Ellen Amara Albright , Rachel Fleck King, Quin Shingai, Grace Marie Wilkinson 

Phosphorus (P) release from lakebed sediments may fuel phytoplankton blooms, especially in shallow waterbodies. A primary mechanism that controls internal P loading is the size and chemical composition of the sediment P pool. However, variation in sediment P within and among shallow lakes remains poorly quantified. We measured the degree of spatial heterogeneity in the size and composition of sediment P pools, both within and among seven shallow lakes. There was a 1.6x difference in total sediment P among the study lakes, and P composition varied across lakes based on differences in watershed soils and land cover and lake basin morphology. Differences in sediment P composition among lakes imply that the dominant mechanisms of internal loading will also vary among lakes. We also found that higher mobile P (as a fraction of total P) in the profundal sediments was positively correlated with long-term mean chlorophyll-a concentrations (p=0.04), indicating the reciprocal relationship between sediment P composition and phytoplankton biomass in shallow lakes. Additionally, we measured substantial within-lake heterogeneity in total and loosely-bound sediment P within each lake. Concentrations were positively correlated with water depth such that extrapolating measurements from the deep site alone could overestimate whole-lake mean P concentrations, reinforcing that single station methods produce unreliable estimates of mean sediment P stocks. Our results provide insight into the magnitude and pattern of inter- and intra-lake variation in sediment P pools that should be accounted for when sampling, scaling measurements, and modeling sediment P dynamics.