Search

Shelf seas play an important role in the global carbon cycle, absorbing atmospheric carbon dioxide (CO2) and exporting carbon (C) to the open ocean and sediments. The magnitude of these processes is poorly constrained, because observations are typically interpolated over multiple years. Here, we used 298500 observations of CO2 fugacity (fCO(2)) from a single year (2015), to estimate the net influx of atmospheric CO2 as 26.2 +/- 4.7 Tg C yr(-1) over the open NW European shelf. CO2 influx from the atmosphere was dominated by influx during winter as a consequence of high winds, despite a smaller, thermally-driven, air-sea fCO(2) gradient compared to the larger, biologically-driven summer gradient. In order to understand this climate regulation service, we constructed a carbon-budget supplemented by data from the literature, where the NW European shelf is treated as a box with carbon entering and leaving the box. This budget showed that net C-burial was a small sink of 1.3 +/- 3.1 Tg C yr(-1), while CO2 efflux from estuaries to the atmosphere, removed the majority of river C-inputs. In contrast, the input from the Baltic Sea likely contributes to net export via the continental shelf pump and advection (34.4 +/- 6.0 Tg C yr(-1)).

A detailed analysis of the internal stoichiometry of a temperate latitude shelf sea system is presented which reveals strong vertical and horizontal gradients in dissolved nutrient and particulate concentrations and in the elemental stoichiometry of those pools. Such gradients have implications for carbon and nutrient export from coastal waters to the open ocean. The mixed layer inorganic nutrient stoichiometry shifted from balanced N:P in winter, to elevated N:P in spring and to depleted N:P in summer, relative to the Redfield ratio. This pattern suggests increased likelihood of P limitation of fast growing phytoplankton species in spring and of N limitation of slower growing species in summer. However, as only silicate concentrations were below potentially limiting concentrations during summer and autumn the stoichiometric shifts in inorganic nutrient N:P are considered due to phytoplankton nutrient preference patterns rather than nutrient exhaustion. Elevated particulate stoichiometries corroborate non-Redfield optima underlying organic matter synthesis and nutrient uptake. Seasonal variation in the stoichiometry of the inorganic and organic nutrient pools has the potential to influence the efficiency of nutrient export. In summer, when organic nutrient concentrations were at their highest and inorganic nutrient concentrations were at their lowest, the organic nutrient pool was comparatively C poor whilst the inorganic nutrient pool was comparatively C rich. The cross-shelf export of these pools at this time would be associated with different efficiencies regardless of the total magnitude of exchange. In autumn the elemental stoichiometries increased with depth in all pools revealing widespread carbon enrichment of shelf bottom waters with P more intensely recycled than N, N more intensely recycled than C, and Si weakly remineralized relative to C. Offshelf carbon fluxes were most efficient via the inorganic nutrient pool, intermediate for the organic nutrient po

The wintertime spatial distribution of carbonate parameters in outer estuarine and coastal waters around Ireland is described from total alkalinity (TA) and dissolved inorganic carbon (DIC) data collected between 2010 and 2013. Due to predominantly limestone bedrock of their river catchments, the River Shannon and Barrow, Nore and Suir River system export high concentrations (>3800 μmol kg−1) of TA to their estuarine and inshore coastal waters where estuarine alkalinity decreases with increasing salinity. TA is lower in rivers with a non-calcareous bedrock, with positively correlated alkalinity-salinity relationships in both the Lee and Foyle outer estuaries. Winter pCO2 in the Shannon, Barrow/Nore/Suir and Lee estuaries is supersaturated relative to atmospheric CO2, while pCO2 in the outer Liffey estuary is slightly lower than atmospheric CO2 in three consecutive winters, indicative of a CO2 sink. Winter pCO2 is close to atmospheric equilibrium along the western shelf and through the centre of the Irish Sea, while it is a CO2 sink across the North Channel. While aragonite was supersaturated in most Irish waters, it was close to undersaturation in both the Lee estuary, attributed to its low alkalinity freshwater source, and Barrow/Nore/Suir estuary related to the flux of high concentrations of DIC from this river system. The seasonal impacts on inorganic carbon chemistry was also investigated by comparing winter and summer data collected between 2009 and 2013 along two transects in western coastal waters and along the western shelf edge. DIC was ~60 μmol kg−1 lower in summer relative to winter in the coastal transects and 39 μmol kg−1 lower along the shelf edge, accompanied by depleted nutrients and supersaturation of dissolved oxygen during summer, indicative of primary production. TA was generally higher in summer relative to winter corresponding with a decrease in nitrate, indicating that primary production dominated the TA distribution over calcification. An exc

Global mass balance calculations indicate the majority of particulate organic carbon (POC) exported from shelf seas is transferred via downslope exchange processes. Here we demonstrate the downslope flux of POC from the Hebrides Shelf is approximately 3-to-5-fold larger per unit length/area than the global mean. To reach this conclusion we quantified the offshore transport of particulate and dissolved carbon fractions via the “Ekman Drain”, a strong downwelling feature of the NW European Shelf circulation, and subsequently compared these fluxes to simultaneous regional air-sea CO2 fluxes and on-shore wind-driven Ekman fluxes to constrain the carbon dynamics of this shelf. Along the shelf break we estimate a mean offshelf total carbon (dissolved + particulate) flux of 4.2 tonnes C m−1 d−1 compared to an onshelf flux of 4.5 tonnes C m−1 d−1. Organic carbon represented 3.3% of the onshelf carbon flux but 6.4% of the offshelf flux indicating net organic carbon export. Dissolved organic carbon represented 95% and POC 5% of the exported organic carbon pool. When scaled along the shelf break the total offshelf POC flux (0.007 Tg C d−1) was found to be three times larger than the regional air-sea CO2 ingassing flux (0.0021 Tg C d−1), an order of magnitude larger than the particulate inorganic carbon flux (0.0003 Tg C d−1) but far smaller than the DIC (2.03 Tg C d−1) or DOC (0.13 Tg C d−1) fluxes. Significant spatial heterogeneity in the Ekman drain transport confirms that offshelf carbon fluxes via this mechanism are also spatially heterogeneous. This article is protected by copyright. All rights reserved.