Elevated phosphorus inputs to Loch Leven during storm events - implications for load estimation and catchment management

Loch Leven is a eutrophic lowland lake in Scotland with an area of 13 square km and average depth of 3.9 m. After much remediation work during the last several decades, diffuse sources now dominate estimated nutrient loads. Phosphorus loads from the catchment of Loch Leven have, historically, been calculated on the basis of water samples collected from inflow streams on an 8-day sampling interval, but it was not known whether this was sufficient to provide accurate load estimates. During this study, automatic sampling instruments were used to sample one stream, the Green’s Burn, every two hours. Several high-flow events and periods of low flow were captured in 2006. Flow data, with a sampling interval of 15 minutes, were also available. During short periods (less than 24 hours) of elevated stream flow, concentrations of both total phosphorus and soluble reactive phosphorus were also elevated, so that loading rates during these events were greatly increased, in comparison to base flow conditions. Based on this study, we estimate that up to 80% of the annual phosphorus load is received by the lake during these high flow events, and that conventional sampling methods will, therefore, greatly underestimate total phosphorus loads from this catchment.