The seasonal cycling and physico-chemical speciation of iron on the Celtic and Hebridean shelf seas

Birchill, A. J., A. Milne, E. M. S. Woodward, A. L. Annett, W. Giebert, S. Ussher, P. J. Worsfold, D. Rusiecka, E. P. Achterberg, C. Harris, M. Gledhill, M. C. Lohan (2017). Seasonal iron depletion in temperate shelf seas. Geophysical Research Letters. doi: 10.1002/2017GL073881 Klar, J., W. B. Homoky, P. J. Statham, A. J. Birchill, E. Harris, E. M. S. Woodward, B. Silburn, M. Cooper, R. H. James, D. P. Connelly (2017). Stability of dissolved and soluble Fe (II) in shelf sediment pore waters and release to an oxic water column. Biogeochemistry. doi:10.1007/s10533-017-0309-x Hopwood, M. J., A. J. Birchill, M. Gledhill, A. Milne, E. P. Achterberg (2017). A 4 method comparison for the measurement of Fe(II) at nanomolar concentrations in coastal seawater. Frontiers in Chemistry. doi.org/10.3389/fmars.2017.00192Shelf seas represent an important source of iron (Fe) to the open ocean. Additionally, shelf seas are highly productive environments which contribute to atmospheric carbon dioxide drawdown and support large fisheries. The work presented in this thesis describes the seasonal cycle of Fe in the Celtic and Hebridean Shelf Seas, and determines the physico-chemical speciation of Fe supplied from oxic margins. The results from repeated field surveys of the central Celtic Sea showed a nutrient type seasonal cycling of dissolved Fe (< 0.2 µm; dFe), which is surprising in a particle rich shelf system, suggesting a balance of scavenging and remineralisation processes. Coincident drawdown of dFe and nitrate (NO3-) was observed during the phytoplankton spring bloom. During the bloom, preferential drawdown of soluble Fe (< 0.02 µm; sFe) over colloidal Fe (0.02-0.2 µm; cFe) indicated greater bioavailability of the soluble fraction. Throughout summer stratification, it is known that NO3- is drawn down to < 0.02 µM in surface waters. This study revealed that both dFe and labile particulate Fe (LpFe) were also seasonally drawn down to < 0.2 nM. Consequently, it is hypothesised that the availability of Fe seasonally co-limits primary production in this region. At depth both dFe and NO3- concentrations increased from spring to autumn, indicating that remineralisation is an important process governing the seasonal cycling of dFe in the central Celtic Sea. In spring, summer and autumn, distinctive intermediate nepheloid layers (INL) were observed emanating from the Celtic Sea shelf slope. The INLs were associated with elevated concentrations of dFe (up to 3.25 ± 0.16 nM) and particulate Fe (up to 315 ± 1.8 nM) indicating that they are a persistent conduit for the supply of Fe to the open ocean. Typically > 15% of particulate Fe was labile and 60-90% of dFe was in the colloidal fraction. Despite being < 50 km from the 200 m isobath, the concentration of dFe was < 0.1 nM in surface waters at several stations. Broadly, the concentration of nutrients in surface waters described an oligotrophic environment where co-limitation between multiple nutrients, including Fe, appears likely. Over the Hebridean shelf break, residual surface NO3- concentrations (5.27 ± 0.79 µM) and very low concentrations of dFe (0.09 ± 0.04 nM) were observed during autumn, implying seasonal Fe limitation. The dFe:NO3- ratio observed is attributed to sub-optimal vertical supply of Fe relative to NO3- from sub-surface waters. In contrast to the shelf break, surface water in coastal regions contained elevated dFe concentrations (1.73 ± 1.16 nM) alongside low NO3-. Seasonal Fe limitation is known to occur in the Irminger and Iceland Basins; therefore, the Hebridean shelf break likely represents the eastern extent of sub-Arctic Atlantic seasonal Fe limitation, thus indicating that the associated weakening of the biological carbon pump exists over a wider region of the sub-Arctic Atlantic than previously recognised. These key findings demonstrate that the availability of Fe to phytoplankton may seasonally reach limiting levels in temperate shelf waters and that oxic margins persistently supply Fe dominated by colloidal and particulate fractions to the ocean

Exploring ocean biogeochemistry using a lab-on-chip phosphate analyser on an underwater glider

The ability to make measurements of phosphate (PO43–) concentrations at temporal and spatial scales beyond those offered by shipboard observations offers new opportunities for investigations of the marine phosphorus cycle. We here report the first in situ PO43– dataset from an underwater glider (Kongsberg Seaglider) equipped with a PO43– Lab-on-Chip (LoC) analyser. Over 44 days, a 120 km transect was conducted in the northern North Sea during late summer (August and September). Surface depletion of PO43– (<0.2 μM) was observed above a seasonal thermocline, with elevated, but variable concentrations within the bottom layer (0.30–0.65 μM). Part of the variability in the bottom layer is attributed to the regional circulation and across shelf exchange, with the highest PO43– concentrations being associated with elevated salinities in northernmost regions, consistent with nutrient rich North Atlantic water intruding onto the shelf. Our study represents a significant step forward in autonomous underwater vehicle sensor capabilities and presents new capability to extend research into the marine phosphorous cycle and, when combined with other recent LoC developments, nutrient stoichiometry

Simultaneous assessment of oxygen- and nitrate-based net community production in a temperate shelf sea from a single ocean glider

The continental shelf seas are important at a global scale for ecosystem services. These highly dynamic regions are under a wide range of stresses, and as such future management requires appropriate monitoring measures. A key metric to understanding and predicting future change are the rates of biological production. We present here the use of an autonomous underwater glider with an oxygen (O2) and a wet-chemical microfluidic total oxidised nitrogen (NOx-NO3-+NO2-) sensor during a spring bloom as part of a 2019 pilot autonomous shelf sea monitoring study. We find exceptionally high rates of net community production using both O2 and NOx-water column inventory changes, corrected for air-sea gas exchange in case of O2. We compare these rates with 2007 and 2008 mooring observations finding similar rates of NOx-consumption. With these complementary methods we determine the O2:N amount ratio of the newly produced organic matter (7.8g±0.4) and the overall O2:N ratio for the total water column (5.7g±0.4). The former is close to the canonical Redfield O2:N ratio of 8.6g±1.0, whereas the latter may be explained by a combination of new organic matter production and preferential remineralisation of more reduced organic matter at a higher O2:N ratio below the euphotic zone

Evidence of nitrification associated with globally distributed pelagic jellyfish

Often considered detrimental to the environment and human activities, jellyfish blooms are increasing in several coastal regions worldwide. Yet, the overall effect of these outbreaks on ecosystem productivity and structure are not fully understood. Here we provide evidence for a so far unanticipated role of jellyfish in marine nitrogen cycling. Pelagic jellyfish release nitrogen as a metabolic waste product in form of ammonium. Yet, we observed high rates of nitrification (NH4+ → NO3−, 5.7–40.8 nM gWW−1 [wet weight] h−1) associated with the scyphomedusae Aurelia aurita, Chrysaora hysoscella, and Chrysaora pacifica and low rates of incomplete nitrification (NH4+ → NO2−, 1.0–2.8 nM gWW−1 h−1) associated with Chrysaora fulgida, C. hysoscella, and C. pacifica. These observations indicate that microbes living in association with these jellyfish thrive by oxidizing the readily available ammonia to nitrite and nitrate. The four studied species have a large geographic distribution and exhibit frequent population outbreaks. We show that, during such outbreaks, jellyfish‐associated release of nitrogen can provide more than 100% of the nitrogen required for primary production. These findings reveal a so far overlooked pathway when assessing pelagic nitrification rates that might be of particular relevance in nitrogen depleted surface waters and at high jellyfish population densities

A Comparison between Four Analytical Methods for the Measurement of Fe(II) at Nanomolar Concentrations in Coastal Seawater

Dissolved Fe(II) in seawater is deemed an important micronutrient for microbial organisms, but its analysis is challenging due to its transient nature. We conducted a series of Fe(II) method comparison experiments, where spikes of 5 to 31 nM Fe(II) were added to manipulated seawaters with varying dissolved oxygen (37 to 156 μM) concentrations. The observed Fe(II) concentrations from four analytical methods were compared: spectrophotometry with ferrozine, stripping voltammetry, and flow injection analysis using luminol (with, and without, a pre-concentration column). Direct comparisons between the different methods were undertaken from the derived apparent Fe(II) oxidation rate constant (k1). Whilst the two luminol based methods produced the most similar concentrations throughout the experiments, k1 was still subject to a 20–30% discrepancy between them. Contributing factors may have included uncertainty in the calibration curves, and different responses to interferences from Co(II) and humic/fulvic organic material. The difference in measured Fe(II) concentrations between the luminol and ferrozine methods, from 10 min–2 h after the Fe(II) spikes were added, was always relatively large in absolute terms (>4 nM) and relative to the spike added (>20% of the initial Fe(II) concentration). k1 derived from ferrozine observed Fe(II) concentrations was 3–80%, and 4–16%, of that derived from luminol observed Fe(II) with, and without, pre-concentration respectively. The poorest comparability of k1 was found after humic/fulvic material was added to raise dissolved organic carbon to 120 μM. A luminol method without pre-concentration then observed Fe(II) to fall below the detection limit (<0.49 nM) within 10 min of a 17 nM Fe(II) spike addition, yet other methods still observed Fe(II) concentrations of 2.7 to 3.7 nM 30 min later. k1 also diverged accordingly with the ferrozine derived value 4% of that derived from luminol without pre-concentration. These apparent inconsistencies suggest that some inter-dataset differences in measured Fe(II) oxidation rates in natural waters may be attributable to differences in the analytical methods used rather than arising solely from substantial shifts in Fe(II) speciation

Rapid prototyping Lab-on-Chip devices for the future: A numerical optimisation of bulk optical parameters in microfluidic systems

Nuclear reactor process control is typically monitored for pure β-emitting radionuclides via manual sampling followed by laboratory analysis, leading to delays in data availability and response times. The development of an in situ microfluidic Lab on Chip (LoC) system with integrated detection capable of measuring pure β-emitting radionuclides presents a promising solution, enabling a reduction in occupational exposure and cost of monitoring whilst providing improved temporal resolution through near real-time data acquisition. However, testing prototypes with radioactive sources is time-consuming, requires specialist facilities/equipment, generates contaminated waste, and cannot rapidly evaluate a wide range of designs or configurations. Despite this, modelling multiple design parameters and testing their impact on detection with non-radioactive substitutes has yet to be adopted as best practice. The measurement of pure β emitters in aqueous media relies on the efficient transport of photons generated by the Cherenkov effect or liquid scintillators to the detector. Here we explore the role of numerical modelling to assess the impact of optical cell geometry and design on photon transmission and detection through the microfluidic system, facilitating improved designs to realise better efficiency of integrated detectors and overall platform design. Our results demonstrate that theoretical modelling and an experimental evaluation using non-radiogenic chemiluminescence are viable for system testing design parameters and their impact on photon transport. These approaches enable reduced material consumption and requirement for specialist facilities for handling radioactive materials during the prototyping process. This method establishes proof of concept and the first step towards numerical modelling approaches for the design optimisation of microfluidic LoC systems with integrated detectors for the measurement of pure β emitting radionuclides via scintillation-based detection

Les chrétiens ont-ils tous le même baptême ?

A realistic estimation of uncertainty is an essential requirement for all analytical measurements. It is common practice, however, for the uncertainty estimate of a chemical measurement to be based on the instrumental precision associated with the analysis of a single or multiple samples, which can lead to underestimation. Within the context of chemical oceanography such an underestimation of uncertainty could lead to an over interpretation of the result(s) and hence impact on, e.g., studies of biogeochemical cycles, and the outputs from oceanographic models. Getting high quality observational data with a firm uncertainty assessment is therefore essential for proper model validation. This paper describes and compares two recommended approaches that can give a more holistic assessment of the uncertainty associated with such measurements, referred to here as the “bottom up” or modeling approach and the “top down” or empirical approach. “Best practice” recommendations for the implementation of these strategies are provided. The “bottom up” approach combines the standard uncertainties associated with each stage of the entire measurement procedure. The “top down” approach combines the uncertainties associated with day to day reproducibility and possible bias in the complete data set and is easy to use. For analytical methods that are routinely used, laboratories will have access to the information required to calculate the uncertainty from archived quality assurance data. The determination of trace elements in seawater is a significant analytical challenge and iron is used as an example for the implementation of both approaches using real oceanographic data. Relative expanded uncertainties of 10 – 20% were estimated for both approaches compared with a typical short term precision (rsd) of≤5%

Physical and biogeochemical controls on seasonal iron, manganese, and cobalt distributions in Northeast Atlantic shelf seas

Dissolved (<0.2 μm) trace metals (dTMs) including iron (Fe), manganese (Mn), and cobalt (Co) are micronutrients that (co-) limit phytoplankton growth in many ocean regions. Here, we present the spatial and seasonal distributions of dFe, dMn, and dCo on the Northeast Atlantic continental margin (Celtic Sea), along a transect across the shelf and two off-shelf transects along a canyon and a spur. Waters on the continental shelf showed much higher dTM concentrations (dFe 0.07–6.50 nmol L−1, average 1.41 ± 0.96 nmol L−1, n = 138; dMn 0.868–14.8 nmol L−1, 2.75 ± 2.37 nmol L−1, n = 148; dCo 54.8–217 pmol L−1, 109 ± 32 pmol L−1, n = 144) than on the slope (dFe 0.03–1.90 nmol L−1, 0.65 ± 0.43 nmol L−1, n = 454; dMn 0.223–1.14 nmol L−1, 0.58 ± 0.20 nmol L−1, n = 458; dCo 27.3–122 pmol L−1, 71.7 ± 11.7 pmol L−1, n = 441), attributed to strong dTM contributions from a low-salinity endmember, i.e., riverine discharge. Benthic sedimentary input via reductive dissolution (especially for dFe and dMn), delineated by short-lived radium (Ra) isotopic activities (223Raxs and 224Raxs), was only prominent at a station (Site A) characterized by fine sediments. On the continental slope, dMn levels at depth were mainly determined by the formation of insoluble Mn oxides and the intrusion of Mediterranean Outflow Waters. In contrast, dFe and dCo concentrations at depth were balanced by the regeneration from remineralization of sinking organic particles and scavenging removal. In addition, bottom and intermediate nepheloid layers along the slope illustrated both elevated dTM concentrations and Ra isotopic activities. The presence of nepheloid layers is especially significant along the canyon transect relative to the spur transect, demonstrating the importance of slope topography on the off-shelf transport of dTMs into the Northeast Atlantic Ocean. As a seasonal stratified shelf sea, dTMs and nutrients showed synchronized seasonal variations on the shelf, indicating the influence of biological processes in addition to source effects. Surface dFe and dCo were depleted in summer due to enhanced biological uptake, while sub-surface dFe and dCo were elevated in summer and autumn ascribed to the remineralization of sinking organic particles. In contrast, surface dMn levels were predominantly controlled by the seasonal variations in photoreduction, while sub-surface dMn concentrations were relatively constant throughout the year. The combined effects of fluvial and benthic sources, topographical controls, and biological processes shape the seasonal variations of dTM distributions. Such seasonal variations in dTMs and biological activities can affect the biological carbon pump on the Northeast Atlantic continental margin, and may further influence the carbon cycle in the Atlantic Ocean via the dynamic dTM exchange between continental margins and the open ocean

Ocean circulation and biological processes drive seasonal variations of dissolved Al, Cd, Ni, Cu, and Zn on the Northeast Atlantic continental margin

Nutrients and nutrient-like dissolved trace metals (dTMs) are essential for the functioning of marine organisms and therefore form an important part of ocean biogeochemical cycles. Here, we report on the seasonal distributions of dissolved zinc (dZn), nickel (dNi), copper (dCu), cadmium (dCd), aluminum (dAl), and nutrients on the Northeast Atlantic continental margin (Celtic Sea), which is representative for temperate shelf seas globally. Variations in surface water dTM and nutrient concentrations were mainly regulated by seasonal changes in biological processes. The stoichiometry of dTMs (especially for dCu and dZn) and nutrients on the continental shelf was additionally affected by fluvial inputs. Nutrients and dTMs at depth on the continental slope were determined by water mass mixing driven by ocean circulation, without an important role for local remineralization processes. The Mediterranean Outflow Waters are especially important for delivering Mediterranean-sourced dTMs to the Northeast Atlantic Ocean and drive dTM:nutrient kinks at a depth of ~1000 m. These results highlight the importance of riverine inputs, seasonality of primary production and ocean circulation on the distributions of nutrients and nutrient-like dTMs in temperate continental margin seas. Future climate related changes in the forcing factors may impact the availability of nutrients and dTMs to marine organisms in highly productive continental shelf regions and consequently the regional carbon cycle

Phytoplankton responses to dust addition in the Fe–Mn co-limited eastern Pacific sub-Antarctic differ by source region

The seasonal availability of light and micronutrients strongly regulates productivity in the Southern Ocean, restricting biological utilization of macronutrients and CO 2 drawdown. Mineral dust flux is a key conduit for micronutrients to the Southern Ocean and a critical mediator of multimillennial-scale atmospheric CO 2 oscillations. While the role of dust-borne iron (Fe) in Southern Ocean biogeochemistry has been examined in detail, manganese (Mn) availability is also emerging as a potential driver of past, present, and future Southern Ocean biogeochemistry. Here, we present results from fifteen bioassay experiments along a north–south transect in the undersampled eastern Pacific sub-Antarctic zone. In addition to widespread Fe limitation of phytoplankton photochemical efficiency, we found further responses following the addition of Mn at our southerly stations, supporting the importance of Fe–Mn co-limitation in the Southern Ocean. Moreover, addition of different Patagonian dusts resulted in enhanced photochemical efficiency with differential responses linked to source region dust characteristics in terms of relative Fe/Mn solubility. Changes in the relative magnitude of dust deposition, combined with source region mineralogy, could hence determine whether Fe or Mn limitation control Southern Ocean productivity under future as well as past climate states. </p