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Nutrients in the Coastal Waters of Scotland

This project provides a strategic simulation tool to identify maritime areas which could be at risk of eutrophication as a result of Scottish nutrient discharges. The tool provides spatially resolved output and is capable of discriminating between different types and locations of nutrient inputs, to enable scenario analyses of different nutrient reduction options.

Model Development

An extension ('sc278') of the European Regional Seas Ecosystem Model ( ERSEM) was chosen as the basis for the project. The extended model domain was divided into boxes covering the entire European shelf from Brittany northwards. The water column was divided into two layers, on the basis of water depth and oceanographic characteristics ( Fig. 1).

The ERSEM model describes the specific processes that affect the seasonal cycle of C, N, P and Si in the physical context of the temperate European shelf seas.

These chemical processes are in turn linked to biological processes taking place in the water column and the benthic layer. Fig. 2. shows the flow of orgnanic matter, organism and nutrients in the aquatic environment.

One of the main forcing data sets required for the current application was the nutrient input from land sources to all of the coastal grid cells of the model. For Scotland, daily inputs of both inorganic and organic forms of nitrogen, phosphorus, carbon and silicon from riverine and direct discharges to the sea were resolved by source (urban waste water, industrial, aquaculture and agriculture plus geological).

Although Scotland represents 25% of the land area of the British Isles (including the Republic of Ireland), it contributes 42% of the freshwater runoff (averaged over 1984, 1987 and 1990). However, the N and P load from Scotland is only around 15% of the British Isles total (approximately 140,000 tonnes N and 14,000 tonnes P). Scotland contributes a disproportionate amount of Si to the British Isles total loading, presumably due to the terrain and geology. On a Europe-wide basis, Scotland contributes less than 10% of the total N and P loading, but 26% of the Si loading.

Reference runs of the ERSEM were carried out using 1984, 1987 and 1990 climatological forcing (transport, irradiance and agricultural, plus geological nutrient inputs) together with nutrient inputs from urban waste and industrial sources set at the levels estimated for 1999, and from aquaculture in 2001. The model was then run for three nutrient load reduction scenarios and the results compared to those from the reference runs. The three scenarios were:

  • 75% reduction in Scottish urban waste water
  • 50% (OSPAR defined) reduction in all Scottish inputs
  • 50% reduction in Scottish aquaculture inputs

Results

The impacts of nutrient load reductions were largely confined to the immediate locality of the inputs (at least on the scales resolved by the model; Fig. 3). The wider Scottish east coast area was the only one of a number of larger regional areas examined which exhibited impacts in excess of the natural climate-driven variability demonstrated by the reference run results.

At a local scale, the model identified the Clyde Sea, and especially the Forth/Tay river plume as areas which should be examined in more detail and at a finer spatial scale for evidence of eutrophication, since the simulated impact of the nutrient load reductions was in excess of the natural climatological variability.

The model indicated that the impact of a 50% reduction in nutrient discharges from Scottish salmon farming was likely to be small (3% or less change in assessment criteria) both locally at the scale of the model, and regionally ( Fig. 4). This was below the natural variability in the system in the affected areas. The OSPAR data were results from the 1996 ASMO Workshop on eutrophication modelling, where Europe-wide 50% reductions in total nitrogen and phosphorus emissions were imposed on the nd130 version of ERSEM with 1985 climatology. Areas 1-14 are the composite areas made up of at least three individual ERSEM boxes. Areas 35, 65 and 75 are the individual ERSEM boxes on the Scottish east coast of the same numbers (Inverness Firth, Forth/Tay and Farne Islands respectively), and areas 47, 74 and 100 are the ERSEM boxes on the west coast of the same numbers (Skye, Clyde and Solway respectively). ASMO nd130 results were not available for the individual ERSEM boxes, or for the Skagerrak and Norwegian Coast (areas 5 and 6).

The impact of Scotland's nutrients on coastal waters appears to be considerably less than that of some other nations. In this project, a 50% reduction in Scottish nutrients produced a 5% or less mean change in overall water quality in Scottish east coast waters (equivalent to 0.2-10 gC m-2 year-1 change in net primary

production). Previous simulations of a European-wide 50% reduction in nutrient inputs, using the North Sea version of ERSEM, produced a similar change in Scottish waters, but around a 15% or greater change in Belgian, Dutch and German coastal waters (equivalent to 39-77 gC m-2 year-1 net primary production).