Nitrogen driven lakes: The Shropshire and Cheshire Meres?

Abstract

Chemical and morphological data from eighteen lakes (meres) in the West Midlands, U.K. were examined to determine the relative importance of nitrogen and phosphorus in determining algal biomass. The meres are characterised by high phosphorus concentrations, with mean annual total phosphorus in excess of 300 mug/l. A comparison of indicators of nutrient limitation (absolute nutrient concentrations, nutrient ratios, nutrient enrichment bioassays and heterocyst numbers) sometimes showed contradictory results confirming the necessity for the use of multiple indicators on repeated sampling occasions. Five of the meres showed strong evidence for nitrogen limitation, while two meres showed strong evidence of phosphorus limitation. A number of the meres showed temporal variation in nutrient limitation and were not clearly limited by a single nutrient. We examined the extent to which high phosphorus within epilimnion waters during summer is a function of deoxygenation of the hypolimnion and long water residence times. No relationship was found and we surmise that the high phosphorus concentrations generally found within the meres are the accumulative effects of historic land use.