Project overview
Stormwater ponds are designed to trap runoff and the pollutants it carries, including particulate and dissolved phosphorus. Much of this phosphorus is expected to settle into pond sediments before pond water is discharged into receiving waters. However, research has shown that many stormwater ponds promote anoxic (low-oxygen) conditions, which can trigger the release of phosphorus from pond sediments. This mobilized phosphorus can then be flushed out of the pond, contributing to ecological damage in lakes, streams, and rivers.
A better understanding of the factors that cause anoxic conditions would help pond managers improve phosphorus retention in stormwater ponds. This study investigated the roles of free-floating aquatic vegetation (primarily Wolffia and Lemna species, commonly known as duckweed) and submerged aquatic vegetation. Specifically, do these plants influence phosphorus cycling in stormwater ponds?
Research questions
- What factors determine the distribution and abundance of submerged aquatic vegetation and duckweed in stormwater ponds?
- How do submerged aquatic vegetation and duckweed impact stormwater pond function and water quality?
- Which methods are effective for managing duckweed?
- Could mechanical duckweed harvesting be a cost-effective method for removing phosphorus from stormwater ponds?
Research findings
- Ponds tended toward either high duckweed cover, high submerged aquatic vegetation cover, or high phytoplankton levels with moderate duckweed and submerged aquatic vegetation cover. Factors associated with high duckweed cover included wind sheltering, shallower depths, smaller surface areas, and elevated levels of phosphorus. Factors associated with high submerged aquatic vegetation cover included water clarity, deeper water, and larger surface areas.
- Higher levels of duckweed cover were associated with low levels of dissolved oxygen, leading to the mobilization of phosphorus from pond sediments. Submerged aquatic vegetation has the opposite effect; it releases oxygen into the water. Dense duckweed cover was demonstrated to suppress the growth of submerged aquatic vegetation.
- Aggressive phosphorus reduction efforts, sustained over multiple years, accomplished by repeated alum applications, was demonstrated to effectively reduce duckweed cover. Shoreline vegetation removal (trees, brush, cattails) produced mixed results. Herbicidal duckweed treatment was associated with elevated total phosphorus levels.
- Mechanical removal of duckweed led to increased light availability, greater abundance of submerged aquatic vegetation, and increased dissolved oxygen levels. Similar to dredging, mechanical removal of duckweed also removes phosphorus from the system. Appendix VII of this project’s final report suggests that this concept could be optimized to achieve phosphorus removal at a cost of $500 per pound.
Key innovations/contributions
This research adds to our understanding of free-floating and submerged aquatic plants’ impacts on pond phosphorus cycling. This new knowledge implies that strategic management of pond vegetation could reduce phosphorus export and maximize phosphorus retention.
What does this mean for Minnesota?
A better understanding of the role plants play in stormwater pond dynamics helps stormwater pond managers make more informed decisions and improve phosphorus capture. Additionally, this study explores the potential use of duckweed as a phosphorus sponge to remove legacy phosphorus from stormwater ponds. If economically feasible, this approach could extend the functional lifespan of aging ponds, reducing costs and reducing nutrient pollution to Minnesota’s surface waters.
Reports and Presentations
- Research Brief 2026
- Final Report 2026
- Final Report Appendix II & III, Appendix VII
- Project update 2024 .pdf