In freshwater lakes across the Midwest and around the globe, nutrient loading frequently occurs after an excess of nutrients – such as phosphorus and nitrogen — enters the watershed. When the aquatic environment becomes overly enriched with nutrients, this leads to harmful algal blooms and creates conditions that can contribute to low oxygen “dead zones.” Researchers have made major progress studying nutrient runoff in recent years, but it’s often uncertain from where exactly the nutrients are coming.
Researchers worked to examine the ratios of oxygen isotopes in inorganic phosphate molecules in manure samples collected from the Grand Lake St. Marys watershed.
Grand Lake St. Marys in northwest Ohio is no exception to this trend, increasingly experiencing conditions with nutrient accumulation and low oxygen in lake water. The waterbody is the largest inland lake in the state, acting as a reservoir for drinking water and providing tourism activities important to the local economy. Meanwhile, watersheds draining into the lake are highly agricultural, with 90% of the landscape dedicated to livestock. Scientists and decision-makers have a need to determine specific sources and locations of these nutrient influxes to help manage this bloom impacted system.
To identify major contributors to algal blooms, researchers can analyze manure from livestock operations and look for patterns. However, existing analytical methods, such as genomic sequencing, are cost prohibitive and generally unreliable when it comes to identifying specific livestock species.
Recently, a team of researchers led by Dr. Melanie Marshall of Wright State University found stable isotope analysis to be a promising new method of tracing nutrient sources. With their latest study, they aimed to determine whether this emerging technology can differentiate among manure sources and locate nutrient “hotspots.” The research was funded through the Ohio Department of Higher Education’s Harmful Algal Bloom Research Initiative (ODHE HABRI).
Researchers put this method to the test by first collecting manure samples from livestock operations within the Grand Lake St. Marys watershed. Then, they examined the ratios of oxygen isotopes in inorganic phosphate molecules in the manure, making comparisons among them as well as with data in established literature. The team also collected stream samples from Chickasaw Creek and other waterways that empty into Grand Lake.
An assessment of different types of agricultural manure from across the watershed indicates statistically significant differences among their isotopic signatures, researchers found. This finding encourages further development of isotopic methods as a way to trace nutrient sources. Researchers also plan to evaluate which of the previously analyzed manures may be contributing to each of the stream locations sampled. Results from the project have contributed to the growing database of isotopic ratios related to nutrient sources in bodies of water. The findings will hopefully lead to more “focused” management and mitigation efforts across the region.
To learn more about more completed HABRI projects, read the 2024 HABRI annual report.
Ohio Sea Grant is supported by The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES) School of Environment and Natural Resources, Ohio State University Extension, and NOAA Sea Grant, a network of 34 Sea Grant programs nation-wide dedicated to the protection and sustainable use of marine and Great Lakes resources. Stone Laboratory is Ohio State’s island campus on Lake Erie and is the research, education, and outreach facility of Ohio Sea Grant and part of CFAES School of Environment and Natural Resources.