The response of native and invasive aquatic macrophytes to water quality conditions after aluminum sulfate treatments

Abstract

Aluminum sulfate (alum) treatments are a common method to reduce internal phosphorus loading, which improves water clarity and potentially enhances macrophyte growth. Despite the increasing use of alum to improve lake water quality, few studies directly examine macrophyte responses to alum treatments. We investigated the effects of alum treatments on water quality and aquatic macrophytes in five Minnesota lakes and two Wisconsin lakes in the USA. These lakes each had several years of pre- and post-alum treatment data for water quality and macrophytes, spanning 2011 to 2023. We assessed, for pre-, during-, and post-alum-treatment differences, for epilimnetic total phosphorus, Secchi depth, and total, native, and invasive plant frequency of occurrence. Using gamma mixed-effects regression models, we found that total epilimnetic phosphorus decreased and Secchi depth increased significantly after alum treatments. There was a significant increase in the frequency of occurrence of both total and native macrophytes following alum treatment and the subsequent increase in clarity. Invasive macrophyte species responses were variable, with curly-leaf pondweed decreasing and Eurasian watermilfoil exhibiting mixed outcomes after alum treatment. Planning for aquatic plant response should be part of alum treatment planning, and integrated management strategies that combine nutrient reduction with targeted invasive species control are needed to sustain ecological balance.