Development of a 2D horizontal biogeochemical model for the Irish Sea DYMONIS

Abstract

Le Gall, A. C., Hydes, D. J., Kelly-Gerreyn, B. A., and Slinn, D. J. 2000. Development of a 2D horizontal biogeochemical model for the Irish Sea DYMONIS. – ICES Journal of Marine Science, 57: 1050–1059. DYMONIS is a numerical model designed to test the interaction of processes linking increased inputs of nutrients to possible increases in phytoplankton production (eutrophication). Originally developed for the North Sea, it has been applied to the Irish Sea. A basic process, which must be modelled before more complex ones, is represented by the salinity field, which passively traces the mixing of river and sea water. To fit the observed data, this requires a 1.5-fold increase in the current estimate of rain inputs. This is reasonable, given the uncertainty in estimates of such inputs. Comparison with model output of observed salinities from the long-term data series from the Isle of Man ‘‘Cypris’’ station shows that real weather effects on circulation have a greater effect on salinity than probable changes in river discharges. Nutrient concentrations are determined by the mixing of river and ocean waters, removal by autotrophic organisms and return from detrital phases. The model shows that the minimum nitrate concentration in winter in the southern Irish Sea is a consequence of the varying balance of these processes with time. It can be demonstrated that long-term increases in the concentration of nitrate off the Isle of Man could be a simple artefact of the increase in river and atmospheric inputs and of the essentially estuarine nature of the Irish Sea. 2000 International Council for the Exploration of the Sea