Abstract
Harmful algal blooms (HABs) in surface waters world-wide decreases their aesthetic, recreational value, and use as potable source waters. This is important in the face of the expanding human population that relies on access to clean water. To remediate HABs requires whole-lake nutrient budgets including internal loading which can contribute significantly to HABs. I used a dual approach to quantify the internal phosphorus (P) load at Willow Creek Reservoir (WCR), OR. I calculated the volume-weighted concentration of P from field-collected samples during summer anoxia and from laboratory incubations of sediment cores collected from various sites in WCR; both commonly used approaches, but rarely applied together. The load calculated from field collected samples was 1.7 fold higher than that calculated from sediment core incubations indicating that the latter would severely underestimate internal loading rates in WCR. I also found a large inter-annual difference between 2014 and 2015 that was likely related to annual precipitation and reservoir drawdown. The comparison of internal loading along with external P sources is recommended as a balanced approach to lake management when applying remediation efforts of in-lake controls as well as within the watershed.