Alum treatments improve water quality and increase native macrophyte occurrence, but invasive macrophyte responses are mixed

Hembre KM, Newman RM, Bajcz AW. 2026. Alum treatments improve water quality and increase native macrophyte occurrence, but invasive macrophyte responses are mixed. Lake Reserv Manag. XXX–XXX.Aluminum sulfate (alum) treatments are commonly used to reduce internal phosphorus loading in lakes, which improves water clarity and potentially enhances macrophyte growth. Despite increasing use of alum to improve lake water quality, few studies have examined subsequent macrophyte responses to alum treatments. We investigated the effects of alum treatments on water quality and aquatic macrophytes in 7 lakes in the upper midwestern United States. These lakes each had several years of pre- and post-alum treatment data for water quality and macrophytes, spanning 2011 to 2023, and received 2 or more split-dose alum treatments (total dose divided across 2 or more treatment years). Six lakes were also actively managed to control invasive aquatic plants. We assessed for pre-, during-, and post-alum-treatment differences in epilimnetic total phosphorus (TP), Secchi depth, and total, native, and invasive plant frequency of occurrence. Generalized regression models indicated that epilimnetic TP decreased and Secchi depth increased significantly after alum treatments (P < 0.001). Frequency of occurrence of both total and native macrophytes increased significantly (P < 0.001) following alum treatment, coincident with the subsequent increase in water clarity. Invasive macrophyte responses were variable, with curly-leaf pondweed showing no significant response and Eurasian watermilfoil exhibiting mixed outcomes after alum treatment. Management with herbicides likely influenced invasive plant responses. Planning for aquatic plant responses should be a key component of the alum treatment strategy. Integrated management approaches that combine nutrient reduction with targeted invasive species control are essential for achieving long-term improvements in water quality and macrophyte communities.