A Machine Learning and Remote Sensing-Based Model for Algae Pigment and Dissolved Oxygen Retrieval on a Small Inland Lake

Excessive algae growth can lead to negative consequences for ecosystem function, economic opportunity, and human and animal health. Due to the cost-effectiveness and temporal availability of satellite imagery, remote sensing has become a powerful tool for water quality monitoring. The use of remotely sensed products to monitor water quality related to algae and cyanobacteria productivity during a bloom event may help inform management strategies for inland waters. To evaluate the ability of satellite imagery to monitor algae pigments and dissolved oxygen conditions in a small inland lake, chlorophyll-a, phycocyanin, and dissolved oxygen concentrations are measured using a YSI EXO2 sonde during Sentinel-2 and Sentinel-3 overpasses from 2019 to 2022 on Lake Mendota, WI. Machine learning methods are implemented with existing algorithms to model chlorophyll-a, phycocyanin, and Pc:Chla. A novel machine learning-based dissolved oxygen modeling approach is developed using algae pigment concentrations as predictors. Best model results based on Sentinel-2 (Sentinel-3) imagery achieved R2 scores of 0.47 (0.42) for chlorophyll-a, 0.69 (0.22) for phycocyanin, and 0.70 (0.41) for Pc:Chla. Dissolved oxygen models achieved an R2 of 0.68 (0.36) when applied to Sentinel-2 (Sentinel-3) imagery, and Pc:Chla is found to be the most important predictive feature. Random forest models are better suited to water quality estimations in this system given built in methods for feature selection and a relatively small data set. Use of these approaches for estimation of Pc:Chla and dissolved oxygen can increase the water quality information extracted from satellite imagery and improve characterization of algae conditions among inland waters. Agricultural runoff and wastewater discharge has fueled nutrient pollution in Lake Mendota over the last century. As a result, algae blooms have become a common summertime occurrence on Lake Mendota. Algae blooms are often made up of different algae species. Green algae are typically harmless, but cyanobacteria (blue-green algae) can produce a range of toxins harmful to human and animal health. The ability to discriminate between cyanobacteria and green algae during a bloom may be useful for lake managers and public health officials in making decisions about closing waterfront areas and communicating with the public. In recent years, satellite imagery has become a powerful tool for monitoring water quality. In this study, we build models that use imagery from two satellites to estimate the abundance of cyanobacteria versus green algae in Lake Mendota. We also find that our algae estimates can be used to model dissolved oxygen, an important water quality indicator that cannot be directly measured from satellite imagery. The methods presented for satellite-based monitoring of algae pigments, the Pc:Chla ratio, and dissolved oxygen has the potential to increase the water quality information extracted from satellite imagery, better characterize algae blooms, and inform management strategies for Lake Mendota. Chlorophyll-a and phycocyanin are sampled from 2019 to 2022 on Lake Mendota, WI Sentinel-2 and Sentinel-3 are used to model chlorophyll-a, phycocyanin, and Pc:Chla A model based in situ data allows for satellite-based estimates of dissolved oxygen