Oceanic controls on the primary production of the northwest European continental shelf under recent past and potential future conditions

Abstract

In this paper we clearly demonstrate that changes in oceanic nutrientsare a first or-der factor in determining changes in the primary production of thenorthwest Europeancontinental shelf on time scales of 5–10 yr. We present a series ofcoupled hydrody-namic ecosystem modelling simulations, using the POLCOMS-ERSEM system.Theseare forced by both re-analysis data and a coupled ocean-atmospheregeneral circu-lation model (OA-GCM) representative of possible conditions in 2080–2100under anSRES A1B emissions scenario, along with the corresponding presentday control. TheOA-GCM forced simulations show a substantial reduction in surfacenutrients in theopen-ocean regions of the model domain, comparing future and presentday time-slices. This arises from a large increase in oceanic stratification.Tracer transportexperiments identify a substantial fraction of on-shelf water originatesfrom the open-ocean region in the south of the domain, where this increase is largest,and indeedthe on-shelf nutrient and primary production are reduced as this wateris transportedon shelf. This relationship is confirmed quantitatively by comparingchanges in win-ter nitrate with total annual nitrate uptake. The reduction in primaryproduction bythe reduced nutrient transport is mitigated by on-shelf processesrelating to tempera-ture, stratification (length of growing season) and recycling. Regionsless exposed toocean-shelf exchange in this model (Celtic Sea, Irish Sea, EnglishChannel, and south-ern North Sea) show a modest increase in primary production (of 5–10%) comparedwith a decrease of 0–20 % in the outer shelf, central and northernNorth Sea. Thesefindings are backed up by a boundary condition perturbation experimentand a simplemixing model.