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

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 directly examined subsequent macrophyte responses to alum treatments. We investigated the effects of alum treatments on water quality and aquatic macrophytes in seven lakes in the upper midwestern USA. These lakes each had several years of pre- and post-alum treatment data for water quality and macrophytes, spanning 2011 to 2023, and received two or more split-dose alum treatments (total dose divided across two or more treatment years). Most lakes were also being actively managed to control invasive aquatic plants with herbicide. 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. Using generalized regression models, we found 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 response. To maximize the water quality effectiveness of alum treatments, planning for aquatic plant responses should be a key component of the treatment strategy. These data and analysis files are in support of the below manuscript submitted to Lake and Reservoir Management.