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Marine diazotrophs convert dinitrogen (N2) gas into bioavailable nitrogen (N), supporting life in the global ocean. In 2012, the first version of the global oceanic diazotroph database (version 1) was published. Here, we present an updated version of the database (version 2), significantly increasing the number of in situ diazotrophic measurements from 13 565 to 55 286. Data points for N2 fixation rates, diazotrophic cell abundance, and nifH gene copy abundance have increased by 184 %, 86 %, and 809 %, respectively. Version 2 includes two new data sheets for the nifH gene copy abundance of non-cyanobacterial diazotrophs and cell-specific N2 fixation rates. The measurements of N2 fixation rates approximately follow a log-normal distribution in both version 1 and version 2. However, version 2 considerably extends both the left and right tails of the distribution. Consequently, when estimating global oceanic N2 fixation rates using the geometric means of different ocean basins, version 1 and version 2 yield similar rates (43–57 versus 45–63 Tg N yr−1; ranges based on one geometric standard error). In contrast, when using arithmetic means, version 2 suggests a significantly higher rate of 223 ± 30 Tg N yr−1 (mean ± standard error; same hereafter) compared to version 1 (74 ± 7 Tg N yr−1). Specifically, substantial rate increases are estimated for the South Pacific Ocean (88 ± 23 versus 20 ± 2 Tg N yr−1), primarily driven by measurements in the southwestern subtropics, and for the North Atlantic Ocean (40 ± 9 versus 10 ± 2 Tg N yr−1). Moreover, version 2 estimates the N2 fixation rate in the Indian Ocean to be 35 ± 14 Tg N yr−1, which could not be estimated using version 1 due to limited data availability. Furthermore, a comparison of N2 fixation rates obtained through different measurement methods at the same months, locations, and depths reveals that the conventional 15N2 bubble method yields lower rates in 69 % cases compared to the new 15N2 dissolution method. This up

Ocean deoxygenation threatens ocean productivity, carbon cycling and marine ecosystems. Shelf seas are highly dynamic regions, which contributes to their high productivity and also makes monitoring and constraining their oxygen status a challenge. Here, using the temperate Celtic shelf sea (April and July 2015) as a case study, we present high-resolution ocean glider observations of turbulence and biogeochemical parameters, demonstrating the potential of these autonomous platforms for environmental monitoring. We estimate vertical turbulent oxygen fluxes be 25% higher in summer than in spring, due to the presence of subsurface chlorophyll and associated oxygen maxima at the seasonal thermocline. We demonstrate that glider-based estimates were able to constrain similar bottom layer respiration rates as those derived from traditional ship-based measurements. We suggest ocean gliders are useful monitoring tools that can aid sustainable management of shelf sea ecosystems.

The Northwest European Shelf is positioned directly beneath the North Atlantic Storm Track, within which the frequency and intensity of transient storms are modulated by large-scale climatic oscillations. In temperate shelf seas, the impact of storms on the physical environment has received considerable attention, but the effect on biogeochemistry is less studied. Here, we use output from a multidecadal (1982–2015) coupled physical-biogeochemical model supported by observations from ocean gliders to investigate phytoplankton growth throughout the winter-spring transition. We define two separate phytoplankton growth events: the spring bloom, defined as the exponential growth following seasonal stratification, and the prebloom, occurring before stratification, and accounting for up to 22% of the total spring growth. Our results support the paradigm that light is a first-order control, with the spring bloom initiating up to 22 days after stratification onset should light levels be too low to trigger the bloom. The prebloom is heavily influenced by the phase of the Atlantic Multidecadal Oscillation (AMO), demonstrated by an acceleration in the rate of increase of total chlorophyll concentrations (±90% confidence limit) from 7.6 ± 2.8 mg m−2 d−1 (during a positive AMO) to 13.1 ± 4.3 mg m−2 d−1 (negative AMO), due to modulation of periods of ephemeral stratification that occur between successive storms. We propose that phytoplankton growth in prebloom events might help buffer the lag between phytoplankton supply and larval recruitment, particularly during years when the spring bloom is delayed.

We used a combined ion pairing - organic matter speciation model (NICA-Donnan) to predict the organic complexation of iron (Fe) at ambient pH and temperature in the Celtic Sea. We optimized our model by direct comparison with Fe speciation determined by Adsorptive Cathodic Stripping Voltammetry using the added Fe-binding ligand 1-nitroso-2-naphthol (HNN) in the presence and absence of natural organic matter. We compared determined Fe speciation with simulated titrations obtained via application of the NICA-Donnan model with four different NICA parameter sets representing a range of binding site strengths and heterogeneities. We tested the assumption that binding sites scale to dissolved organic carbon (DOC) concentrations in marine waters. We found that a constant low DOC concentration resulted in an improved fit of our titration data to the simulated titrations, suggesting that inputs of autochthonous marine DOM may not increase the heterogeneity or concentrations of Fe binding sites. Using the optimal parameter set, we calculated pFe(III)´ (−log(∑Fe (OH)i3−i)) and apparent Fe(III) solubility (SFe(III)app) at ambient pH andtemperature in the water column of the Celtic Sea. SFe(III) app was defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM formed at a free Fe (Fe3+)concentration equal to the limiting solubility of Fe hydroxide (Fe(OH) 3(s)). SFe(III)app was within range of the determined dissolved Fe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at our most offshore stations. Our study supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding by organic matter, although the overall number of metal binding sites in the marine environment may not be directly scalable to DOC concentrations.

The filamentous diazotrophic cyanobacterium Trichodesmium is responsible for a significant fraction of marine di-nitrogen (N2) fixation. Growth and distribution of Trichodesmium and other diazotrophs in the vast oligotrophic subtropical gyres is influenced by iron (Fe) and phosphorus (P) availability, while reciprocally influencing the biogeochemistry of these nutrients. Here we use observations across natural inverse gradients in Fe and P in the North Atlantic subtropical gyre (NASG) to demonstrate how Trichodesmium acclimates in situ to resource availability. Transcriptomic analysis identified progressive upregulation of known iron-stress biomarker genes with decreasing Fe availability, and progressive upregulation of genes involved in the acquisition of diverse P sources with decreasing P availability, while genes involved in N2 fixation were upregulated at the intersection under moderate Fe and P availability. Enhanced N2 fixation within the Fe and P co-stressed transition region was also associated with a distinct, consistent metabolic profile, including the expression of alternative photosynthetic pathways that potentially facilitate ATP generation required for N2 fixation with reduced net oxygen production. The observed response of Trichodesmium to availability of both Fe and P supports suggestions that these biogeochemically significant organisms employ unique molecular, and thus physiological responses as adaptations to specifically exploit the Fe and P co-limited niche they construct.

Temperate shelf seas are productive areas with the potential to export high quantities of particulate organic carbon (POC), as sinking particles, to the sediments or off-shelf to the open ocean. The amount of carbon which can be exported depends partly on the amount of POC produced and on the remineralization processes occurring on the sinking material. Here, we assessed the relative seasonal importance of microbial respiration and bacterial production associated with suspended, slow- and fast-sinking particle fractions. The three fractions were collected in the Celtic Sea above and below the seasonal thermocline in November 2014, April and July 2015 using Marine Snow Catchers. The slow-sinking fraction had higher microbial respiration and bacterial production rates than the fast-sinking fractions, and these two fractions sustained rates of microbial respiration and bacterial production between 1 and 3 orders of magnitude lower than the suspended fraction. This low contribution of the slow- and fast-sinking fractions was consistent with their low contribution to the POC concentration at the two depths sampled. The POC-specific respiration rates associated with the slow- and fast-sinking fractions were low (median 0.17 and 0.08d1, respectively), indicating low-sinking particle degradation. Our results indicate that 5% of the POC in surface waters can be exported below the thermocline.

The GO-SHIP nutrient manual covers all aspects of nutrient analysis from basic sample collection and storage, specifically for Continuous Flow analysis using an Auto-Analyzer, and describes some specific nutrient methods for Nitrate, Nitrite, Silicate, Phosphate and Ammonium that are in use by many laboratories carrying out at-sea analysis and repeat hydrography sections across the world. The focus is on segmented flow analyzers not flow injection analyzers. It also covers laboratory best practices including quality control and quality assurance (QC/QA) procedures to obtain the best results, and suggests protocols for the use of reference materials (RM) and certified reference materials (CRMs).

Using underwater gliders we have identified canyon driven upwelling across the Celtic Sea shelf-break, in the vicinity of Whittard Canyon. The presence of this upwelling appears to be tied to the direction and strength of the local slope current, which is in itself highly variable. During typical summer time equatorward flow, an unbalanced pressure gradient force and the resulting disruption of geostrophic flow can lead to upwelling along the main axis of two small shelf break canyons. As the slope current reverts to poleward flow, the upwelling stops and the remnants of the upwelled features are mixed into the local shelf water or advected away from the region. The upwelled features are identified by the presence of sub-pycnocline high salinity water on the shelf, and are upwelled from a depth of 300 m on the slope, thus providing a mechanism for the transport of nutrients across the shelf break onto the shelf.