Sediment phosphorus composition controls hot spots and hot moments of internal loading in a temperate reservoir

Spatial heterogeneity in sediment phosphorus pools and phosphatase activity in a eutrophic reservoir

Agriculture is necessary for food production, but agricultural inputs of phosphorus (P) to waterways can lead to harmful algal blooms in downstream reservoirs. Some of the P that enters these water bodies can be stored in reservoir sediments and later contribute to internal P loading, supplementing external P loads carried in from rivers. Increased P can lead to harmful algal blooms. However, how P is cycling in the sediment of these water bodies varies spatially and temporally has been relatively unstudied. Our objective was to understand how P concentration and form vary spatiotemporally, as well as how P is processed in the sediment of the reservoir. We sampled 30 locations in both August and October 2018 around Milford Reservoir (Kansas), a man-made eutrophic reservoir with frequent harmful algal blooms. We collected water chemistry samples, field measurements of temperature, dissolved oxygen, and pH, and sediment samples to analyze for P chemical speciation and phosphatase enzyme activity. We show that P release by phosphatase activity was higher under anaerobic and basic conditions, which subsequently affects spatiotemporal variation in sediment P pools. We found that low oxygen positively influenced phosphatase activity and sediment P pools, and may drive high internal P loading and harmful algal blooms in the summer months. This research increased our understanding of P cycling in a reservoir highly impacted by agricultural inputs and contributed to a small but growing body of research on internal P loading in midwestern reservoirs.