Marine environment: ecosystem modelling

Virtually all of the research undertaken by Marine Scotland Science results in the development of a biological model because the goal of scientists is to find out meaningful, relevent information which describes the relationships between different aspects of life in the sea.

Biological models (created the development of ecosystem models and the design and implementation of Biophysical modelling tools) are a way for a scientist to show what biological processes look like. In general, these processes are expressed in numbers - the essence of biological phenomena are captured as formulae or equations.

Biological models can be as simple as a general relationship between food and growth, or as complex as a description of the detailed interactions within a highly structured ecosystem. No matter how simple or complicated a model is, it will always be just a snapshot of the natural process it is trying to replicate. The challenge is to identify and retain the most relevant elements.

Eosystem Modelling

Marine Scotland Science (MSS) has worked in collaboration with other organisations to design, assemble and test ecosystem models of sea lochs and shelf waters.

These ecosystem models are used to develop mathematical relationships which simulate the transfer of energy and mass (e.g. carbon, nutrients) between and within the various trophic levels in the ecosystem and they can be used for a variety of applications; a recent example is the modelling work carried out to investigate the behaviour of Nutrients in the Coastal Waters of Scotland.

Biophysical Monitoring

The bio-physical models developed by Marine Scotland Science are Individual-Based Models (IBMs) of biological processes, linked to Lagrangian particle tracking models which provide the spatial and temporal context for the biological processes. Marine Scotland Science's long track record in bio-physical modelling has been applied to a wide range of ecological investigations.

Marine Scotland Science has used bio-physical models to explain the long term trends in abundance of the copepod Calanus finmarchicus in the North Sea, as a function of ecological and climatic change, and their population dynamics in the North Atlantic. Bio-physical models have suggested a linkage between North Sea and west of Scotland monkfish populations and distant spawning areas on the north east Atlantic continental shelf and Rockall.

These models have been used to explore the transport of squid, crab larvae, and the distribution of salmon smolts in the sea.

Our most recent applications include the use of bio-physical models to investigate the relative importance of stock structure and environmental factors on the recruitment of cod and haddock, and a sea lice dispersal model in a Scottish sea loch.

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.

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. 

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. 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. 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).