Estimates of internal loading of phosphorus in a western US reservoir using 3 methods

Abstract Burnet SH, Wilhelm FM. 2021. Estimates of internal loading of phosphorus in a western US reservoir using 3 methods. Lake Reserv Manage. 37:261–274. We used 3 methods to estimate internal loading of total and dissolved phosphorus (TP/DP) in 2 years in a stratified eutrophic western US reservoir with high internal loading of phosphorus. We calculated the internal load from (i) net increases of TP/DP from reservoir water column samples, (ii) gross TP mass balances, and (iii) gross TP/DP loading estimated from laboratory-incubated sediment cores and the anoxic reservoir area. We also present a cost comparison and spreadsheet to allow managers to estimate costs for each method. The highest internal loads were estimated in 2015 from the net in situ samples (999 mg/m2/yr) and the gross annual mass balance (850 mg/m2/yr). Gross estimates from sediment cores were 1.2× higher in 2015 than in 2014 due to differences in anoxic area resulting from a drought and record reservoir drawdown. Costs were highest for the mass balance method and lowest for the sediment core method. Given the small monetary difference between the sediment core and in situ methods, we suggest managers obtain in situ data to estimate internal loading because this yields a net estimate most reflective of whole-lake dynamics. Our data provide an important benchmark of internal loading in a human-made reservoir given the general paucity of such estimates in this large geographic region.