Internal phosphorus loading in Esthwaite Water: considering the role of weather and climate

Abstract

Esthwaite Water is a small, stratifying lake in the northwest of the United Kingdom. It has experienced decades of nutrient enrichment from its catchment, including both point and diffuse sources, and as a result is meso-eutrophic. Various management actions have been undertaken in the past to address the catchment nutrient sources and while some improvement in surface-water phosphorus (P) concentrations has occurred, significant internal P loading from the bed sediments takes place during the summer. This study considers internal loading of soluble reactive P (SRP) between the hypolimnion and epilimnion. It combines data from a previous investigation of internal SRP loading (Mackay et al. 2014a) with long-term monitoring data to consider how variability in physical forcing leads to intra- and inter-annual variation in SRP fluxes and how the influence of climate change might affect these fluxes over longer timescales. Inter-annual variability in the hypolimnetic flux was found to drive changes in the overall SRP budget. The variation in this flux is associated with changes in anoxia and stratification drivers, which in turn are influenced by inter-annual differences in weather conditions. Over longer timescales, increasing strength and length of stratification is occurring alongside an increase in hypolimnetic anoxia, which is likely to have implications for the duration of sediment P release and the size of the hypolimnetic SRP pool. Better constraining of our estimates of internal SRP fluxes and their role in promoting poor water quality in the lake is therefore essential when deciding on whether active management of internal loading is justified.