Study shows lakes damaged by cattle

New techniques for measuring phosphorus loadings, which represent the main threat of freshwater pollution, in Irish lakes suggest high concentrations can be strongly linked to cattle numbers in surrounding areas.

A team from Trinity College Dublin led by Dr Ken Irvine of the Department of Zoology has devised the new models for assessing phosphorus loads and applied them to 30 Irish lakes over two years. Their results show a strong association between cattle density and average "total phosphorus", and underline the negative impact cattle-farming has on the quality of Irish lakes.

"Phosphorus concentrations were strongly correlated with cattle numbers, suggesting that the dairy industry is a main contributory factor to nutrient enrichment of Irish lakes," he said.

Cattle farming was probably the most important single contributor to nutrient enrichment of fresh waters in lowland areas, and could even be adopted as a useful index of the state of lakes known as their "trophic status".

Trophic status in peatland areas is determined by factors such as overgrazing by sheep and forestry plantations, he told the limnologists' conference.

Many methods of measuring phosphorus arising from slurry, fertiliser, sewage and other wastes flowing into fresh water are difficult to deploy effectively. This is because, for example, it can be discharged from very diffuse sources through surface and ground water. It may be associated with high rainfall or be at much higher concentration in winter than during the summer.

The Trinity team has taken an alternative approach to long-term and continuous measurement of nutrients and the way they make their way into lakes by taking into account the geology of the surrounding catchment, topography and land use.

The conference was told that aerating the deepest zones of lakes and reservoirs, particularly in summer time, was beginning to be used widely in the industrialised world to improve raw drinking-water quality and to restore cold-water fish habitats. Scientists from the University of British Columbia, Canada, outlined the most effective aerators available.