Carbon exchange between a shelf sea and the ocean: The Hebrides Shelf, west of Scotland

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

Carbon exchange between a shelf sea and the ocean: The Hebrides Shelf, west of Scotland

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

Ocean acidification and calcium carbonate saturation states in the coastal zone of the West Antarctic Peninsula

The polar oceans are particularly vulnerable to ocean acidification; the lowering of seawater pH and carbonate mineral saturation states due to uptake of atmospheric carbon dioxide (CO2). High spatial variability in surface water pH and saturation states (Ω) for two biologically-important calcium carbonate minerals calcite and aragonite was observed in Ryder Bay, in the coastal sea-ice zone of the West Antarctic Peninsula. Glacial meltwater and melting sea ice stratified the water column and facilitated the development of large phytoplankton blooms and subsequent strong uptake of atmospheric CO2 of up to 55 mmol m-2 day-1 during austral summer. Concurrent high pH (8.48) and calcium carbonate mineral supersaturation (Ωaragonite ~3.1) occurred in the meltwater-influenced surface ocean. Biologically-induced increases in calcium carbonate mineral saturation states counteracted any effects of carbonate ion dilution. Accumulation of CO2 through remineralisation of additional organic matter from productive coastal waters lowered the pH (7.84) and caused deep-water corrosivity (Ωaragonite ~0.9) in regions impacted by Circumpolar Deep Water. Episodic mixing events enabled CO2-rich subsurface water to become entrained into the surface and eroded seasonal stratification to lower surface water pH (8.21) and saturation states (Ωaragonite ~1.8) relative to all surface waters across Ryder Bay. Uptake of atmospheric CO2 of 28 mmol m-2 day-1 in regions of vertical mixing may enhance the susceptibility of the surface layer to future ocean acidification in dynamic coastal environments. Spatially-resolved studies are essential to elucidate the natural variability in carbonate chemistry in order to better understand and predict carbon cycling and the response of marine organisms to future ocean acidification in the Antarctic coastal zone

The elemental stoichiometry (C, Si, N, P) of the Hebrides Shelf and its role in carbon export

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 pool and least efficient for the particulate pool. N loss from the shelf however was most efficient via the dissolved organic nutrient pool. Mass balance calculations suggest that 28% of PO43−, 34% of NO3− and 73% of Si drawdown from the mixed layer fails to reappear in the benthic water column thereby indicating the proportion of the nutrient pools that must be resupplied from the ocean each year to maintain shelf wide productivity. Loss to the neighbouring ocean, the sediments, transference to the dissolved organic nutrient pool and higher trophic levels are considered the most likely fate for these missing nutrients

Climate Change and the Marine Environment: Impact of Increased CO2 Concentration and Ocean Acidification on the Genetic Diversity of a Key Phytoplankton Species

This study investigated the effects of ocean acidification on the genetic diversity of Synechococcus. A mesocosm experiment simulating ocean acidification was carried out by scientists at Plymouth Marine Laboratory at the Large Scale Facility in Bergen, Norway, in May 2006. Mesocosm bags were seeded with C02 to replicate the rise in atmospheriC CO2 concentrations to the level of 700ppm as projected by the IS92a "business as usual" scenario, and were run in conjunction with background bags seeded with air. Changes in Synechococcus microdiversity between these two treatments and a fjord sample at the start of the experiment were assessed using the rpoC1-RFLP method, whereby clone libraries were constructed for three sampling dates. The novel primer pair lpoC1-39F-L4a1lpoC1-462R-L4 was tested for its ability to amplify coastal strains of Synechococcus. Multivariate statistical analyses revealed that there was no significant link between ocean acidification and Synechococcus microdiversity. Moreover, It was not possible to distinguish between treatment and mesocosm effects, with a BIO-ENV analysis revealing that microdiversity was the most highly correlated (p=O.S71) with nutrient concentrations, particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations within the mesocosm bags. However, the significance level of the sample statistic was low (20%). Therefore, it was found that increasing the number of independent observations by sampling replicate mesocosms is necessary in order to ascertain whether decreasing seawater pH affects Synechococcus microdiversity. However, changes to abundance and occurrence of the 17 RFLP types observed were evident, the most notable being the detection of four novel RFLP types unique to the acidified bag on the 21 st of May. Based upon phylogenetic analysis of the rpoC1 sequences of each RFLP type, it was found that all RFLP types clustered into two putative subclades within the previously defined Synechococcus clade I, which encompasses isolates found in coastal locations.Faculty of Scienc

Rapid increase of observed DIC and pCO(2) in the surface waters of the North Sea in the 2001-2011 decade ascribed to climate change superimposed by biological processes

The CO2 system in the North Sea over the 2001-2011 decade was investigated using four comprehensive basin-wide datasets covering the late summer periods of 2001, 2005, 2008 and 2011. We find that rises in surface water DIC and pCO(2) exceeded concurrent rises in atmospheric pCO(2), which we attribute primarily to biological activity in late summer. After accounting for this biological signal, the observed ocean acidification occurs at a rate that is consistent with concurrent atmospheric and open ocean CO2 increases over the 2001-2011 decade. Nevertheless, we do find a consistent reduction in CO2 undersaturation in the NNS and an increase in CO2 supersaturation in the SNS. We propose that the synergistic effects of increasing atmospheric pCO(2) and subsequent decrease in seawater buffering capacity, together with rising sea surface temperatures in the future oceans, may reduce the strength of the North Sea as a CO2 sink. Such a reduction would diminish the efficiency of this region as a continental shelf pump with respect to uptake of CO2 by the sea. Ultimately this would constitute a positive feedback mechanism, i.e. enhancing the airborne fraction of anthropogenic CO2 and thus the net rate of increase of atmospheric pCO(2) and subsequent global climate change. (C) 2015 Elsevier B.V. All rights reserved.</p

Rapid increase of observed \DIC\ and pCO2 in the surface waters of the North Sea in the 2001-2011 decade ascribed to climate change superimposed by biological processes

Cycles of metals and carbon in the oceans - A tribute to the work stimulated by Hein de BaarInternational audienceAbstract The \CO2\ system in the North Sea over the 2001-2011 decade was investigated using four comprehensive basin-wide datasets covering the late summer periods of 2001, 2005, 2008 and 2011. We find that rises in surface water \DIC\ and pCO2 exceeded concurrent rises in atmospheric pCO2, which we attribute primarily to biological activity in late summer. After accounting for this biological signal, the observed ocean acidification occurs at a rate that is consistent with concurrent atmospheric and open ocean \CO2\ increases over the 2001-2011 decade. Nevertheless, we do find a consistent reduction in \CO2\ undersaturation in the \NNS\ and an increase in \CO2\ supersaturation in the SNS. We propose that the synergistic effects of increasing atmospheric pCO2 and subsequent decrease in seawater buffering capacity, together with rising sea surface temperatures in the future oceans, may reduce the strength of the North Sea as a \CO2\ sink. Such a reduction would diminish the efficiency of this region as a continental shelf pump with respect to uptake of \CO2\ by the sea. Ultimately this would constitute a positive feedback mechanism, i.e. enhancing the airborne fraction of anthropogenic \CO2\ and thus the net rate of increase of atmospheric pCO2 and subsequent global climate change