A water quality study of Lough Gowna

Abstract

Lough Gowna, located on the borders of Counties Cavan and Longford is an important coarse fish angling lake, and also has stocks of brown trout. The lake attracts thousands of British, European and local anglers and has been the venue for the All Ireland Course Angling Competition. Lough Gowna is popular for swimming, water skiing and power boat racing. The lake is also used for abstraction of public water supplies by Longford and Cavan County Councils. Suitable water quality criteria to protect the above mentioned uses are; (i) EC (Quality of Salmonid Waters) Regulations, 1988. (ii) EC (Quality of Surface Water Intended for the Abstraction of Drinking Water) Regulations, 1989. The water quality monitoring data complied by Cavan and Longford County Councils, the E.P.A. and the Northern Regional Fisheries Board is reviewed and compared with studies carried out in the 1970’s and 1980’s by An Foras Forbartha. High phosphorus and chlorophyll a concentrations and very low transparency levels together with the dominance of blue-green algae in the phytoplankton indicate the lake is eutrophic. Stratification of the lake in the summer months gives rise to severe deoxygenation in the hypolimnion. “Excessive” and “permissible” annual phosphorus loadings for Lough Gowna were calculated using Vollenweider’s equation (Vollenweider 1976). The “permissible” annual loading of phosphorus is 174 mg P rh 2 yr' 1 and the “excessive” annual loading is 349 mg P m 2 yr"1; this corresponds to 2.24 tonnes P and 4.5 tonnes P respectively. The actual orthophosphorus loading is 527 mg P m 2 yr"1. This is calculated from average lake orthophosphorus concentrations using another equation by Vollenweider (Vollenweider 1980). A large scale fish kill in Lough Gowna in July 1993 was caused by deoxygenation due to eutrophication and resulted in 99.9% perch fry mortality. This prompted a trial in 1994 using barley straw to inhibit the growth of algae in the lake. The results of the trial were inconclusive as the straw failed to decompose completely and was not replaced with fresh straw. There was some evidence to indicate the algae in the treated section of the lake were under stress. As there are no direct industrial or sewage discharges in the catchment area , agricultural wastes are the most likely cause of eutrophication in Lough Gowna. Controlling disposal of farm wastes under Section 12 of the Local Government (Water Pollution) Act, 1977 should help to limit the impact of agricultural activity on Lough Gowna. A phosphorus budget for the lake should be prepared to establish the extent to which farm wastes and the use of artificial fertilisers should be controlled.