Shallow Sediment as a Phosphorus Reservoir in an Oligotrophic Lake

Lake sediment is an important reservoir for storage of phosphorus (P). Skaneateles Lake, a pristine oligotrophic, midsize lake in New York State, USA, is the primary unfiltered water source for the city of Syracuse and nearby communities. We conducted a lake‐wide survey of shallow sediments in Skaneateles Lake to inform understanding of the potential role of sediment in P storage and release into the water column. A total of 103 shallow samples of surface sediments were collected along 17 transects around the lake margin and analyzed for sediment and pore water elemental concentration, grain size, total organic carbon, nitrogen, carbonate content, stable isotopes of carbon, and mineral content. Reduction‐oxidation (redox) processes of Fe‐bound and Mn‐bound P appear to be a primary control of P storage and release. Sediment P concentrations are positively correlated with water depth and negatively correlated with Ca. Grain size and sediment P distributions around the lake are heterogeneous and patchy. Pore water concentrations reveal strong but variable partitioning of P, apparently controlled by sediment redox conditions. Pore water P concentrations (mean 90 μg L−1) are elevated compared with the oligotrophic water column (∼5 μg L−1) and may be an important source of nutrients to the water column of the lake. Plain Language Summary Phosphorus is an important limiting nutrient in freshwater lakes. This study was conducted on an oligotrophic lake in central NY, Skaneateles Lake. The lake is a major municipal water supply and has recently experienced harmful algal blooms despite its historically low levels of nutrients. In this study, we examine the chemical and physical properties of shallow water sediments of Skaneateles Lake to understand the distribution of nutrients. We spatially characterize the surface sediment using metrics such as particle size, mineral content, nutrient and chemical content, pore water chemistry, and organic matter content. We find a dynamic system where nutrients likely cycle quickly in and out of the sediments and rapidly become available for up‐take by phytoplankton. Key Points A midsize lake with recently degrading water quality reveals a complex relationship between sediment, pore water P, and the water column Surface sediment P varies heterogeneously in an oligotrophic lake and has a positive relationship with water depth, Fe, and Al Shallow sediments at the lake margin likely provide a dynamic environment for P cycling