Modelling Irish Transitional and Coastal Systems

to determine nutrient reduction measures to achieve good status

Eutrophication as a consequence of nutrient enrichment from agriculture and wastewater discharges remains the most prominent issue currently impacting water quality (DECLG 2015) with both sources contributing 93 per cent of nitrogen load and 88 per cent of phosphorus load. Diffuse discharge from agriculture generates the majority of both nutrients whilst municipal discharges release almost a third of phosphorus.

DCPM is a combined model developed by Aldridge et al. (2010) based on two previous models which individually modelled phytoplankton (Painting et al. 2007) and macroalgae (Aldridge and Trimmer 2009). Models such as DCPM give an overall perspective on lumped nutrient reductions required within an entire waterbody and are compromised in their spatial resolution by the completely mixed reactor approach. The identification of suitable nutrient reductions to limit phytoplankton and macroalgae growth has been made feasible through the application of models such as these to the range of options.

The MSN_WQ coupled depth integrated hydrodynamic, solute transport and biogeochemical model has been utilised to simulate phytoplankton dynamics in a complex harbour with promising results (Hartnett and Nash 2004, Nash et al. 2011, Hartnett et al. 2011). The model was utilised to simulate nutrient, oxygenation and phytoplankton growth data before and after the commissioning of a wastewater treatment plant in Cork harbour Hartnett et al. (2011). Trophic status classifications using numerical model data corroborated the EPA’s previous classification for the same waterbodies.

Given the high percentage of total N and P load which can be directly attributed to agriculture and wastewater discharges, any efforts to limit eutrophication and oxygen depletion in transitional and coastal waters must involve nutrient load reduction modelling in order to determine the magnitude of reductions necessary to bring about good status under the Water Framework Directive (WFD).

Given sufficient time and resources, high resolution water quality modelling would be the ideal. It is not practicable to assemble individual detailed water quality models such as MSN_WQ for each estuary in Ireland for each cycle of the Water Framework Directive. In such cases, a simple box model such as DCPM is ideal as management tool to determine the nutrient load reductions necessary to bring transitional and coastal waters into compliance.

No effort has been made to date nationally or internationally to benchmark DCPM against a higher resolution 2D water quality model. Benchmarking is vital to build confidence in the capabilities of DCPM in advance of widespread application of the model to transitional waters. No direct link has been made in an Irish context between the previous measures implemented under the 1st cycle of river basin management planning in Ireland or nitrates action plans and the achievement of good status in transitional and coastal waters. Such an exercise would highlight the most effective measure of the 1st programme of measures which should be rolled out on a wider basis in the upcoming 2nd programme of measures.