Impact of physical and biological processes on temporal variations of the ocean carbon sink in the mid-latitude North Atlantic (2002–2016)

The ocean is currently a significant net sink for anthropogenically remobilised CO2, taking up around 24% of global emissions. Numerical models predict a diversity of responses of the ocean carbon sink to increased atmospheric concentrations in a warmer world. Here, we tested the hypothesis that increased atmospheric forcing is causing a change in the ocean carbon sink using a high frequency observational dataset derived from underway pCO2 (carbon dioxide partial pressure) instruments on ships of opportunity (SOO) and a fixed-point mooring between 2002 and 2016. We calculated an average carbon flux of 0.013 Pg yr−1 into the ocean at the Porcupine Abyssal Plain (PAP) site, consistent with past estimates. In spite of the increase in atmospheric pCO2, monthly average seawater pCO2 did not show a statistically significant increasing trend, but a higher annual variability, likely due to the decreasing buffer capacity of the system. The increasing led to an increasing trend in the estimated CO2 flux into the ocean of 0.19 ± 0.03 mmol m−2 day−1 per year across the entire 15 year time series, making the study area a stronger carbon sink. Seawater pCO2 variability is mostly influenced by temperature, alkalinity and dissolved inorganic carbon (DIC) changes, with 77% of the annual seawater pCO2 changes explained by these terms. DIC is in turn influenced by gas exchange and biological production. In an average year, the DIC drawdown by biological production, as determined from nitrate uptake, was higher than the DIC increase due to atmospheric CO2 dissolution into the surface ocean. This effect was enhanced in years with high nutrient input or shallow mixed layers. Using the rate of change of DIC and nitrate, we observed Redfieldian carbon consumption during the spring bloom at a C:N ratio of 6.2 ± 1.6. A comparison between SOO and PAP sustained observatory data revealed a strong agreement for pCO2 and DIC. This work demonstrates that the study area has continued to absorb atmospheric CO2 in recent years with this sink enhancing over time. Furthermore, the change in pCO2 per unit nitrate became larger as surface buffer capacity changed

Arctic freshwater fluxes: sources, tracer budgets and inconsistencies

The net rate of freshwater input to the Arctic Ocean has been calculated in the past by two methods: directly, as the sum of precipitation, evaporation and runoff, an approach hindered by sparsity of measurements, and by the ice and ocean budget method, where the net surface freshwater flux within a defined boundary is calculated from the rate of dilution of salinity, comparing ocean inflows with ice and ocean outflows. Here a third method is introduced, the geochemical method, as a modification of the budget method. A standard approach uses geochemical tracers (salinity, oxygen isotopes, inorganic nutrients) to compute “source fractions” that quantify a water parcel's constituent proportions of seawater, freshwater of meteoric origin, and either sea ice melt or brine (from the freezing-out of sea ice). The geochemical method combines the source fractions with the boundary velocity field of the budget method to quantify the net flux derived from each source. Here it is shown that the geochemical method generates an Arctic Ocean surface freshwater flux, which is also the meteoric source flux, of 200±44 mSv (1 Sv=106 m3 s−1), statistically indistinguishable from the budget method's 187±44 mSv, so that two different approaches to surface freshwater flux calculation are reconciled. The freshwater export rate of sea ice (40±14 mSv) is similar to the brine export flux, due to the “freshwater deficit” left by the freezing-out of sea ice (60±50 mSv). Inorganic nutrients are used to define Atlantic and Pacific seawater categories, and the results show significant non-conservation, whereby Atlantic seawater is effectively “converted” into Pacific seawater. This is hypothesized to be a consequence of denitrification within the Arctic Ocean, a process likely becoming more important with seasonal sea ice retreat. While inorganic nutrients may now be delivering ambiguous results on seawater origins, they may prove useful to quantify the Arctic Ocean's net denitrification rate. End point degeneracy is also discussed: multiple property definitions that lie along the same “mixing line” generate confused results

Atlantic water influx and sea-ice cover drive taxonomic and functional shifts in Arctic marine bacterial communities

The Arctic Ocean is experiencing unprecedented changes because of climate warming, necessitating detailed analyses on the ecology and dynamics of biological communities to understand current and future ecosystem shifts. Here, we generated a four-year, high-resolution amplicon dataset along with one annual cycle of PacBio HiFi read metagenomes from the East Greenland Current (EGC), and combined this with datasets spanning different spatiotemporal scales (Tara Arctic and MOSAiC) to assess the impact of Atlantic water influx and sea-ice cover on bacterial communities in the Arctic Ocean. Densely ice-covered polar waters harboured a temporally stable, resident microbiome. Atlantic water influx and reduced sea-ice cover resulted in the dominance of seasonally fluctuating populations, resembling a process of “replacement” through advection, mixing and environmental sorting. We identified bacterial signature populations of distinct environmental regimes, including polar night and high-ice cover, and assessed their ecological roles. Dynamics of signature populations were consistent across the wider Arctic; e.g. those associated with dense ice cover and winter in the EGC were abundant in the central Arctic Ocean in winter. Population- and community-level analyses revealed metabolic distinctions between bacteria affiliated with Arctic and Atlantic conditions; the former with increased potential to use bacterial- and terrestrial-derived substrates or inorganic compounds. Our evidence on bacterial dynamics over spatiotemporal scales provides novel insights into Arctic ecology and indicates a progressing Biological Atlantification of the warming Arctic Ocean, with consequences for food webs and biogeochemical cycles

Overview of the MOSAiC expedition: Physical oceanography

Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present alongdrift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean

The effect of phytoplankton growth on nitrogen cycling in a non-turbid estuary, Southampton Water, UK

Stable isotope tracer incubation techniques were used to estimate the uptake of nitrogen by phytoplankton growth in the Southampton Water estuarine system over the productive spring-summer period in 2001 and 2002.  Nutrient uptake experiments were carried out at approximately 2 week intervals with water samples collected from three stations in the estuary, representative of coastal waters of the Solent, the mid-estuary and brackish waters of the system.  In 2001, separate subsamples were incubated with 13C-HCO3 and 15N-NO3-, 15N-NH+4, and 15N-urea, and were deployed in situ at 4 photic depths plus a dark incubation.  In 2002, separated subsamples were incubated with 15N-NO-3 and 15N-NH4+, and were deployed at 2 photic depths.  In addition, in 2002 the release of DON and the regeneration of NH4+ were also quantified.  Vertical profiles of temperature, salinity and irradiance were made at each station, and water samples from 5 depths were collected for later analysis of nutrients (nitrate, ammonium, urea, and silicate) and chlorophyll a.  Chlorophyll a concentrations of up to 64 &mu;g L-1 were measured, and generally increased from the coastal waters towards the upper estuary.  Nutrient concentration were also higher in the inner estuary and appear to be affected by the seasonal cycle of phytoplankton growth, showing lower concentrations during the summer months.  Uptake rates of nitrogen within the Southampton Water estuary were comparable with value reported in the literature for other similar estuaries and coastal systems, and showed that during extensive phytoplankton blooms within these systems comparatively elevated uptake rates can be reached.  In general, the temporal variation in the uptake of N-nutrients was consistent with the seasonal variations in the Chl-a levels, showing higher rates between April/May to August during the two years investigated.  Uptake rates during both years were higher in the inner estuary relative to the coastal waters of the system.  Results showed that ammonium was the dominant source of nitrogen, contributing on average &gt;60% to the total nitrogen uptake.  Nitrate and urea however, showed dominant contributions on some occasions.  The 15N-tracer experiment carried out during this investigation demonstrated that phytoplankton activity can remove nitrate, ammonium and urea from the euphotic water column at rates of up to &tilde;9, 28 and 51% h-1 of the ambient levels, and can potentially increase by threefold during periods of high water slack.  15N-incubation experiments showed that an average of 74±32% of the nitrate taken up by phytoplankton was released as DON, and that ammonium uptake rates within correction for isotope dilution represented an average of 31±9% of those corrected.  Results thus demonstrated that the impact of phytoplankton growth on the nitrogen levels can be underestimated if the gross uptake of nitrate and ammonium are not taken into account.</p

Advective pathways of nutrients and key ecological substances in the Arctic

The Arctic Ocean is undergoing remarkable environmental changes due to global warming. The rise in the Arctic near-surface air temperature during the past decades is twice as high as the global average, a phenomenon known as the “Arctic Amplification”. As a consequence the Arctic summer sea ice extent has decreased by more than 40 % in recent decades, and moreover a year-round sea ice loss in extent and thickness was recorded. By opening up of large areas formerly covered by sea ice, the exchange of heat, moisture and momentum between the ocean and atmosphere intensified. This resulted in changes in the ocean circulation and the water masses impacting the marine ecosystem. We investigate these changes by using a large set of hydrographic and biogeochemical data of the entire Arctic Ocean. To better quantify the current changes in the Arctic ecosystem we will combine our observational data analysis with model simulations using a very high resolution (1/12°) biogeochemical atmosphere-sea ice-ocean model from our partners at the National Oceanographic Center in the UK (Yevgeni Aksenov and Stefanie Rynders)

Summertime Amino Acid and Carbohydrate Patterns in Particulate and Dissolved Organic Carbon Across Fram Strait

Amino acids (AA) and carbohydrates (CHO) are important components of the marine organic carbon cycle. Produced mainly by phytoplankton as part of the particulate organic carbon (POC) fraction, these compounds can be released into the outer medium where they become part of the dissolved organic carbon (DOC) pool and are rapidly taken up by heterotrophs (e.g., bacteria). We investigated the quantity and quality of POC and DOC, AA and CHO composition in both pools in three different water masses in the Fram Strait (Arctic Ocean) in summer 2017. Polar Waters and Atlantic Waters showed similar concentrations of particulate and dissolved AA and CHO, despite Polar Waters showing the highest DOC concentrations. In Mixed Waters, where the two water masses mix with each other and with melting sea ice, the concentrations of particulate and dissolved AA and CHO were highest. AA and CHO composition differed substantially between the particulate and dissolved fractions. The particulate fraction (>0.7 μm) was enriched in essential AA and the CHO galactose, xylose/mannose, and muramic acid. In the dissolved fraction non-essential AA, several neutral CHO, and acidic and amino CHO were enriched. We further investigated different size fractions of the particulate matter using a separate size fractionation approach (0.2–0.7 μm, 0.7–10 μm and >10 μm). The chemical composition of the 0.2–0.7 μm size-fraction had a higher contribution of non-essential AA and acidic and amino sugars, setting them apart from the 0.7–10 μm and >10 μm fractions, which showed the same composition. We suggest that the relative differences observed between different size fractions and DOC with regards to AA and CHO composition can be used to evaluate the state of organic matter processing and evaluate the contribution of autotrophic phytoplankton or more heterotrophic biomass. In the future, changing conditions in the Central Arctic Ocean (Atlantification, warming, decreasing ice concentrations) may increase primary production and consequently degradation. The AA and CHO signatures left behind after production and/or degradation processes occurred, could be used as tracers after the fact to infer changes in microbial loop processes and food web interactions

Counteracting Contributions of the Upper and Lower Meridional Overturning Limbs to the North Atlantic Nutrient Budgets: Enhanced Imbalance in 2010

The North Atlantic Basin is a major sink for atmospheric carbon dioxide (CO2) due in part to the extensive plankton blooms which form there supported by nutrients supplied by the three-dimensional ocean circulation. Hence, changes in ocean circulation and/or stratification may influence primary production and biological carbon export. In this study, we assess this possibility by evaluating inorganic nutrient budgets for 2004 and 2010 in the North Atlantic based on observations from the transatlantic A05-24.5°N and the Greenland-Portugal OVIDE hydrographic sections, to which we applied a box inverse model to solve the circulation and estimate the across-section nutrient transports. Full water column nutrient budgets were split into upper and lower meridional overturning circulation (MOC) limbs. According to our results, anomalous circulation in early 2010, linked to extreme negative NAO conditions, led to an enhanced northward advection of more nutrient-rich waters by the upper overturning limb, which resulted in a significant nitrate and phosphate convergence north of 24.5°N. Combined with heaving of the isopycnals, this anomalous circulation event in 2010 favored an enhancement of the nutrient consumption (5.7 ± 4.1 kmol-P s−1) and associated biological CO2 uptake (0.25 ± 0.18 Pg-C yr−1, upper-bound estimate), which represents a 50% of the mean annual sea–air CO2 flux in the region. Our results also suggest a transient state of deep silicate divergence in both years. Both results are indicative of a MOC-driven modulation of the biological carbon uptake (by the upper MOC limb) and nutrient inventories (by the lower MOC limb) in the North Atlantic