How Does Dissolved Organic Matter Impact Harmful Algal Blooms?

Researchers from Miami University, funded by the Ohio Department of Higher Education (ODHE) Harmful Algal Bloom Research Initiative, demonstrated various effects of dissolved organic matter, and findings are informing statewide efforts.

In Ohio, dissolved organic matter — a mixture made up of decomposed plant matter, bacteria, and algae — can come from multiple sources upstream of harmful algal blooms. These can include agricultural fields, wetlands, and numerous connecting tributaries.

Previous research indicated that dissolved organic matter may contribute to blooms, yet scientists don’t know how exactly how these materials affect cyanobacteria and their toxins.

Recently, researchers funded by the Ohio Department of Higher Education (ODHE) Harmful Algal Bloom Research Initiative aimed to support management efforts of Ohio agencies and Governor Mike Dewine’s H2Ohio Initiative regarding harmful algal blooms by determining the relative importance of dissolved organic matter.

The team, led by Dr. Craig Williamson of Miami University, performed a two-year study employing laboratory experiments, a large-scale mesocosm experiment, and analysis of data from high frequency, on-site sensors. Researchers studied how a variety of different factors affected phytoplankton communities (microscopic algae) and algal toxin production.

“Natural resource management is becoming increasingly difficult due to converging human-caused impacts, Williamson said. “This work investigated impacts to lakes and reservoirs at the convergence of nutrient enrichment and increased dissolved organic matter from runoff.”

In laboratory experiments, the team found that adding terrestrially derived dissolved organic matter, or tDOM, significantly increased the concentration of a cyanobacteria pigment that is used to indicate cyanobacteria presence and biomass (size). Adding tDOM from manure leachate significantly increased microcystin concentrations in water from Sandusky Bay and Grand Lake St. Marys. Further, adding nutrients to water with tDOM resulted in a greater increase in microcystin concentration compared to when nutrients alone were added. Researchers are still evaluating data from the mesocosm study to evaluate the effects of tDOM manure leachate and nutrients on toxin production.

“Interestingly, not all phytoplankton communities responded the same to treatments, with some more resilient than others,” Williamson said. “The information gleaned from this work can be used to identify lakes and reservoirs more at risk to harmful algal blooms.”

Crucially, some best management practices being deployed in Ohio to reduce nutrient runoff from agricultural lands can actually result in higher dissolved organic matter output to downstream rivers and lakes, the study found.

Results from this study will help determine to what extent and how tDOM should be included in nutrient reduction strategies and implications for monitoring HABs with high frequency sensors. Findings will inform work being done by the Ohio Environmental Protection Agency, the Ohio Department Agriculture, and NOAA’s Great Lakes Observing System. The team also collaborated with the H2Ohio initiative to discuss how to incorporate dissolved organic matter into wetland monitoring.

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.