Lough Sheelin and its catchment Water Quality Status and Nutrient Loadings 1998-2005

Lough Sheelin is a trout fishery located in counties Cavan, Meath and Westmeath, with a surface area of 1855 ha, and a total catchment area of c.24,900 ha. The lake is one of the twelve lakes in Western Europe capable of supporting substantial stocks of large wild brown trout (O'Grady, 2000).

The catchment is characterised by intensive agriculture, there are two Local Authority wastewater Treatment Plants in the catchment at Oldcastle, Co. Meath and Ballyjamesduff, Co. Cavan. The soils in the Lough Sheelin catchment have a poor hydraulic infiltration capacity, surface run-off is frequent causing manures and fertilisers to be washed off to surface water streams. The lake has shown signs of eutrophication since the early 1970's.

This study found that there was a modest decrease in the total phosphorus loadings to the lake over the period 1988-2005. This suggests that phosphorus losses from the Sheelin catchment to the lake are slowly declining, however the rate of change in nutrient loadings to the lake is so slow that the aim of restoring Lough Sheelin to a premium wild brown trout fishery will not be realised in any reasonable time scale. There is a clear relationship between rainfall and nutrient loadings to the lake. However it is thought there is a considerable time lag between reduction in nutrient levels being applied to the land and any reduction in nutrient loadings to the lake. Inlake chlorophyll levels also decreased over the study period, but this is also affected by unrelated factors e.g. zebra mussels.

Whilst the Mountnugent Stream and the Ross River have the greatest influence on nutrient loadings, phosphorus exports from the Bellsgrove and Schoolhouse catchments also contribute substant nutrient loadings to the lake which are disproportionate with the flow of these streams. The impact of large shock loadings to the system is highly significant, with one single three day event giving rise to 87% of the Bellsgrove's total phosphorus loadings in 2004.

Intensive agricultural industries continue to present the principal threat to Lough Sheelin. Point sources are also a factor and Oldcastle wastewater treatment plant is in need of significant upgrade with added phosphorus removal facilities. Ballyjamesduff waste water treatment plant was upgraded in 1999, there has been a substantial expansion in the domestic loading to the town since then. Industrial discharges also contribute to the problems in the feeder streams and the lake. Indications are that landspreading practices in the catchment have improved somewhat, however there is still considerable evidence of landspreading taking place before high rainfall events. As populations in the catchment increase, lack of capacity within the wastewater plants have the potential to undermine any improvements to landspreading practices.

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