likely due to differences in the air–sea gas exchange, which is a major term for both mass balances. Oxygen has a residence time with respect to gas exchange of about 1 month while the CO2 gas exchange response time is more like a year. Therefore the biologically induced oxygen saturation anomaly responds fast enough to record annual changes, whereas that for CO2 does not. Phytoplankton pigment analysis from the upper ocean shows lower chlorophyll a concentrations and changes in plankton community composition (greater relative abundance of picoplankton) in the year after the warm water patch entered the area than in previous and subsequent years. Our analysis of multiple physical and biological processes that may have caused the ANCP decrease after warm water entered the area suggests that it was most likely due to the temperature-induced changes in biological processes.

Assessing Phytoplankton Activities in the Seasonal Ice Zone of the Greenland Sea Over an Annual Cycle

AbstractIn seasonal ice zones (SIZs), such as the one of the Greenland Sea, the sea ice growth in winter and subsequent melting in summer influence the phytoplankton activity. However, studies assessing phytoplankton activities over complete annual cycles and at a fine temporal resolution are lacking in this environment. Biogeochemical‐Argo floats, which are able to sample under the ice, were used to collect physical and biogeochemical data along vertical profiles and at 5‐day resolution during two complete annual cycles in the Greenland Sea SIZ. Three phytoplankton activity phases were distinct within an annual cycle: one under ice, a second at the ice edge, and a third one around an open‐water subsurface chlorophyll maximum. As expected, the light and nitrate availabilities controlled the phytoplankton activity and the establishment of these phases. On average, most of the annual net community production occurred equally under ice and at the ice edge. The open‐water subsurface chlorophyll maximum phase contribution, on the other hand, was much smaller. Phytoplankton biomass accumulation and production thus occur over a longer period than might be assumed if under ice blooms were neglected. This also means that satellite‐based estimates of phytoplankton biomass and production in this SIZ are likely underestimated. Simulations with the Arctic‐based physical‐biologically coupled SINMOD model suggest that most of the annual net community production in this SIZ results from local processes rather than due to advection of nitrate from the East Greenland and Jan Mayen Currents.

Biological and physical controls on the oxygen cycle in the Kuroshio Extension from an array of profiling floats

The Kuroshio Extension (KE) is a current associated with the largest CO2 flux into the Pacific Ocean, a broad region of uptake that extends across the Pacific basin between the subarctic and subtropical regions. The relative importance of the biological and physical processes controlling this sink is uncertain. Because oxygen is stoichiometrically linked to changes in dissolved inorganic carbon due to photosynthesis and respiration and subject to many of the same physical drivers as the CO2 flux, in situ oxygen measurements help to determine the processes driving this large CO2 flux. We analyzed data from eight Argo profiling floats equipped with oxygen sensors to estimate oxygen fluxes in the upper ocean of the KE region (approximate bounds: 25°N to 45°N, 135°E to 155°E). In situ air calibrations of these sensors allowed us to accurately measure air-sea oxygen differences, which largely control the flux of oxygen to and from the atmosphere. To characterize distinct biogeographical regions in the Kuroshio Extension and to accommodate seasonal north-south shifts in the location of the regional boundaries, we averaged oxygen measurements from different floats along isopycnal surfaces into 3 regions based on temperature-salinity relationships: North KE, Central KE, and South KE. Using these regional concentration time series, we determined the physical fluxes using an upper ocean layered model and calculated the residual oxygen flux, a combination of column-integrated net community production and physical processes unexplained by the model. The annual oxygen budget is largely a balance of air-sea exchange and the residual oxygen term. Residual oxygen fluxes are -5.4 ± 1.1, -5.9 ± 0.1, and 2.1 ± 2.2 mol O2 m-2 yr-1 (where negative is a loss from the upper ocean) for the North, Central, and South KE regions, respectively. The North and Central KE are regions of mode water formation, which balances the large air-sea fluxes into the ocean. The South KE oxygen residual indicates a biologically produced flux to the atmosphere in two out of three years that agrees with previous estimates of subtropical annual net community production (ANCP) but exhibits high interannual variability. This study suggests that physical processes are the primary drivers for the annual uptake of the gases oxygen and carbon dioxide in the Central KE region where the annual CO2 uptake is strongest.

Seaglider Surveys at Ocean Station Papa: Oxygen Kinematics and Upper‐Ocean Metabolism

AbstractUnderstanding variability in annual net community production (NCP) and factors affecting its estimation are important priorities for the study of ocean biogeochemistry. The time evolution of dissolved oxygen is useful proxy for NCP if the physical tendency can be isolated and removed; a key challenge is the resolution of the terms contributing to this tendency. Here oxygen balances are examined in the upper 200 m at Ocean Station Papa (50°N, 145°W) using data from Seaglider surveys June 2008 to January 2010. Sampling of horizontal gradients of oxygen, temperature, and salinity during these surveys allows the inference of monthly three‐dimensional advection and turbulent diffusion of oxygen. The resulting monthly oxygen balances show strong variability with depth, with similarities to the temperature balances shallower than 100 m and salinity balances deeper. The estimated annual NCP of 2.2 ± 1.2 mol C·m−2·year−1 in the top 120 m is in agreement with contemporary studies in the subarctic North Pacific Ocean. Horizontal advection in the surface layer is found to be small as previously assumed but important at greater depths: Assuming zero horizontal advection in the top 120 m would have resulted in an overestimate of annual NCP by 50%. Horizontal and vertical advection together dominated the oxygen balance in the permanent pycnocline. Results emphasize the three‐dimensional nature of the oxygen balance, even in this relatively quiescent location, and further demonstrate the viability of autonomous spatial surveys for resolving physical oxygen transport terms.

Mechanisms for a nutrient-conserving carbon pump in a seasonally stratified, temperate continental shelf sea

Continental shelf seas may have a significant role in oceanic uptake and storage of carbon dioxide (CO2) from the atmosphere through a ‘continental shelf pump’ mechanism. The northwest European continental shelf, in particular the Celtic Sea (50°N 8°W), was the target of extensive biogeochemical sampling from March 2014 to September 2015 as part of the UK Shelf Sea Biogeochemistry research programme (UK-SSB). Here, we use the UK-SSB carbonate chemistry and macronutrient measurements to investigate the biogeochemical seasonality in this temperate, seasonally stratified system. Following the onset of stratification, near-surface biological primary production during spring and summer removed dissolved inorganic carbon and nutrients, and a fraction of the sinking particulate organic matter was subsequently remineralised beneath the thermocline. Water column inventories of these variables throughout 1.5 seasonal cycles, corrected for air-sea CO2 exchange and sedimentary denitrification and anammox, isolated the combined effect of net community production (NCP) and remineralisation on the inorganic macronutrient inventories. Overall inorganic inventory changes suggested that a significant fraction (>50%) of the annual NCP of around 3 mol-C m–2 yr–1 appeared to be stored within a long-lived organic matter (OM) pool with a lifetime of several months or more. Moreover, transfers into and out of this pool appeared not to be in steady state over the one full seasonal cycle sampled. Accumulation of such a long-lived and potentially C-rich OM pool is suggested to be at least partially responsible for the estimated net air-to-sea CO2 flux of ∼1.3 mol-C m–2 yr–1 at our study site, while providing a mechanism through which a nutrient-conserving continental shelf pump for CO2 could potentially operate in this and other similar regions.

Assessment of Export Efficiency Equations in the Southern Ocean Applied to Satellite‐Based Net Primary Production

AbstractCarbon export efficiency (e‐ratio) is defined as the fraction of organic carbon fixed through net primary production (NPP) that is exported out of the surface productive layer of the ocean. Recent observations for the Southern Ocean suggest a negative e‐ratio versus NPP relationship, and a reduced dependency of export efficiency on temperature, different than in the global domain. In this study, we complement information from a passive satellite sensor with novel space‐based lidar observations of ocean particulate backscattering to infer NPP over the entire annual cycle, and estimate Southern Ocean export rates from five different empirical models of export efficiency. Inferred Southern Ocean NPP falls within the range of previous studies, with a mean estimate of 15.8 (± 3.9) Pg C yr−1 for the region south of 30 S during the 2005–2016 period. We find that an export efficiency model that accounts for silica(Si)‐ballasting, which is constrained by observations with a negative e‐ratio versus NPP relationship, shows the best agreement with in situ‐based estimates of annual net community production (annual export of 2.7 ± 0.6 Pg C yr−1 south of 30 S). By contrast, models based on the analysis of global observations with a positive e‐ratio versus NPP relationship predict annually integrated export rates that are ∼ 33% higher than the Si‐dependent model. Our results suggest that accounting for Si‐induced ballasting is important for the estimation of carbon export in the Southern Ocean.

Physical and Biological Drivers of Biogeochemical Tracers Within the Seasonal Sea Ice Zone of the Southern Ocean From Profiling Floats

AbstractHere we present initial findings from nine profiling floats equipped with pH, O2, , and other biogeochemical sensors that were deployed in the seasonal ice zone (SIZ) of the Southern Ocean in 2014 and 2015 through the Southern Ocean Carbon and Climate Observations and Modelling (SOCCOM) project. A large springtime phytoplankton bloom was observed that coincided with sea ice melt for all nine floats. We argue this bloom results from a shoaling of the mixed layer depth, increased vertical stability, and enhanced nutrient and light availability as the sea ice melts. This interpretation is supported by the absence of a springtime bloom when one of the floats left the SIZ in the second year of observations. During the sea ice covered period, net heterotrophic conditions were observed. The rate of uptake of O2 and release of dissolved inorganic carbon (derived from pH and estimated total alkalinity) and is reminiscent of biological respiration and is nearly Redfieldian for the nine floats. A simple model of mixed layer physics was developed to separate the physical and biological components of the signal in pH and O2 over one annual cycle for a float in the Ross Sea SIZ. The resulting annual net community production suggests that seasonal respiration during the ice covered period of the year nearly balances the production in the euphotic layer of up to 5 mol C m−2 during the ice free period leading to a net of near zero carbon exported to depth for this one float.