Numerical modelling of the shelf break ecosystem: reproducing benthic and pelagic measurements

A coupled pelagic-benthic biogeochemical model, embedded in a turbulence-closure formulation is employed for the Goban Spur shelf-break area (northeast Atlantic). Our main objectives are to examine the impact of in situ atmospheric conditions on ecosystem dynamics, to reproduce biogeochemical distributions in the water column and the sediments, and to derive a nitrogen budget for the area. Given a data set of atmospheric forcing conditions at 3-h intervals, the model successfully explains the time evolution of the temperature field. Most biochemical water column properties are reasonably well simulated, both in timing and in magnitude. Some of the short-term variability, apparent in the data, can be reproduced, suggesting that this may result from variability in the in situ atmospheric forcing. In summer, intermittent mixing events generate increased ammonium and nitrate concentrations in the upper water column, consistent with observations. These short-term nutrient injections substantially increase euphotic zone production, mainly by stimulating new production. The model also reproduces a set of benthic nutrient profiles, measured on two occasions, both qualitatively and quantitatively. The results suggest that there is a significant variability in benthic properties. A tentative nitrogen budget for the Goban Spur shelf break area is proposed. The sediments account for about 7% of organic nitrogen respiration; about 42% occurs in the euphotic zone, and the remaining 50% takes place in the water column below the euphotic zone. About 3% of the annual primary production of organic nitrogen is denitrified in the sediments and is replenished from lateral sources in the model. Nitrification mainly takes place in the water column below the euphotic zone (66%); sedimentary nitrification and ammonium oxidation in the euphotic zone both account for 17%. Over the year, only 55% of euphotic zone nitrogen assimilation is based on the in situ regenerated inorganic nitrogen, the remainder is mainly supplied by mixing from below the euphotic zone, either in the form of nitrate (72% ) or ammonium (28%). The implications of these nitrogen pathways in the euphotic zone on the measured f-ratio are discussed.