Modelled carbon fluxes as validated by field dta on the north Norwegian shelf during the productive period in 1994

Abstract

Abstract A 3-dimensional coupled hydrodynamic and biological model has been used to study the effect of the physics on the productivity and the carbon export of the ecosystem outside Trams county, northern Norway. The horizontal grid point distance is 4 km. The ecosystem model consists of eight state variabies (nitrate, ammonium, silicate, diatoms, flagellates, microzooplankton, fast sinking detritus, slow sinking detritus) and assumes that nitrogen and silicate are the limiting nutrients. Mseasured mesozooplankton biomass (mainly Calanus finmarchicus) is used to impose the effect of grazing. The parameters used by the model are mostly taken from the literature, but monthly measurements of CTD, nutrients, chlorophyll, micro and mesozooplankton, and sedimentation have been used to validate the model output. The measured data vvere compared and contrasted with two model scenarios (i.e. without and with advection), where the latter run was in best agreement with the field data. This may be due to a more realistic physical setting, since without advection, the nutrient supply comes only via vertical mixing, while with advection nutrients are being advected into the euphotic zone via topographically steered upwelling. These upwelling “hot spots” are found downstream with an elevated annual production 160 g C m{-su2}, of which 70% is new production. Comparing the partitioning of carbon between the components in the pelagic food webs in the Bering Sea and the north Norwegian shelf, the highest similarity in species assemblages may exist between the outer Bering Sea shelf and the north Norwegian shelf. The mesozooplankton carbon flow in both areas is calculated to approximately 60 g C m-2 yr1. An important difference in the routing of the flow is present, where at the Norwegian shelf mesozooplankton grazing in the model was fuelled exclusively by microzooplankton. The modelled carbon fluxes outside North Norway during the study period may be influenced by topography, and the highest phytoplankton crop is found near the high productive area, close to the shelf break after the spring bloom. The banks (as Nordvestbanken) receive much of their water from the adjacent shelf region, which in general b.2,s less biomass than the shelfbreak. The trenches like Malangsdjupet receive most of their water from the shelfbreak and will therefore in general tend to have net import of biomass which sinks out to deeper layers where the residence time is longer.