Interannual variations in atmospheric forcing determine trajectories of hypolimnetic soluble reactive phosphorus supply in a eutrophic lake

1. We tested the hypotheses that variability in soluble reactive phosphorus (SRP) loading from the hypolimnion to the epilimnion of a small, eutrophic, temperate zone lake is significant at both interannual and subannual scales, and that this variability is influenced by changes in lake thermal structure. We calculated weekly hypolimnetic fluxes of SRP, during the stratified periods of 2008 and 2009 in Esthwaite Water, U.K., and compared them with the SRP fluxes from external sources. 2. As a result of variations in the supply of SRP from the hypolimnion, which differed by c. 80% between years, we found a 30% difference in overall SRP loading to the epilimnion between the 2 years. Despite similarities in mean summer meteorological conditions, relatively subtle differences in weather conditions during the midsummer of only 0.7 m s−1 in wind speed and 8 W m−2 in solar radiation between the 2 years resulted in two diverging trajectories of hypolimnetic SRP loading. 3. In the first year, there was a high thermal lake stability of 102 J m−2 on average, and a relatively shallow epilimnion until the late summer, giving rise to prolonged deep-water anoxia and a large accumulation of SRP that was mixed into the surface waters at the end of the summer. In contrast, 30% lower thermal stability and 20% deeper epilimnetic mixing in the second year resulted in the arrest and early decline of deep-water anoxia and SRP accumulation, leading to lower SRP fluxes to the epilimnion. 4. Understanding this meteorologically driven variability in SRP supply is crucial for predicting nutrient loading in lakes. It is likely to become especially relevant to lakes around the globe for which the importance of internal loading is predicted to increase due to the effects of climate change.