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

DYMONIS is a numerical model designed to test the interaction of processes linking increased inputs of nutrients to possible increases in phytoplankto n production (eutrophication). Originally developed for the North Sea, it h as been applied to the Irish Sea. A basic process, which must be modelled b efore more complex ones, is represented by the salinity held, which passive ly 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. Compariso n with model output of observed salinities from the long-term data series f rom the Isle of Man "Cypris" station shows that real weather effects on cir culation have a greater effect on salinity than probable changes in river d ischarges. Nutrient concentrations are determined by the mixing of river an d ocean waters, removal by autotrophic organisms and return from detrital p hases. The model shows that the minimum nitrate concentration in winter in the southern Irish Sea is a consequence of the varying balance of these pro cesses with time. It can be demonstrated that long-term increases in the co ncentration of nitrate off the Isle of Man could be a simple artefact of th e increase in river and atmospheric inputs and of the essentially estuarine nature of the Irish Sea. (C) 2000 International Council for the Exploratio n of the Sea.