Science Inventory

Modeling nearshore total phosphorus in Lake Michigan using linked hydrodynamic and water quality models

Citation:

Pauer, J., M. Rowe, W. Melendez, T. Hollenhorst, L. Lowe, P. Alsip, D. Robertson, AND S. Pothoven. Modeling nearshore total phosphorus in Lake Michigan using linked hydrodynamic and water quality models. ECOLOGICAL MODELLING. Elsevier Science BV, Amsterdam, Netherlands, 493:110718, (2024). https://doi.org/10.1016/j.ecolmodel.2024.110718

Impact/Purpose:

Lake Michigan, similar to most of the other Great Lakes, is experiencing nutrient related issues, such as excessive nuisance and toxic algae. The most recent amendment of the Great Lakes Water Agreement calls for the assessment of the nearshore waters of the Great Lakes. We have developed a total phosphorus (TP) model to estimate nearshore phosphorus concentrations in Lake Michigan.TP model results were similar to a Nutrient-Phytoplankton-Zooplankton-Detrital-Mussel (NPZDM) model and fit the observational data reasonably well. Model sensitivity analysis shows the importance of tributary phosphorus loadings and lake circulation on nearshore phosphorus concentrations, especially close to the mouths of major tributaries. The simplicity of our TP modeling approach makes it a useful tool to describe nearshore phosphorus concentrations under existing and alternative futures, such as future landscapes and climates This work supports the Nutrients and Harmful Algae Blooms topic under EPA/ORD’s Safe and Sustainable Water Research. Understanding nearshore nutrient related pollution is important to US EPA Great Lakes National Program Office, Regions 2 and Region 5 and Great Lakes states such as Michigan, Wisconsin, and New York.

Description:

Although the offshore water of Lake Michigan has been below the Great Lakes Water Quality Agreement (GLWQA) total phosphorus (TP) spring target concentration of 7 µg L−1 for several decades, higher TP concentrations occur in the nearshore, contributing to the resurgence of Cladophora and other nutrient related issues. The most recent update of the GLWQA specifically calls for the assessment of current nearshore conditions. Such assessment would require an intensive monitoring program supported by water quality models. Here we applied multiple versions of a phosphorus-based model linked to an unstructured-grid hydrodynamic model. We focus on the nearshore regions of Lake Michigan around the Grand and Muskegon rivers – a region with strong riverine TP influence and intensive monitoring. Results from a TP model were compared to observational data and to a previously published Phosphorus-based Nutrient–Phytoplankton–Zooplankton–Detrital–Mussel (NPZDM) model. Model results and observational data show that parts of the nearshore can be well above the target TP concentrations but, due to the dynamic nature of this region, the TP concentrations can change rapidly. The models’ skill statistics in predicting individual observations were variable, but it was able to simulate temporal and spatial trends and captured the distribution of observations in our study area. The similarity between the results of the TP and NPZDM models demonstrated the TP concentrations in this nearshore area are driven by hydrodynamics and river TP loads, which are likely the reasons for the higher observed TP concentrations. Simplicity, transparency, and ease of use of the TP model make it a useful tool for supporting nearshore assessments and estimating existing and future nearshore TP concentrations.