Description: Our study followed the seasonal cycling of soluble (SFe), colloidal (CFe), dissolved (DFe), total dissolvable (TDFe), labile particulate (LPFe) and total particulate (TPFe) iron in the Celtic Sea (NE Atlantic Ocean). Preferential uptake of SFe occurred during the spring bloom, preceding the removal of CFe. Uptake and export of Fe during the spring bloom, coupled with a reduction in vertical exchange, led to Fe deplete surface waters (〈0.2 nM DFe; 0.11 nM LPFe, 0.45 nM TDFe, 1.84 nM TPFe) during summer stratification. Below the seasonal thermocline, DFe concentrations increased from spring to autumn, mirroring NO3- and consistent with supply from remineralised sinking organic material, and cycled independently of particulate Fe over seasonal timescales. These results demonstrate that summer Fe availability is comparable to the seasonally Fe limited Ross Sea shelf, and therefore is likely low enough to affect phytoplankton growth and species composition.

Description: Iron (Fe), cobalt (Co), and vitamin B12 addition experiments were performed in the eastern Equatorial Pacific/Peruvian upwelling zone during the 2015 El Niño event. Near the Peruvian coastline, apparent photosystem II photochemical efficiencies (Fv/Fm) were unchanged by nutrient addition and chlorophyll‐a tripled in untreated controls over two days, indicating nutrient replete conditions. Conversely, Fe amendment further away from the coastline in the high nitrate, low Fe zone significantly increased Fv/Fm and chlorophyll‐a concentrations. Mean chlorophyll‐a was further enhanced following supply of Fe+Co and Fe+B12 relative to Fe alone, but this was not statistically significant; further offshore, reported Co depletion relative to Fe could enhance responses. The persistence of Fe limitation in this system under a developing El Niño, as previously demonstrated under non‐El Niño conditions, suggests that diminished upwelled Fe is likely an important factor driving reductions in offshore phytoplankton productivity during these events.

Description: Anthropogenic activities have resulted in enhanced lead (Pb) emissions to the environment over the past century, mainly through the combustion of leaded gasoline. Here, we present the first combined dissolved (DPb), labile (LpPb) and particulate (PPb) Pb dataset from the Northeast Atlantic (Celtic Sea) since the phasing out of leaded gasoline in Europe. Concentrations of DPb in surface waters have decreased by 4-fold over the last four decades. We demonstrate that anthropogenic Pb is transported from the Mediterranean Sea over long distances (〉2500 km). Benthic DPb fluxes exceeded the atmospheric Pb flux in the region, indicating the importance of sediments as a contemporary Pb source. A strong positive correlation between DPb, PPb and LpPb indicates a dynamic equilibrium between the phases and the potential for particles to ‘buffer’ the DPb pool. This study provides insights into Pb biogeochemical cycling and demonstrates the potential of Pb in constraining ocean circulation patterns.

Description: Marine carbonate chemistry measurements have been carried out annually since 2009 during UK research cruises along the Extended Ellett Line (EEL), a hydrographic transect in the northeast Atlantic Ocean. The EEL intersects several water masses that are key to the global thermohaline circulation, and therefore the cruises sample a region in which it is critical to monitor secular physical and biogeochemical changes. We have combined results from these EEL cruises with existing quality-controlled observational data syntheses to produce a hydrographic time series for the EEL from 1981 to 2013. This reveals multidecadal increases in dissolved inorganic carbon (DIC) throughout the water column, with a near-surface maximum rate of 1.800.45 mu molkg(-1)yr(-1). Anthropogenic CO2 accumulation was assessed, using simultaneous changes in apparent oxygen utilization (AOU) and total alkalinity (TA) as proxies for the biogeochemical processes that influence DIC. The stable carbon isotope composition of DIC (C-13(DIC)) was also determined and used as an independent test of our method. We calculated a volume-integrated anthropogenic CO2 accumulation rate of 2.80.4mgCm(-3)yr(-1) along the EEL, which is about double the global mean. The anthropogenic CO2 component accounts for only 316% of the total DIC increase. The remainder is derived from increased organic matter remineralization, which we attribute to the lateral redistribution of water masses that accompanies subpolar gyre contraction. Output from a general circulation ecosystem model demonstrates that spatiotemporal heterogeneity in the observations has not significantly biased our multidecadal rate of change calculations and indicates that the EEL observations have been tracking distal changes in the surrounding North Atlantic and Nordic Seas.

Description: Low dissolved iron (DFe) concentrations limit primary production in most high-nutrient low-chlorophyll (HNLC) regions. Increased recycling of iron (Fe) relative to nitrogen (N) by zooplankton may help to sustain phytoplankton production in these conditions. We concurrently determined rates of DFe and ammonium (NH4 +) recycling by natural mesozooplankton communities in HNLC conditions of the Northeast Atlantic. NH4 + excretion remained constant and ranged between 14.2-54.1nmol NH4 + mg dry weight-1h-1. Fe recycling ranged between 6-138 pmol DFe mg dry weight-1h-1 during the first hour and decreased thereafter, reflecting the transition from the loss of phytoplankton-derived Fe to basal DFe excretion. Mesozooplankton-driven nutrient recycling was estimated to support 6-59 and 〈1-13 of the respective phytoplankton requirements for DFe and N; DFe:N regeneration ratios were 5-26 times larger than those required by phytoplankton. Our data suggest that Fe recycling by grazing organisms has the potential to reduce the intensity of HNLC conditions. © 2012. American Geophysical Union. All Rights Reserved.

Description: Particulate organic carbon (POC) generated by primary production and exported to depth, is an important pathway for carbon transfer to the abyss, where it is stored over climatically significant timescales. These processes constitute the biological carbon pump. A spectrum of particulate sinking velocities exists throughout the water column, however numerical models often simplify this spectrum into suspended, fast and slow sinking particles. Observational studies suggest the spectrum of sinking speeds in the ocean is strongly bimodal with >85 POC flux contained within two pools with sinking speeds of 〈10 m day -1 and >350 m day -1. We deployed a Marine Snow Catcher (MSC) to estimate the magnitudes of the suspended, fast and slow sinking pools and their fluxes at the Porcupine Abyssal Plain site (48°N, 16.5°W) in summer 2009. The POC concentrations and fluxes determined were 0.2μ g C L -1 and 54 mg C m -2 day -1 for fast sinking particles, 5μ g C L -1 and 92μ mg C m -2 day -1 for slow sinking particles and 97 g C L -1 for suspended particles. Our flux estimates were comparable with radiochemical tracer methods and neutrally buoyant sediment traps. Our observations imply: (1) biomineralising protists, on occasion, act as nucleation points for aggregate formation and accelerate particle sinking; (2) fast sinking particles alone were sufficient to explain the abyssal POC flux; and (3) there is no evidence for ballasting of the slow sinking flux and the slow sinking particles were probably entirely remineralised in the twilight zone. Copyright 2012 by the American Geophysical Union.

Description: Aeolian dust transport from the Saharan/Sahel desert regions is considered the dominant external input of iron (Fe) to the surface waters of the eastern (sub-) tropical North Atlantic Ocean. To test this hypothesis, we investigated the sources of dissolved Fe (DFe) and quantified DFe fluxes to the surface ocean in this region. In winter 2008, surface water DFe concentrations varied between 〈0.1 nM and 0.37 nM, with an average of 0.13 ̃ 0.07 nM DFe (n = 194). A strong correlation between mixed layer averaged concentrations of dissolved aluminum (DAl), a proxy for dust input, and DFe indicated dust as a source of DFe to the surface ocean. The importance of Aeolian nutrient input was further confirmed by an increase of 0.1 nM DFe and 0.05 μM phosphate during a repeat transect before and after a dust event. An exponential decrease of DFe with increasing distance from the African continent, suggested that continental shelf waters were a source of DFe to the northern part of our study area. Relatively high Fe:C ratios of up to 3 ° 10°5 (C derived from apparent oxygen utilization (AOU)) indicated an external source of Fe to these African continental shelf waters. Below the wind mixed layer along 12°N, enhanced DFe concentrations (〉1.5 nM) correlated positively with apparent oxygen utilization (AOU) and showed the importance of organic matter remineralization as an DFe source. As a consequence, vertical diffusive mixing formed an important Fe flux to the surface ocean in this region, even surpassing that of a major dust event. © 2012. American Geophysical Union.

Description: Concentrations of dissolved iron (DFe, 0.2μm) were determined at two stations in the Biscay Abyssal Plain (North East Atlantic) in March 2002. DFe concentrations in the surface layer (0.23–0.34 nM) were typical of winter conditions in this area. At 1000 m, DFe concentrations increased to 0.62–0.86 nM. This feature is consistent with the production of DFe by remineralization of the biogenic material. However, at this depth, Mediterranean Outflow Water (MOW) could be an additional source of DFe. Below 2500 m, DFe concentrations were constant (0.75 ± 0.04 nM). An interesting feature of the profiles was the intermediate maximum of DFe (1.19–1.12 nM) around 2000 m, associated with the Labrador Sea Water (LSW). We suggest that the iron enrichment of LSW occurred when this water mass reached the continental margin, likely in the vicinity of the Goban plateau. Vertical distributions were highly dependent on water masses encountered.

Description: Measurements performed on a cruise within the central Iceland Basin in the high-latitude (〉55 degrees N) North Atlantic Ocean during late July to early September 2007 indicated that the concentration of dissolved iron (dFe) in surface waters was very low, with an average of 0.093 (〈0.010-0.218, n = 43) nM, while nitrate concentrations ranged from 2 to 5 mu M and in situ chlorophyll concentrations ranged from 0.2 to 0.4 mg m(-3). In vitro iron addition experiments demonstrated increased photosynthetic efficiencies (F(v)/F(m)) and enhanced chlorophyll accumulation in treatments amended with iron when compared to controls. Enhanced net growth rates for a number of phytoplankton taxa including the coccolithophore Emiliania huxleyi were also observed following iron addition. These results provide strong evidence that iron limitation within the postspring bloom phytoplankton community contributes to the observed residual macronutrient pool during summer. Low atmospheric iron supply and suboptimal Fe:N ratios in winter overturned deep water are suggested to result in the formation of this seasonal high-nutrient, low-chlorophyll (HNLC) condition, representing an inefficiency of the biological (soft tissue) carbon pump in the region.

Description: In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (〈1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect of increasing phytoplankton biomass” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.