Biogeochemical Cycling Of Carbon, Nitrogen And Phosphorus In The North Sea - CANOPY- Final report

Biogeochemical Cycling Of Carbon, Nitrogen And Phosphorus In The North Sea - CANOPY- Final report

The CANOPY project aimed to determine the importance of the internal cycling processes of uptake and regeneration of carbon, nitrogen and phosphorus in the Southern Bight of the North Sea (SBNS), a marine area receiving riverine inputs from the Scheldt and Rhine rivers and influenced by inflowing English channel waters. The SBNS area was characterized by the presence of 2 large coastal river plumes with high levels of nutrients, and a central part with lower nutrient levels. Dissolved inorganic N (DIN and P (DIP) levels varied seasonally and were minimal for highest phytoplankton biomass, in spring and late summer. Additionally, organic N and P represented important fractions of the total N and P. In the central waters, dissolved organic N (DON), P (DOP) and particulate organic P (POP) dominated the N and P pools in summer, but their relative importance was more limited in the river plumes. For the pelagic system, DIC, DIN and DIP varied seasonally and were highest during the spring phytoplankton bloom, and for some stations, at the end of the summer phytoplankton bloom. Additionally, the uptake of DIN and primary production (PP) were well correlated with a C:N ratio of 6.5 very close to the Redfield ratio. In general, the regeneration of DIN largely exceeded the uptake except in spring, where a more balanced situation was observed. The regeneration of DIP also represented a large fraction of the DIP uptake. Heterotrophic processes are thus very active in the SBNS. They clearly dominate the pelagic N cycling, and are in balance with uptake processes for DIP. At ecosystem scale, it was demonstrated that the Scheldt plume was net heterotrophic during most years and acted as an important source of CO2 for the atmosphere. In contrast, the Scheldt plume also acted as an active sink for both DIN and DIP probably due to active denitrification and physical adsorption processes in this organic matter and SPM rich environment. In contrast, the central waters were on the yearly average autotrophic and acted as a sink for CO2. Again in contrast, there was a net production of DIN and DIP. The comparison of DIN and DIP budgets with DIC budgets showed that correlations exist but were not straightforward. In general, correlations showed a C:N ratio of 3.5 (<Redfield) and a C:P ratio of 109 (=Redfield). This suggests that whatever the season there is a DIC production sustained by OM with a high C:N ratio, either of terrestrial (estuarine) origin or from the sediment. This is agreement with the fact that consumption of DIC by PP exceeds the net ecosystem DIC production. The other possible cause is a large removal of DIN by denitrification in agreement with the fact that net pelagic DIN production largely exceeds net ecosystem DIN production. However, the correlation lines did not pass through 0 showing that primary production and respiration are not the only players in the DIC-DIN-DIP dynamics with the presence of unrelated DIC, DIN and DIP sources our sinks.