Evidence for internal phosphorus loading in a large prairie reservoir (Lake Diefenbaker, Saskatchewan)

Increasingly, our global water storage is contained in large reservoirs that retain nutrients. Given the value of reservoirs to ecosystem services, it is important to know the risks associated with the mobilization of legacy phosphorus (P) from sediments. From 2011 to 2013, Lake Diefenbaker was a significant sink for P retaining 91% of the external total phosphorus (TP) and 41% of the dissolved reactive phosphorus (DRP) loaded from the tributaries. We investigated if this retained P has the potential to re-enter the water column through internal P loading from sediments. In 2013, we estimated the rates of internal P loading with a specific focus on measuring TP, DRP, and total dissolved iron release from sediments. We estimated that over the whole reservoir, the internal load (based on TP increases) was 169 mg TP m− 2 summer− 1 and 229 mg TP m− 2 winter− 1. Year-round, the mean daily internal P loading rate was 1.8 mg TP m− 2 day− 1, representing ~ 24% of the annual external TP load. Although our internal load estimates are dependent upon sedimentation rates, they do suggest winter internal P loading is important, resulting in a continuous release of P from the sediments into the overlying water year-round. Internal loading from sediments is an unaccounted-for source of P to Lake Diefenbaker and should be considered in future management of this critically important reservoir. Our recommendations for other reservoirs in obtaining realistic estimates of internal P loading include year-round estimates considering both the stratification and flow regimes.