•Seasonality and cross shelf distribution of DOM and its optical properties were examined in the Celtic Sea.•Strong cross shelf gradients in humic DOM components with significant negative ...correlations with salinity and DOC.•Variability in labile DOM protein components linked to biological productivity.
The Celtic Sea is a productive temperate sea located on the Northwest European Shelf. It is an important pathway for the delivery of land-derived material to the North Atlantic Ocean, including dissolved organic matter (DOM). The aim of this study was to determine the seasonal and spatial variability in the magnitude, source and composition of DOM at three sites representing on shelf, central shelf and shelf edge regions in the Celtic Sea, using observations collected during the UK Shelf Sea Biogeochemistry (SSB) research programme (November 2014 – August 2015). The concentration of dissolved organic carbon (DOC) alongside DOM absorbance and fluorescence indices were measured and fluorescence Excitation and Emission Matrices (EEMs) combined with Parallel Factor Analysis (PARAFAC) were used to assess DOM composition and lability. The PARAFAC model identified four unique fluorescent components for autumn (November 2014), winter (March 2015), spring (April 2015) and summer (July 2015) consisting of two humic-like components attributed to terrestrial (C1) and marine sources (C2), and two protein components identified as tyrosine-like (C3) and tryptophan-like (C4) attributed to in situ production. DOC varied seasonally and there were strong cross shelf trends. The protein components (C3 and C4) exhibited large seasonal and within season variability particularly during productive periods. In contrast, there were persistent cross shelf gradients in the CDOM absorption coefficient at 305 nm (a305), the UV specific absorbance at 280 nm (SUVA280), the humification index (HIX), and the humic-like fluorescent components (C1 and C2), which were higher in the on shelf region and decreased towards the shelf edge. The humic-like components and the slope ratio (SR) were significantly correlated with salinity throughout all seasons, indicating a strong influence of terrestrially-derived organic matter in the Celtic Sea, with potentially up to 35% of DOC in the central shelf during winter originating from terrestrial inputs. Results from this study illustrate the importance of monitoring DOM quantitatively and qualitatively for a better understanding of the supply, production, cycling and export of this dynamic organic carbon pool in shelf seas.
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 and temperature 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.
•We determine iron speciation in samples collected from the North-West European Shelf region.•We optimize the NICA-Donnan model for iron binding to dissolved organic matter for the study region.•We predict apparent iron solubility as the sum of soluble inorganic and organic iron species.•Predicted apparent iron solubilities are within range of observed dissolved iron concentrations in waters >500 m deep.
For their size, shelf seas play a disproportionately large role in the oceanic carbon pump. While accounting for only 7% of the surface ocean, the amount of carbon exported from shelf seas ...contributes to between 20 and 50% of total oceanic CO2 storage (Tsunogai et al. 1999). In terms of their socio economic importance, shelf seas support ~ 90% of global fish catches (Pauly et al. 2002), and future projections estimate that up to 4.2 billion people will live within 200 km of the coast by 2030 (Kummu et al. 2016). Shelf seas are the interface between land and ocean and high nutrient inputs and intense physical energy provided mainly by tidal mixing, help to maintain high biological activity on the shelf. Indeed, rates of primary production are up to 3 times greater in shelf seas than in the open ocean (Simpson and Sharples 2012). Through a process known as the continental shelf pump, over 40% of particulate organic matter produced on-shelf is exported to the adjacent ocean (Muller-Karger et al. 2005). The role of dissolved organic matter (DOM) in this pump, in particular, dissolved organic carbon (DOC), is less well understand, despite DOC concentrations in the ocean being 50 times more than carbon in the particulate pool (Eglinton and Repeta 2006), and the amount of carbon in the DOC pool being similar to that of carbon as CO2 in the atmosphere (Hansell and Carlson 1998). Here, to add to the growing body of work on the distribution of DOM in shelf seas, the main goals of this thesis were to (a) map the distribution of DOC and dissolved organic nitrogen (DON) across 5 shelf regions across the Northwest European shelf, and the adjacent North Atlantic; (b) to assess the distribution of DOC and DON in different regions over a seasonal cycle to see if they followed a typical seasonal cycle of production, consumption and loss; (c) to characterise the source of DOM in the Celtic Sea and determine how much of the DOC pool was of terrestrial origin; (d) to assess multi-year trends in DOC and DON in the North Sea specifically and finally (e) for the first time, estimate DOC fluxes between the Celtic Sea and North Atlantic, and compare them with DOC fluxes across the Malin-Hebrides Shelf. DOC and DON distributions across the Northwest European seas were in accordance with global trends, and concentrations decreased with increasing distance from land, highlighting the influence of terrestrially-derived DOM in this region. However, there was large variability within and between regions, highlighting the importance of local controls on DOM production and distribution. In the North Sea, multi-year differences in DOC and DON concentrations showed an overall decline between 2011 and 2015 alongside declines in concentrations of inorganic nutrients, indicating a combination of changing nutrient regimes and variability in the strength of exchange with the North Atlantic. Finally, annual net and springtime surface fluxes from the Celtic Sea to the North Atlantic were significant and higher compared to the Malin-Hebrides Shelf. The estimates were large but within global ranges (Barron and Duarte 2015).
A carbon budget for the northwest European continental shelf seas (NWES) was synthesised using available estimates for coastal, pelagic and benthic carbon stocks and flows. Key uncertainties were ...identified and the effect of future impacts on the carbon budget were assessed. The water of the shelf seas contains between 210 and 230 Tmol of carbon and absorbs between 1.3 and 3.3 Tmol from the atmosphere annually. Off-shelf transport and burial in the sediments account for 60-100% and 0-40% of carbon outputs from the NWES, respectively. Both of these fluxes remain poorly constrained by observations and resolving their magnitudes and relative importance is a key research priority. Pelagic and benthic carbon stocks are dominated by inorganic carbon. Shelf sediments contain the largest stock of carbon, with between 520 and 1600 Tmol stored in the top 0.1 m of the sea bed. Coastal habitats such as salt marshes and mud flats contain large amounts of carbon per unit area but their total carbon stocks are small compared to pelagic and benthic stocks due to their smaller spatial extent. The large pelagic stock of carbon will continue to increase due to the rising concentration of atmospheric CO2, with associated pH decrease. Pelagic carbon stocks and flows are also likely to be significantly affected by increasing acidity and temperature, and circulation changes but the net impact is uncertain. Benthic carbon stocks will be affected by increasing temperature and acidity, and decreasing oxygen concentrations, although the net impact of these interrelated changes on carbon stocks is uncertain and a major knowledge gap. The impact of bottom trawling on benthic carbon stocks is unique amongst the impacts we consider in that it is widespread and also directly manageable, although its net effect on the carbon budget is uncertain. Coastal habitats are vulnerable to sea level rise and are strongly impacted by management decisions. Local, national and regional actions have the potential to protect or enhance carbon storage, but ultimately global governance, via controls on emissions, has the greatest potential to influence the long-term fate of carbon stocks in the northwestern European continental shelf.
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