Spatially resolved monthly riverine fluxes of oxidised nitrogen (nitrate and nitrite) to the European shelf seas, 1960-2005

This report documents the methodology developed for statistically modelling the spatial and temporal patterns of oxidised nitrogen (nitrate plus nitrite) riverine fluxes into the coastal waters of northwestern Europe, over the period 1960-2005. The purpose of the study was to provide boundary data for a modelling study of new primary production in European waters. For the UK and Ireland, monthly freshwater discharges to a set of grid cells around the coastline were modelled from rainfall data and calibrated from detailed analyses performed for a subset of years with contrasting climatology. The mean and long-term trends in nitrate and nitrite content of the river discharges were modelled from Harmonised Monitoring Scheme data and flux estimates for each of the years of contrasting climateology. The product of the discharge rate and nitrogen content provided estimates of the monthly flux to each grid cell. Scandinavian inputs of nitrate and nitrite to the North Sea, Skagerrak and Kattegat were assembled from a composite of statistically modelled freshwater discharge, and recent estimates of nitrogen flux from national monitoring agencies. Fluxes of nitrate and nitrite from the rivers flowing into the North Sea from Germany, Netherlands and Belgium during 1960-2005 were assembled from previous analyses by researchers at the University of Hamburg. Nitrate and nitrite fluxes from French rivers flowing into the English Channel, in particular the Seine, were indirectly derived by correlation with the River Scheldt, calibrated from published estimates of annual fluxes. The results show the total oxidised nitrogen input to European shelf seas increasing from approximately 0.6Mt pa. in the 1960's to 1.2Mt pa. in the mid-1980's. Recent estimates of the annual flux since 2000 have been approximately 1.1Mt pa. Around 60-70% of the total annual flux to the northwest European shelf enters via the North Sea. Winter input rates are approximately twice those in the summer in all areas except the Skagerrak/Kattegat

Modelling the behaviour of nutrients in the coastal waters of Scotland - an update on inputs from Scottish aquaculture and their impact on eutrophication status

A previous study estimated that salmon farming contributed approximately 6% of Scotland's nitrogen-nutrient input to coastal waters, and 13% of phosphorus (based on 2001 production figures). However, in some areas of the west of Scotland with small freshwater catchment areas and low levels of human habitation, aquaculture inputs represented greater than 80% of the total. In 2002, FRS published results from an ecosystem modelling study involving a collaboration with the Institute for Marine Research, University of Hamburg, and the Macaulay Land Use Research Institute in Aberdeen, to assess the eutrophication impact of various nutrient inputs to Scottish waters. The results suggested that a 50% reduction in aquaculture salmon production would have only a small impact on water quality which would be undetectable against the background of natural variability due to climate variations. Estimating aquaculture nutrient discharge is a difficult task. The 2002 study was based on data relating to the consented biomass of fish at farm sites in sea lochs. Since then, new data have become available on the actual harvest of fish at all sites in Scotland. In this report, we re-assess the salmon production in Scotland in 2001 and the consequent nutrient discharge, and repeat the ecosystem model runs to estimate the impact of reduction scenarios on eutrophication status. The new data indicate that the previous study had overestimated salmon production and nutrient discharge by approximately 18% Scotland wide. Production and discharge at Shetland and in the Southern Hebrides had been under-estimated, whilst that in the Minches had been over-estimated. New runs of the ecosystem model show that the original conclusions on eutrophication impact were sound. A scenario of 50% reduction in salmon production produced regional changes in water quality which were less than 25% of the natural variability due to climate. New runs simulating a cessation of aquaculture showed that even this extreme reduction scenario produced changes in water quality that were less than half the natural variability