A high degree of phosphorus sediment saturation in a shallow reservoir system

This study sought to establish the degree of phosphorus saturation in sediment within a shallow reservoir that has a history of being hypereutrophic with documented harmful cyanobacteria blooms. Five sites were chosen within the Des Lacs Reservoir System for sediment sampling. At each site a transect with five sampling points had sediment sampled at the 0–15 cm and 15–30 cm depth from the sediment–water interface line. The degree of phosphorus saturation was determined by measuring the amount of solution nitric acid extractable phosphorus adsorbed on the sediments using increasing phosphorus concentrations. Adsorption at low phosphorus concentrations indicates low phosphorus saturation while adsorption at only high concentrations indicates high phosphorus saturation. Sediment soil texture, organic matter, and carbon content was analyzed. Sediment within the lakes were measured to contain a mean total phosphorus concentration of 641 mg/kg (174 mg/kg standard deviation) with sediments being at more than 95% of their phosphorus saturation. With a high degree of phosphorus saturation, sediments are unable to readily sequester P from the water column and will internally load phosphorus which can sustain hypereutrophic conditions and cyanobacteria blooms. The high phosphorus sediment saturation is unique compared to other lakes and reservoirs which have lower phosphorus sediment saturation (20–70%) and some ability to sequester phosphorus. Nearly saturated sediment means there is a high risk of continued hypereutrophic conditions and cyanobacteria blooms in the reservoir and solutions such as reducing external phosphorus loading will be less effective.