Pätsch, Johannes, W. Kühn and A. Oschlies

Institute of Oceanography, University Hamburg, Troplowitzstr. 7, D-22529 Hamburg, Germany, E-mail: paetsch@ifm.uni-hamburg.de

Biogeochemical models of the open ocean - important processes on the shelfs neglected?

The influence of riverine carbon and nitrogen loads on the biogeochemistry of the North Atlantic is investigated for the year 1995. We compare two runs of the 3d model CN3 (Pätsch et al., 2002) the first of which one reflects the “normal” approach of modelling open ocean dynamics, i.e. no lateral inputs from the continents and an open lower boundary allowing sinking material to escape after leaving the deepest layer. The second run (“shelf run”) includes river input of DIC, POC, DOC, DIN, PON and DON. Additionally sediment effects were included by recirculating remineralized material into the water column. The carbon data for the river loads were extracted from the global data set by Ludwig et al. (2002). The nitrogen loadings were calculated using the global data set by Seitzinger and Kroeze (1998). For the North Atlantic region 8° – 65° N including the Northwest European shelf and the western boundary shelfs we determined a total C load of 131.640 kt C and a total N load of 7.810 kt N per year, respectively. Whereas the Northwest European Shelf inputs exhibited C:N ratios near the Redfield Ratio (6,6:1) the western boundary inputs had significantly higher C:N ratios of ~20:1. The almost coinciding CO₂ air-sea uptake of both runs (0.59 Gt C y^-1) agreed very well with the corresponding annual flux derived from DpCO₂ data by Takahashi et al. (2002). In contradiction to the expected intensification of the biological pump in highly euthrophic seas the “shelf run” exhibited decreased influx on the Northwest European Shelf and increased atmospheric uptake of CO₂ at the western boundary, especially in the Gulf of Mexico. The latter effect was compensated by a decreased influx of CO₂ along the North Atlantic current. Biological inactivity during winter in higher latitudes vs. production all over the year in nutrient-enriched subtropical regions as well as different hydrodynamical dynamics could explain this apparent paradox.

References

Pätsch, J. et al., 2002: Interannual variability of carbon fluxes at the North Atlantic station ESTOC. Deep-Sea Res. II, 49(1-3): 253-288.

Takahashi T. et. al., 2002: Global sea-air CO₂ flux based on climatological surface ocean pCO₂,and seasonal biological and temperature effects. Deep-Sea Res. II, 49: 1601-1622.

Ludwig, W., P. Amiotte-Suchet and J.L. Probst, 2002: Global data sets fot the atmospheric CO₂ consumption by continental erosion and the riverine fluxes of carbon and sediments to the ocean. (Unpublished).

Seitzinger, S.P. and C. Kroeze, 1998: Global distribution of nitrous oxide production and N inputs in freshwater and coastal marine ecosystems. Glob. Biogeochem. Cycles 12(1): 93-113.