Processes controlling the net carbon consumption of inorganic carbon in the North Sea

Data on dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2) and nutrients have been collected during four cruises carried out in the whole North Sea, each cruise covering one month of each season between 2001 and 2002. The impact of biological activity, air-sea exchange and physical mixing processes on the monthly variations of DIC have been determined for each regions of the North Sea using the International Commission for the Exploration of the Seas (ICES) boxes separation. For the February-July period, the biological activity was the main factor controlling the DIC variations in the upper layer of the whole North Sea and was responsible for a loss of 20 to 50 mmol m-3 month-1. The concomitant atmospheric input increased DIC by 8 to 12 mmol m-3, whereas the mixing term was low, except in the north-western North Sea, where it increased DIC concentrations by approximately 10 mmol m-3.We computed the Net Community Production based on Carbon (DIC) data (NCPC) for the North Sea. For the productive period, the NCPC was higher in the upper layer of the northern and central North Sea than in the coastal and southern areas. Maximum values of 1.0 to 1.5 mol C m-2 month-1 were observed inMay in the northern and central North Sea, whereas maximum of 0.5-0.7 mol C m-2 month-1 were observed in April in the southern and coastal areas of the North Sea.We compared the NCPC to the NCP calculated from nitrogen data from the same cruises and converted into moles of carbon using the Redfield ratios (NCPN). Results showed that in areas where nitrate was depleted by April, i.e. mainly in the central and northern North Sea, DIC consumption continues until July, leading to a large difference between NCPC and NCPN. On the other hand, in coastal areas where nitrate were available during the complete productive period, NCPC and NCPN were very similar. For the whole year in the upper layer of the North Sea, the NCPC of 2.3 mol C m-2 yr-1 was a factor two higher than NCPN and higher than previous calculation of the net carbon production in the North Sea based on nutrient data. We argue that NCP based on nutrient data converted to moles of carbon using the Redfield ratios, leads to underestimation of the net carbon production for the North Sea because of the preferential recycling of nutrients

Four years of dissolved inorganic carbon dynamics along the Belgian coast

CO2 partial pressure (pCO2) of surface seawater along the Belgian coast (from the Scheldt mouth to offshore waters) is measured continuously since September 2000 from an autonomous system deployed on board of the R.V. Belgica. In parallel, 30 campaigns were carried on in that area to measure total alkalinity, dissolved oxygen, chlorophyll-a and nutrients (nitrate and nitrite, silicate and phosphate) from January 2001 to October 2004. We propose then to establish, compare and validate a one box LOICZ budget between dissolved inorganic carbon, phosphorus and nitrogen. And, to determine the CO2 air-sea exchange, the Net Ecosystem Production and calcification with seasonal and annual variability for the outer Scheldt

, Air-sea exchange of nitrogen and carbon over the Northwest-European Continental Shelf

The ecosystem model ECOHAM3 we developped simulates the marine carbon, nitrogen and oxygen cycle.We used the model to evaluate especially the fluxes between the shelf and the atmosphere, i.e. the absorption and release of CO2, the aeolian input of bioavailable nitrogen compounds and the release of molecular nitrogen due to benthic denitrification. The simulations were performed for the mid-nineties, a period with an extremely large variation of the NAO index with the consequence of very different meteorological conditions. Time series of daily riverine nitrogen and carbon loads were used, while for the atmospheric deposition of nitrate and ammonium only annual means from EMEP (European Monitoring and Evaluation Program) were available. According to our model results the Northwest European Shelf acts as a ‘continental shelf pump’, i.e. it effectively absorbs atmospheric CO2 (300 Gmol C yr−1 in the high-NAOI years 1994/95 and 560 Gmol C yr−1 in 1996 with the very low NAOI) and exports dissolved inorganic carbon into the neighbouring open North Atlantic. At the same time the shelf, especially the North Sea, gets a large amount of nitrogen via atmospheric deposition (27 Gmol N yr−1) as well as by river input (50-80 Gmol N yr−1) which is biologically used. A large part of the nitrogen the North Sea is supplied with, including a considerable advective net inflow of organic nitrogen, is released to the atmosphere as molecular nitrogen (about 70 Gmol N yr−1). In spite of this strong benthic denitrification the North Sea acts as net source of dissolved inorganic nitrogen for the North Atlantic; this is a consequence of the net heterotrophy of the North Sea

Variability of the CO2 air sea fluxes in the North Sea

In order to investigate the role of coastal seas in the global carbon cycle, an intense field campaign has been carried out in the North Sea located at the NW European shelf. The partial pressure of CO2 (pCO2) and related parameters have been measured in the surface waters of the North Sea with high spatial and temporal resolution in all 4 seasons consecutively. The variability of the pCO2 and the corresponding CO2 air-sea fluxes will be investigated. Two biogeochemical regions can be identified in the North Sea, and the different control mechanisms such as biology, temperature, terrestrial inputs of the pCO2 will be unraveled. The effects of climate change processes on the CO2 air-sea fluxes will be assessed employing an ecosystem model for three different scenarios: increased temperature, increased CO2 and increased temperature and CO2

Why is pCO2 in the North Sea rising faster than in the Atmosphere?

The carbon cycle of the North Sea, a NW European Shelf sea, has been investigated in detail in 2001/2002 covering the North Sea by a 90 stations grid during all four seasons. Underway measurements of the partial pressure of CO2 (pCO2) complemented the station sampling. In 2005 the 90 stations and surface waters have been resampled again during the same period in summer in order to investigate the temporal variability of the carbon and related nutrient cycles. The atmospheric pCO2 increased from approximately 364ppm in 2001 to 375ppm in 2005, however, the pCO2 observations reveal that the surface water pCO2 increased at a higher rate. This in turn means that the partial pressure difference, being the driver of the CO2 air-to-sea flux, decreased. Similar trends have recently been reported by others for the open North Atlantic Ocean. While necessarily assuming the North Sea carbon cycle to be in steady-state during the initial evaluations of the 2001/2002 data, the consideration of the recent 2005 data allows us to identify an increase of the water column DIC inventory from 2001 to 2005, this in addition to the CO2 exported to the North Atlantic Ocean via the continental shelf pump mechanism. Several hypotheses will be compared for the discrepancy between the rates of increase of the atmospheric and surface water pCO2 in order to define the most likely explanation

The inorganic carbon chemistry in coastal and shelf waters around Ireland

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 exception to this was at two stations along the shelf edge where TA was lower in summer relative to winter (51 μmol kg−1) and coincides with high reflectance in satellite images from a coccolithophore bloom at the time of sampling. While pCO2 was close to atmospheric equilibrium along the shelf edge during winter, this area was a CO2 sink during summer, apart from the stations where calcification was likely occurring resulting in elevated CO2 relative to atmospheric concentrations