Project Highlights

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Identifying Pressures

Three Irish estuaries were modelled with the aim of quantifying the impact on water quality of existing nutrient loading from direct and diffuse inputs. One commonality between the three chosen estuaries was the prevalence of opportunistic macroalgae blooms, although the upstream catchments and thus the causative factors differed.

In the Liffey–Tolka–Dublin Bay system, wastewater was found to be the greatest pressure on water quality. The two other estuarine systems, namely the Argideen Estuary and Clonakilty Harbour, were both found to be under considerable pressure from diffuse nutrient inputs from agriculture.

Informing Policy

A range of scenarios was explored using calibrated models of the three estuarine systems to establish what conditions, if any, could meet ambient water quality standards and reduce macroalgae bloom magnitudes to the compliance limits:

• In the Liffey–Tolka–Dublin Bay system, either reducing river nitrogen and phosphorus inputs by 50% or upgrading wastewater treatment at the main facility to include aerobic granular sludge technologies would reduce macroalgae bloom magnitudes to the compliance limits.
• In Clonakilty Harbour, a 25% reduction in river phosphorus inputs would similarly restore macroalgae compliance.
• In the system including the Argideen Estuary, reducing river nitrogen and phosphorus inputs by 66% would restore compliance with trophic status parameters. However, none of the scenarios considered could reduce macroalgae bloom magnitudes in the Argideen Estuary to within compliance limits.

The Argideen Estuary was modelled using sub-hourly riverine nutrient measurements provided by the Teagasc Agricultural Catchments Programme. Advances in catchment management practices in the Argideen catchment have yielded reductions in flow-normalised phosphorus transfer. In spite of these improvements, macroalgae bloom magnitudes downstream increased because of higher river flows and higher tidally exchanged nutrient concentrations.

Developing Solutions

This project entailed a detailed assessment of a simple box model, with a view to increasing use of the box model in future. High-, medium- and low- resolution nutrient loading information was applied to the Courtmacsherry, Dublin and Clonakilty models, respectively, facilitating a critical analysis of the consequences of adopting different nutrient loading estimates or measurements in box model calibration and scenario modelling.

Although the simple box model performed well with regard to the simulated water column nutrient and chlorophyll a concentrations and macroalgae wet weights, a number of recommendations are made to increase the confidence in the model results; these actions encompass refinements to upstream riverine and downstream tidal nutrient loadings and concentrations, respectively.

Recommendations on the necessary nutrient load reductions to improve estuarine water quality and the appropriate pressure to target in nutrient load reductions for the three systems studied are provided.