Abstract
Abstract New production of organic matter from photosynthesis based on “new” nitrate transported into the illuminated surface layer fuels temperate ecosystems during periods of stratification when surface waters are nutrient limited. Published observations from the northeastern North Sea show a large spatial heterogeneity in vertical nitrate fluxes and suggest shelf edge mixing may be the major source for new production here during the stratified summer season. In the current study, we further investigate these empirical findings with a numerical model, where physical transports and mixing are evaluated against observations of temperature, salinity, nutrients and dissipation of turbulent kinetic energy. The relatively shallow central North Sea is separated from the deep Norwegian trench by a strong shelf edge current. This shelf edge frontal zone is characterized by a vertical separation of the surface and benthic boundary layers by an intermediate layer exhibiting low turbulence. A new nitrate assimilation model, driven by light and nitrate availability, is developed and applied for quantifying the potential for, and distribution of, new production in the area. New production in the frontal zone above the shelf edge is located in a narrow high productive (~100 mg C m−2 day−1) band. This is in qualitative accordance with observations. The model results also suggest, however, that new production of similar magnitude occurs above the deep Norwegian trench, where a shallow nutricline in combination with mesoscale eddy activity leads to increased transport of nitrate to the surface layer. Increased new production along the shelf edge could potentially impact ecosystem structure and may explain the relatively high species richness and fishing activity recorded in this part of the North Sea.
Highlights
Primary production in the ocean is understood as being comprised of two forms. Dugdale and Goering (1967) designated these as new and regenerated production, where the former is driven by an allochthonous nutrient source and the latter by a nitrogen source that has been regenerated locally, i.e., within the system, itself
∫ accumulated nitrate assimilation (AcNA) = Nitrate assimilation (NA)(N, photosynthetic available radiation (PAR)) dt Physical conditions and nitrate assimilation were analyzed along the five transects in the northeastern North Sea occupied during the VERMIX study
The five transects cover a region above the deep Norwegian trench, where Atlantic water masses gradually mix with less saline water from the Baltic Sea and the southern North Sea
Summary
Primary production in the ocean is understood as being comprised of two forms. Dugdale and Goering (1967) designated these as new and regenerated production, where the former is driven by an allochthonous nutrient (nitrogen) source and the latter by a nitrogen source that has been regenerated (through heterotrophic processes) locally, i.e., within the system, itself. Dugdale and Goering (1967) designated these as new and regenerated production, where the former is driven by an allochthonous nutrient (nitrogen) source and the latter by a nitrogen source that has been regenerated (through heterotrophic processes) locally, i.e., within the system, itself. The differentiation between these two forms is important as it is only new production that can lead to a net increase in the production of organic material. Two regions where total primary production estimates are similar can have vastly different potential for supporting productive food webs and/or contributing to export of organic material from surface to deep waters
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