Spatiotemporal Complexity of Internal Phosphorus Loading in Reservoirs

Abstract:

Internal phosphorus (P) loading can fuel harmful algal blooms in lakes and reservoirs. As such, there is a pressing need to understand the drivers of internal loading in order to properly manage for the hot spots and hot moments of sediment P release. However, the spatiotemporal complexity of internal P loading within individual waterbodies remains poorly quantified, limiting our ability to predict and manage internal P loads at an ecosystem scale. In order to evaluate spatiotemporal variation in sediment P release, we sampled along a longitudinal gradient in a temperate reservoir during three periods of thermal stratification and three mixing events over the course of a year. We measured release rates in ex situ core incubations under ambient temperature and dissolved oxygen conditions. Sediment P release rates varied considerably across seasons. In the spring when the sediment-water interface was oxygenated, all sites had moderate P release (range 1.22 – 2.37 mg P m^-2 day^-1). The observed P release may have been due to decomposition of organic matter from the previous year’s algal bloom and P mineralization overwhelming the sorption capacity of the sediments. In the summer, spatial heterogeneity in the P fluxes was greater as hypolimnetic anoxia developed at the deep site while more shallow sites remained oxygenated. During this time, the shallow sediments retained P while the deep site had the highest flux measured (17.24 + 1.16 mg P m^-2 day^-1), likely due to reductive dissolution redox-sensitive P minerals, which are found in high concentrations in the reservoir sediments. While release rates were highest overall in the summer, there was a surprising degree of anaerobic internal loading in the winter, with the deep site flux at 40% of the peak summer rate. The spatiotemporal patterns of sediment P fluxes in this reservoir illustrate that both aerobic and anaerobic internal loading processes need to be considered when managing for internal loading and that internal loading drivers vary both by location and season within reservoirs.