Exploring Wisconsin Lakes’ Sensitivity to Nutrient Loading and Prioritizing Lakes For Conservation

Abstract

Nutrients such as phosphorus and nitrogen cycle through the environment naturally. However, excessive nutrient concentrations have negative effects on aquatic ecosystems and water quality. While the importance of good water quality has become more apparent over time, and the understanding of water quality protection strategies has improved, nutrient loading continues to be a problem in many states, including Wisconsin. As such, this study aimed to support nutrient pollution management in Wisconsin lakes. The objectives of the study were to (1) model a relationship between in-lake total phosphorus (TP) and water clarity, (2) create a model that will measure lake sensitivity to phosphorus loading, (3) determine which Wisconsin lakes are most and least sensitive to phosphorus loading, and (4) create a prioritized list for lake conservation/restoration efforts based on lakes sensitivity to phosphorus loading. This study used a quantitative approach to prioritize 929 Wisconsin lakes by sensitivity to phosphorus. The model had three components: phosphorus loading sensitivity (S), sensitivity significance (SS), and lake phosphorus sensitivity significance priority score (LPSSn). A mass balance equation by Dillon and Rigler (1975) was used to determine sensitivity (S) to phosphorus loading. Next, the likelihood of reaching that phosphorus loading threshold was calculated using the sensitivity (S) and several other variables, which gave the output SS. Finally, SS was used along with clarity trend data to determine a ranked lake phosphorus sensitivity significance (LPSSn) priority score from 0-100. Lakes near TP loading thresholds received a high LPSSn score, while lakes that far exceeded TP loading thresholds scored lower. Lakes below TP loading thresholds were too few to report significant placement on the LPSSn priority score. Lakes in the Northern Lakes and Forests ecoregion scored significantly higher than the lakes in the Driftless area, Southeastern Wisconsin Till Plains, and North Central Hardwood Forests. Model results will aid decision-makers in determining which waterbodies should be prioritized for conservation and water quality protection. Prioritizing Wisconsin lakes based on sensitivity to phosphorus loading and then identifying the significance of that sensitivity based on land use, surface area, in-lake mean TP concentrations, and other factors will target high quality lakes near tipping points of becoming degraded.