Carbon and Nutrient Monitoring in the Great Lakes Using Satellite Observations

Carbon and nutrients are the foundation of lake food webs and play an important role in the chemical and physical processes that shape aquatic ecosystems and various lake dynamics. Studying these cornerstones can help improve understanding of other lake conditions like harmful algal blooms, hypoxia, and phytoplankton community composition.

The way in which these characteristics are monitored varies, though many rely on a proxy approach wherein parameters are extrapolated from the measurement of a different parameter. An assistant professor at Cleveland State University, Brice Grunert, is working to improve current strategies and take a satellite approach to monitoring the Great Lakes.

“All of our observations use optical components as either direct measurements, like phytoplankton pigments, or as proxies, like using colored or fluorescent dissolved organic matter as an indicator of dissolved organic carbon concentrations,” states Grunert.

Emily Hyland supporting water collection in Lake Michigan, with Milwaukee in the background, for validation of NASA’s Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) sensor in March 2025. (Credit: Brice Grunert / Cleveland State University)

Monitoring Carbon and Nutrients in the Great Lakes using Satellite Data

A good example of one of these studies is an ongoing effort to measure the phosphorus content of the benthic sediments and work with satellite algorithms that calculate sediment loads throughout the water column. This data will be used to estimate internal phosphorus loading from resuspended sediments in Lake Erie’s western basin.

“Our current analysis indicates that this can be done quite effectively as long as those individual pieces, like how much phosphorus would be released to the water column from a certain quantity of sediment, are known,” states Grunert.

Another component of Grunert’s fieldwork right now focuses on validating NASA’s new Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) sensor in the Great Lakes. PACE will make monitoring the composition of different aquatic components like phytoplankton, organic carbon, and sediment types remotely easier, and Grunert’s work will help support the accuracy of this approach.

Satisfactory performance of PACE will help improve and refine lake management and help resource managers and researchers move past proxy monitoring and dive deeper into what is actually happening in the water.

Cyanobacteria surface scum observed in southern Green Bay, Lake Michigan, in July 2024. (Credit: Brice Grunert / Cleveland State University)

Grunert explains, “We can move past indicators like chlorophyll as a bulk proxy of biomass and start understanding who is actually there—is it cyanobacteria, and do we need to worry about toxins, or is it a bloom that may benefit the ecosystem?”

The team samples 120 sites across 40 unique locations in Lake Erie, Lake Michigan, and Lake Superior up to five times per year, measuring total algae (chlorophyll & phycocyanin), turbidity, FDOM, temperature, conductivity, pH, and dissolved oxygen using a YSI EXO2; particle characteristics using a Sequoia Scientific hyper-bb backscattering meter; and remote sensing reflectance and electromagnetic energy, using a SpectraVista Corporation HR-512i radiometer.

Water samples are also collected during these site visits using a spectrophotometer, spectrofluorometer, and total organic carbon analyzers.

“We focus on validating the satellites, so we try to cover biogeochemical gradients. In western Lake Erie, we’ll sample the Maumee River plume and a grid of sites that cover the plume as it mixes into the lake, and then sites influenced more by the Detroit River,” explains Grunert.

Grunert also pulls data from several real-time data buoys deployed in the Great Lakes and managed by other universities or larger agencies that have a public data portal.

A YSI EXO2 water quality sonde and Sequoia Scientific hyper-bb instrument package being deployed in Green Bay, Lake Michigan, in May 2025. (Credit: Brice Grunert / Cleveland State University)

Data Sharing and Applications of Findings

“I focus on bringing this cutting-edge science into the classroom and our lab, to ensure we train the next generation of scientists and stakeholders and maximize the positive research outcomes for our communities, from local to global,” states Grunert.

In these communications, Grunert notes that clarity and brevity are essential to effective communication, particularly when measuring proxies, as there are limitations to what can be concluded about ecosystems.

He continues, “For example, even when we retrieve phytoplankton pigment concentrations, these don’t directly tell us who is in an algal bloom. But they can be important indicators, and enable extending our observations through time, from deployed sensors, and in space and time via satellite observations.”

Sky radiance observations being collected using an SVC HR-512i handheld radiometer as part of a suite of measurements used to calculate remote sensing reflectance for validation of satellite observations. (Credit: Emily Hyland / Cleveland State University)

In larger lake systems like the Great Lakes, satellite imagery-based monitoring is a necessary step to making large-scale observations of the lakes. And while satellite sensors have limitations, pairing satellite observations with in situ measurements can help improve accuracy and improve our understanding of aquatic ecosystems.

“When I show a student or talk to someone in the community about what we can observe with in situ and satellite sensors, the response is always inspired,” states Grunert.

He continues, “It truly is amazing what we can see and say about aquatic ecosystems, sometimes with just a few observations, and translating these observations into meaningful science outcomes and training opportunities for students are the best parts of the job.”

Members of the Carbon & (H2)Optics lab at Cleveland State University after a day of water quality sampling on the R/V Gibraltar III in Lake Erie’s western basin. From left, Emily Hyland, Kendra Herweck, Trevor Holm, Stephanie Ratliff, and Anshula Dhiman. (Credit: Emily Hyland / Cleveland State University)