Harmful algal blooms (HABs) present public health challenges for communities in Iowa and around the globe. These blooms are most closely associated with in-lake phosphorus availability. Management strategies to prevent and mitigate HABs typically focus on watershed sources of phosphorus; however, lakebed sediments also represent a pool of phosphorus that may reenter the water column and fuel HABs. In many systems, this internal phosphorus loading can maintain high, in-lake phosphorus concentrations, even if external nutrient inputs are reduced.It is known that a variable suite of chemical, physical, and biological mechanisms drives internal P loading; however, these control points are poorly understoodand quantified in shallow lakes. The objectives of this research are to (1) quantify the magnitude of gross internal phosphorus loading in shallowlakes, including variation within and among lakes; (2) identify sources of variation in internal phosphorusloading within and among lakes; and (3) evaluate the relative influences of chemical conditions and disturbance on sediment phosphorus loads. In order to meet the proposed objectives, we will build a sediment core incubation system and use it to measure gross internal phosphorus loading on cores from seven study lakes in northwestern Iowa. Replicate cores will be incubated under different oxygen and disturbance treatments to evaluate interactions among hypothesized drivers of internal loading. The results of this research will offer a quantitative understanding of internal phosphorus loading in shallowlakes. Thisis critical knowledge for the prediction and prevention of HABs.This project will additionally enhance our understanding of shallow lake biogeochemistry, offer verified methods to measure sediment phosphorus fluxes, and provide managers with another tool for lake restoration planning and evaluation.