Aluminum sulfate (alum) treatments: A surprising boost for aquatic plant growth

April 19, 2024

by Katie Hembre, Masters student, Water Resources Science

Side by side comparison of two lakes
Poor clarity leads to few plants. Right: Good clarity leads to more plants. Photos provided by: Katie Hembre (2023).

Have you ever marveled at the stark difference between a serene, crystal-clear lake and one resembling a pea-green soup? The answer lies in a delicate balance of nutrients. You may be wondering; how do you fix it? Well, one of the first steps is to control external sources, such as runoff carrying leaves and fertilizer. Once these external sources have been managed, many decision-makers have turned to alum treatments as a promising solution.

Alum treatments have been used for lake management since the early 1970s and today are an important management tool. Alum effectively "traps" excess nutrients and enhances water clarity over an extended period.

The improved clarity fosters a ripple effect throughout the lake's ecosystem. By reducing nutrient levels and reducing the growth of algae, alum treatments create a byproduct: a place for aquatic macrophytes to thrive. Aquatic plants play a crucial role in oxygen production, sediment stabilization, fish and invertebrate habitat, and nutrient cycling, thus  enhancing the health and balance of the entire lake.

Now, let's introduce a challenge to our story: aquatic invasive plants, such as curly leaf pondweed and Eurasian watermilfoil. Unlike many native plants, these invaders thrive in both murky and clear waters, presenting a formidable threat to the balance we seek to achieve in our lakes.

Intriguingly, these invasive species can exploit the improved clarity conditions fostered by alum treatments. With their ability to outcompete native aquatic plants, they can form dense canopies and grow aggressively, often before the natives can establish themselves. This creates a complication in our pursuit of the ideal lake ecosystem, as it challenges the delicate equilibrium we strive to maintain.

Consequently, lake managers often apply herbicide treatments and employ various nuisance plant management strategies. These efforts aim to give native plants a fighting chance to outcompete the invasives.

To unravel the intricacies of these interactions following alum treatment, our study aims to determine how lakes in Minnesota and Wisconsin are responding to alum treatments.

Photo of Katie Hembre
Graduate student Katie Hembre collecting water quality data. Photo Credit: Maija Weaver (2023).

Graduate student, Katie Hembre, has spent the last two summers alongside her lab mates assessing the water quality and plant communities in nine MN and WI lakes across the Upper Mississippi Basin. Collaborating closely with each lake’s management team, Katie has compiled a comprehensive database of water quality and plant abundance spanning from 2011 to 2023. These data enable us to gain a deeper understanding of how lake ecosystems are reacting to alum treatments. 

More specifically, our study's data point to an overall trend of improvement despite variability in response among lakes. Alum treatments have decreased epilimnetic phosphorus, increased clarity (Secchi depth), and increased total and native plant abundance. Responses of invasive plants were variable and lake-specific, likely due to invasive management.

Our study aims to provide insight into the relationships between alum applications, water quality dynamics, and native and invasive aquatic plants. By shedding light on these complex interactions, we hope to provide valuable insights that can inform more effective lake management and conservation strategies in the future.

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