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The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

Two experiments were performed during the wet and dry seasons to quantify dissolved carbon dynamics and fluxes in the Shark River, a tidal estuary flowing through the largest contiguous mangrove forest in North America (Everglades National Park, Florida, USA). During these experiments, between 80% and 87% of the total dissolved carbon pool consisted of inorganic carbon (DIC). Carbon inputs from mangroves to the estuary were slightly higher during the wet season, whereas alkalinity inputs were comparable during the two experiments. Longitudinal dissolved carbon fluxes to the coastal ocean were slightly higher during the wet season (13.53 ± 0.76 × 105 mol day−1 during the wet and 11.70 ± 0.32 × 105 mol day−1 during the dry), whereas longitudinal alkalinity flux was comparable during both experiments (10.64 ± 0.74 in the wet vs. 9.88 ± 0.30 × 105 mol day−1 during the dry season). Overall, DIC production in surface water, porewater, and groundwater was dominated by oxic mineralization of mangrove‐derived organic matter and carbonate dissolution. Carbonate dissolution was the most important alkalinity production process in the system. The experiments show that regardless of the season and hydro‐climatic conditions, Shark River receives large inputs of dissolved carbon from the upstream marsh, mangroves, and carbonate dissolution, and that per area, it exports a greater amount of dissolved carbon than many other mangrove‐dominated estuaries in the world. Key Points: Most of the dissolved carbon in the mangrove estuary (between 80% and 87%) exist as dissolved inorganic carbonLongitudinal dissolved carbon fluxes to the coastal ocean were ~16% higher during the wet season compared to the dry season, and air‐water CO2 fluxes were similar between the two seasonsAerobic respiration and sulfate reduction of mangrove‐derived organic carbon along with carbonate dissolution were the main processes contributing dissolved inorganic carbon and alkalinity to Shark River [ABSTRACT FROM AUTHOR]

Uncertainties in carbon chemistry variability still remain large in the Gulf of Mexico (GoM), as data gaps limit our ability to infer basin-wide patterns. Here we configure and validate a regional high-resolution ocean biogeochemical model for the GoM to describe seasonal patterns in surface pressure of CO2 (pCO2), aragonite saturation state (ΩAr), and sea–air CO2 flux. Model results indicate that seasonal changes in surface pCO2 are strongly controlled by temperature across most of the GoM basin, except in the vicinity of the Mississippi–Atchafalaya river system delta, where runoff largely controls dissolved inorganic carbon (DIC) and total alkalinity (TA) changes. Our model results also show that seasonal patterns of surface ΩAr are driven by seasonal changes in DIC and TA, and reinforced by the seasonal changes in temperature. Simulated sea–air CO2 fluxes are consistent with previous observation-based estimates that show CO2 uptake during winter–spring, and CO2 outgassing during summer–fall. Annually, our model indicates a basin-wide mean CO2 uptake of 0.35 molm-2yr-1 , and a northern GoM shelf (< 200 m) uptake of 0.93 molm-2yr-1. The observation and model-derived patterns of surface pCO2 and CO2 fluxes show good correspondence; thus this study contributes to improved constraints of the carbon budget in the region. [ABSTRACT FROM AUTHOR]

Uncertainties in carbon chemistry variability still remain large in the Gulf of Mexico (GoM), as data gaps limit our ability to infer basin-wide patterns. Here we configure and validate a regional high-resolution ocean-biogeochemical model for the GoM to describe seasonal patterns in surface pressure of CO2 (pCO2), aragonite saturation state (ΩAr), and air-sea CO2 flux during 2005-2014. Model results indicate that seasonal changes in surface pCO2 are strongly controlled by temperature across most of the GoM basin, except in the vicinity of the Mississippi-Atchafalaya River System delta, where runoff largely controls dissolved inorganic carbon (DIC) and total alkalinity (TA) changes. Our model results also show that seasonal patterns of surface ΩAr are driven by seasonal changes in DIC and TA, and reinforced by the seasonal changes in temperature. Simulated air-sea CO2 fluxes are consistent with previous observation-based estimates that show CO2 uptake during winter-spring, and CO2 outgassing during summer-fall. Annually, our model indicates a basin-wide mean CO2 uptake of 0.35 mol m-2 yr-1, and a northern GoM shelf (< 200 m) uptake of 0.93 mol m-2 yr-1. The observation and model-derived patterns of surface pCO2 and CO2 fluxes show good correspondence, thus contributing to improved constraints of the carbon budget in the region. [ABSTRACT FROM AUTHOR]

As part of an effort to monitor changes in inorganic carbon chemistry of the coastal ocean, near-synoptic cruises are being conducted in the Northern Gulf of Mexico and along the East Coast of the United States. Here we describe observations obtained on a cruise in the summer of 2012 and compare them with results from a cruise following a similar track in 2007. The focus is on describing spatial patterns of aragonite saturation state ( Ω Ar ). This parameter is an indicator of ecosystem health, in particular for calcifying organisms. The results show large-scale regional trends from different source waters at the northeastern and southwestern edges of the domain, along with the modulating effects of remineralization/respiration and riverine inputs. The broader patterns and changes over five years along the coast can be well described by the impacts of large-scale circulation, notably changes in source water contributions. Changes in the well-buffered Loop Current and Gulf Stream with high Ω Ar impact the waters in the southern part of the study area. The less buffered southward coastal currents with low Ω Ar originating from the Labrador Sea and Gulf of St. Lawrence impact the Ω Ar patterns in the Northern regions. The expected 2% average decrease in Ω Ar in the surface mixed layer due to increasing atmospheric CO 2 levels over the 5-year period is largely overshadowed by local and regional variability from changes in hydrography and mixed layer dynamics. [ABSTRACT FROM AUTHOR]

Distributions of total alkalinity (TA), dissolved inorganic carbon (DIC), and other parameters relevant to the marine inorganic carbon system were investigated in shelf and adjacent ocean waters during a U.S. Gulf of Mexico and East Coast Carbon cruise in July-August 2007. TA exhibited near-conservative behavior with respect to salinity. Shelf concentrations were generally high in southern waters (Gulf of Mexico and East Florida) and decreased northward from Georgia to the Gulf of Maine. DIC was less variable geographically and exhibited strongly nonconservative behavior. As a result, the ratio of TA to DIC generally decreased northward. The spatial patterns of other CO2 system parameters closely followed those of the TA : DIC ratio. All sampled shelf waters were supersaturated with respect to aragonite (saturation state ΩA > 1). The most intensely buffered and supersaturated waters (ΩA > 5.0) were in northern Gulf of Mexico river-plume waters; the least intensely buffered and least supersaturated waters (ΩA < 1.3) were in the deep Gulf of Maine. Due to their relatively low pH, ΩA, and buffer intensity, waters of the northeastern U.S. shelves may be more susceptible to acidification pressures than are their southern counterparts. In the Mid-Atlantic Bight, alongshore mixing tended to increase DIC concentrations southward, but this effect was largely offset by the opposing effects of biogeochemical processing. In the Gulf of Mexico, downstream increases in Loop Current DIC suggested significant contributions from shelf and gulf waters, estimated at 9.1 x 109 mol C d-1. Off the southeastern U.S., along-flow chemical changes in the Florida Current were dominated by mixing associated with North Atlantic subtropical recirculation. [ABSTRACT FROM AUTHOR]

Considerable drawdown of total dissolved inorganic carbon (CT) and oversaturation of oxygen (O2) within a cold (15°C) oligotrophic eddy in the extratropical North Atlantic Ocean (46°N, 20.5°W) indicate that, despite the absence of nitrate (NO3-), the eddy was highly productive. Estimates of net community production using the mass balances of CT and O2 were two to five times greater than those obtained using the mass balance of NO3-. The remineralization rates obtained using the integrated rates of CT and NO3- accumulation and O2 utilization for the upper thermocline waters (35-300-m depth) were in agreement with CT- and O2-based net community production over the same period; however, all the estimates exceeded the NO3--based net community production by a factor of two to five, pointing to a considerable accumulation of NO3- in the upper thermocline in excess of changes in the mixed-layer NO3- inventory. The amount of this excess NO3- suggests that a considerable fraction of the net community production was not supported by the mixed-layer NO3- inventory and that an external source of NO3- must be present. Of the various mechanisms that might explain the inequity between NO3- drawdown in the surface layer and NO3- accumulation in the upper thermocline, N2 fixation is the most viable yet surprising mechanism for producing such excess NO3- in this oligotrophic eddy. A significant fraction of net community production in oligotrophic extratropical waters could be supported by processes that are not fully explored or to date have been considered to be insignificant. [ABSTRACT FROM AUTHOR]