Estimating Summer Internal Phosphorus Loads and Their Drivers in an Eutrophicated Lake in Southern Sweden

Eutrophication remains one of the major drivers of change in freshwater ecosystems worldwide. Phosphorus (P) is of particular concern since it is considered the limiting nutrient for phytoplankton growth in lakes. An excess of P can lead to increased phytoplankton biomass, reduced water clarity, and undesirable ecological changes. Consequently, reducing P loading from anthropogenic sources in the catchment has historically been the main restoration approach for eutrophic lakes. Many lakes have shown improvements in water quality following reductions in external P loading. However, others have not responded as expected, and their recovery has been delayed by years or even decades, often attributed to internal cycling of P. Lake Vombsjön in southern Sweden exemplifies these issues, as it continues to experience high P concentrations despite catchment mitigation efforts. Previous studies have indicated that internal P loading sustains high P concentrations, but this has not been quantified.

In this study, we applied a weekly ecosystem-scale mass-balance approach to quantify the internal P loading of Lake Vombsjön during the summer period and to identify the key physical drivers. We combined weekly monitoring of inflowing and outflowing streams with in-lake measurements of total phosphorus and other water quality parameters. We found that internal P loading fluctuated throughout the summer and, in total, dominated the summer P budget. A substantial proportion of the mobilised P remained within the lake, indicating continued internal retention that may delay recovery. Variations in internal loading were primarily associated with physical processes related to wind conditions and thermal structure. These findings demonstrate that internal P loading is a key factor controlling summer P dynamics in Lake Vombsjön and highlight the importance of accounting for internal P loading when designing management strategies for eutrophic lakes.