When efforts to reduce the drivers leading to harmful algal blooms (HABs) have not taken place or are ineffective, lake managers are faced with the challenge of mitigating the impacts of HABs. One approach to mitigate the impacts of HABs on human health and the environment is to manipulate the lake or reservoir by altering the physical, chemical, or biological environment such that HABs are less likely to form. One way is to add a coagulant or adsorbent to the lake to reduce lake nutrient concentrations or more effectively settle out HAB biomass. Aluminum sulfate, or “alum,” is a metal salt coagulant long used for the coagulation and removal of turbidity in conventional drinking water treatment plants. Alum also has a long history of use for the precipitation of phosphorus from wastewater. Alum has recently been explored as an in situ treatment chemical for the mitigation of HABs in inland lakes. To control HABs, alum is added to reduce phosphorus levels in the lake or reservoir through either precipitation or adsorption. Once phosphorus is associated with aluminum precipitates, the solids settle, and the phosphorus becomes “locked” in the lake sediments. The addition of alum during a bloom may also help remove HAB cells from the water column.