Assimilative Capacity Modeling Page updated: 18 Nov 2008

ENVIRONMENTAL BIOLOGY Research in the School of Life Sciences, Napier University Go back to Environmental Biology front page

This page concerns work currently funded by:

SARF (the Scottish Aquacultural Research Forum) through project SARF012, The development of modelling techniques to improve predictions of assimilative capacity of water bodies utilised for marine caged fish farming, co-ordinated by Napier University and with partners in the Scottish Association for Marine Science (SAMS) and the Scottish Executive's Fisheries Research Service (FRS) at the Marine Laboratory Aberdeen.

The European Commission (DG Fish) through the ECASA project, co-ordinated by SAMS, in which Napier University is a partner. ECASA stands for Ecosystem Approach for Sustainable Aquaculture.

The assimilative capacity of a sea-loch, voe or coastal water is its ability to absorb the waste products associated with mariculture without harming the local marine ecosystem or the farmed fish. It is undesirable to measure harm by allowing it to happen and observing it, because it may be difficult for the marine ecosystem to recover quickly from the damage. This work aims to develope tools to allow potential harm to be predicted, and avoided. These tools are mathematical models that can simulate changes induced in water quality variables by fish farm wastes. These variables are: the concentrations of dissolved plant nutrients (nitrate, ammonium and phosphate) and dissolved oxygen; the transparency of seawater; the amounts of phytoplankton (measured as chlorophyll) and the balance between the two main types of phytoplanktonic micro-organisms. Predicted variables can then be compared with 'Ecological Quality Objectives' (EcoQOs) set by regulators, and the water body and fish farm managed so as to prevent variables from breaching the corresponding EcoQOs.

This strategy is illustrated in the diagram. It uses the DPSIR (Driver-Pressure-State-Impact-Response) concept of anthropogenic impact on ecosystem. The causative agent of ecosystem change is the ecological pressure, measured by an indicator that could be a measure of nutrient loading; the (undesirable) effect of this pressure appears as changes in an impact indicator, such as the amount or composition of phytoplankton. The regulator's EcoQOs are translated into (impact) Ecological Quality Standards (EQS), one of which is shown as a horizontal line that should not be transgressed. The models link pressure to impact and so allow pressure to be managed.

The starting points for the work were the 'Equilibrium Concentration Enhancement' (ECE) and 'Comprehensive Studies Task Team' (CSTT) models. Further details of these are given on the Napier CSTT model page. Both models treat a loch or voe as if it were a single box with well mixed contents in potential exchange with the sea. Fish farm waste, such as ammonium, phosphate or 'biological oxygen demand' from faeces, would accumulate in the box in the absence of exchange. Where exchange occurs, a proportion of the waste is removed each day. The ECE model calculates nutrient concentration under equilibrium conditions - i.e., when the rate at which nutrients are removed by exchange is exactly balanced by the rate at which they are added by fish farms or other local sources such as rivers. The CSTT model calculates the greatest amount of chlorophyll that could be produced if all the ECE nutrient were used by phytoplankton.

Several types of improvement are being made to these models:

• sea lochs will be described in terms of 3 layers rather than the single layer of the simple box model;
• the physics of exchange between lochs and voes will be improved, leading to the ACExR physical model developed at SAMS;
• the ideas in these models will be applied to offshore locations for fishfarms, leading to the OWExR physical model developed at SAMS; this will be augmented by addition of CSTT model biological equations;
• simulation of the seasonal cycle of nutrients (in the absence of phytoplankton) will be described in the sECE model developed at Napier, which will also include submodels for fishfarm production of nutrients and for terrestrial nutrient inputs;
• a seasonally varying, or dynamic, dCSTT model will be developed at Napier;
• an L-ESV model will be developed at Napier to add some other ecosystem state variables to those of the dCSTT model; L-ESV stands for 'loch - ecosystem state vector'.

All these models deal with the 'zone B' scale of the Comprehensive Studies Task Team in 1994, and refers to an area of sea or volume of water in which buoyant waste remains for a few days before dispersion and dilution in the wider ocean. See Napier Mariculture & Environment page for more about scales.

As the models are developed, they will be described in the 'milestone' reports that provide part of the deliverables of the SARF012 project.

The work of ECASA is concerned with:

1. identifying plausible quantitative indicators of the effects of aquaculture on ecosystems through a process of expert working groups, workshops and meetings,
2. identifying indicators of the main drivers of ecosystem change affecting aquaculture,including natural and environmental pressures,
3. assessing both sets of indicators using existing datasets, considering each in the context of appropriate selection criteria,
4. developing a range of tools, particularly models, that encapsulate best process understanding at a wide range of scales,
5. testing these models and indicators in a wide variety of field locations across Europe encompassing major culture species and technologies, and covering a wide spectrum of environment types, selected according to criteria developed during the project,
6. using this data to test and select the final "toolpack" of models and indicators, including appropriate decision support tools to guide users to effective implementation.

The work described on this page also contributes to most of these objectives. Subject to the agreement of the non-ECASA partners and SARF, the models will be considered for inclusion in the ECASA toolbox.

Link to ECASA web page at SAMS DML.

The organization(s) mentioned against the milestone is/are primarily responsible for this part of the work in the SARF012 project. Some of the reports are available to download. They are working documents, however, and should not be considered definitive or error-free. Final results will be communicated to SARF and, it is hoped, published in the scientific literature after peer review.

1. The ACExR model:
   • Milestone: RRE EXCHANGE: Report on the parameterization of water exchange and internal mixing for RREs (SAMS).
   • Report: Gillibrand, P.A. & Inall, M.E. (2006). Improving Assimilative Capacity Modelling for Scottish Coastal Waters: I. A Model of Physical Exchange in Scottish Sea Lochs. Marine Physics Report No. 167, March 2006. Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban. Download pdf (about 0.4 MB).
2. The OWExR model:
   • Milestone: OFFSHORE ZONE B PARAMETERIZATION: Report on the parameterization of water exchange, volume and thickness for zone B boxes in offshore waters (SAMS).
   • Report: Gillibrand, P.A. (2006). Improving Assimilative Capacity Modelling for Scottish Coastal Waters: II. A Model of Physical Exchange for Open Water Sites. Marine Physics Report No. 168, June 2006. Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban. Download pdf (about 0.9 MB).
3. The sECE model:
   • Milestone: SEASONAL ECE MODEL: Description of the seasonal ECE model and auxiliary models and their implementation as a Matlab script. (NUE).
   • Report: Portilla, E. & Tett, P. (2006a). Report on the Seasonal Equilibrium Concentration Enhancement (sECE) model, August 2006. School of Life Sciences, Napier University, Edinburgh. Download pdf (about 2.5 MB).(corrected 30 Nov 06.)
4. The dCSTT model:
   • Milestone: DYNAMIC CSTT MODEL: Description of the combined, dynamic, ECE-CSTT model and auxiliary models and their implementation as a Matlab script (NUE).
   • Report: Portilla, E. & Tett, P. (2006b). Report on the dynamic CSTT (dCSTT) model: SARF012 milestone 4. November 2006. School of Life Sciences, Napier University, Edinburgh. Download pdf (about 2.0 MB).
5. Final report and the L-ESV model:
   • Milestone: MODEL TESTS: Final report undating the descriptions of ECE, dynamic coupled ECE/CSTT, and describing the prototype ESV model, including the validation of the models against field data, examples of their use to estimate assimimilative capacity, and model algorithms as Matlab scripts (NUE/SAMS/FRS).
   • Report: Portilla, E., Tett, P., Gillebrand, P., Inall, P., Gubbins, M. & Amundrod, T. (2008) SARF012 Assimilative Capacity Models: Milestone 5 report on the ACExR and LESV models. School of Life Sciences, Napier University, Edinburgh. Download pdf (as at 18 Nov 2008: chapters 1-4 only).

• CSTT model web page at Napier University;
• Mariculture and the Environment research in Napier University;
• Environmental Biology homepage in Napier University;
• SAMS homepage and ECASA homepage;
• FRS homepage;
• SARF homepage;
• SEPA aquaculture homepage;