Ellen Amara Albright , Grace Marie Wilkinson 

Phosphorus (P) flux across the sediment-water interface in lakes and reservoirs responds to external perturbations within the context of sediment characteristics. Lentic ecosystems experience profound spatiotemporal heterogeneity in the mechanisms that control sediment P fluxes, likely producing hot spots and hot moments of internal loading. However, spatiotemporal variation in P fluxes remains poorly quantified, particularly in the context of sediment chemistry as a controlling variable. We measured P flux rates and mobile sediment P forms along the longitudinal gradient of a temperate reservoir every two months from February to October of 2020. Both aerobic and anaerobic processes mobilized sediment P throughout the year. High flux rates at littoral sampling sites (8.4 and 9.7 mg P m-2 day-1) occurred in late summer under oxic conditions in the overlying water and mobilized labile organic P. High fluxes at the profundal site coincided with hypolimnetic anoxia under ice cover and in mid-summer (11.2 and 17.2 mg P m-2 day-1, respectively) and released redox-sensitive P. Several high fluxes substantially skewed the flux rate distribution, providing evidence of hot spots and hot moments of internal loading. We further scaled the measured sediment P flux rates to representative areas of the lakebed to estimate internal P loads at an ecosystem scale. We found that P release from littoral sites under oxic conditions in the overlying water had an outsized impact on total loads. Our findings demonstrate the importance of considering spatial and seasonal variation in sediment P pools and fluxes in order to more accurately estimate internal loads and identify the dominant biogeochemical mechanisms involved.