CONNECTING LAND-USE WITH WATER QUALITY: SOURCES, SINKS AND TIME- BOMBS

The use of nutrient export coefficients provides a convenient means to estimate diffuse export of nutrients from catchments to lakes. Within individual catchments it is common to calibrate measured nutrients in surface waters with coefficients of intensity of land use, but the estimation of nutrient loads from one catchment based on export coefficients from another, albeit similar one, can be problematic. In the Lough Carra catchment in the west of Ireland, use of seemingly reasonable export coefficients, led to the estimation of about three times the measured load of phosphorus to the lake. This discrepancy is likely caused by low drainage density, high chemical buffering capacity of podzolic soils of the catchment and low historical intensification of the catchment relative to many other Irish grasslands. Nevertheless, profiles of lake sediment indicate progressive eutrophication of the lake, with associated reductions in Fe:P ratios indicative of impact from diffuse P loads. Lough Carra is one of the few remaining calcareous lakes in Europe managed to protect populations of wild trout (Salmo trutta). There is a high risk that continuation of current landuse in the catchment of the lake is an ecological time-bomb that will effect undesirable and persistent changes.