Using sediments to assess the resistance of a calcareous lake to diffuse nutrient loading

The capacity of lake sediments to retain phosphorus can provide an important buffer to eutrophication. Concentrations of total phosphorus (TP) in the surface sediment of a shallow calcareous lake (Lough Carra, Ireland) varied significantly across three basins and were correlated strongly with iron and manganese, but correlated inversely with calcium carbonate. The concentrations of phosphorus in the surficial sediments of each basin were observed to reflect the mean annual TP of overlying waters, such that the North Basin > South Basin > Mid Basin. Phosphorus sorption experiments on the most TP-enriched sediments predicted a saturation concentration of 0.15 mg P g. Geochemically similar sediments from the northern basin had slightly lower measured TP concentrations (0.10 mg P g), while those sediments with higher Fe concentrations had up to 0.54 mg P g. Decreasing Fe:P in the upper sections of sediment cores taken from each lake basin show clearly a reduction in the ability of the lake sediments to bind P, reducing the resistance of the lake to eutrophication. The reduced capacity of the sediments to adsorb P should be viewed as a warning signal that indicates increasing risk to water quality and current high conservation status of Lough Carra. The risk that the lake could “flip” to an alternative state, with high concentrations of phytoplankton and a loss of extensive charophyte beds, requires that measures are effected that will prevent or reduce nutrient loads to the lake. This will necessitate a reversal of the current trend of increasing catchment pressures from intensification of agriculture and increased rural housing.