DURHAM, N.C. – Organisms floating in currents beneath a river’s surface are exposed to far less sunlight than scientists previously believed, and the light that does reach them is mostly sporadic and short-lived, a new Duke University-led study shows.

The new findings have broad implications for river management – especially in large, murky rivers – since sunlight spurs the growth of algae and plants that feed the aquatic food web and it also helps break down contaminants and deactivate human pathogens that may be in the water.   

“We found that the models scientists traditionally use to predict the light exposure of organisms floating in these rivers consistently overestimated it by between 62% and 1,700% over the actual exposures we measured in field tests,” said John R. Gardner, who led the study while he was a doctoral student at Duke’s Nicholas School of the Environment.

Gardner and his colleagues published their peer-reviewed study July 26 in the journal Limnology and Oceanography.

To conduct their study, they deployed newly invented robotic devices called HydroSpheres along the Upper Mississippi River in Wisconsin and the Neuse River in North Carolina. The HydroSpheres measured light levels and water quality as they drifted passively with currents beneath the rivers’ surfaces.    

Most of the light exposure they measured occurred in momentary bursts, said Gardner, now a National Science Foundation Postdoctoral Fellow in global hydrology at the University of North Carolina at Chapel Hill.

These brief pulses of light, which Gardner likened to “the dappled sunflecks that filter through a thick forest canopy to reach small plants on the forest floor,” accounted for between 62% and 99% of all light recorded by the robotic drifters.

“Knowing that microscopic plants and the chemicals being carried in a river’s current see sunlight in pulses instead of as a steady, dim glow could improve how river managers predict algae growth and contaminant transport,” said Martin W. Doyle, professor of river systems science and policy at Duke’s Nicholas School, who co-authored the new paper.

The study marks the first time sub-surface drifters have been used to measure light in rivers.

By comparing the light measured by HydroSpheres along an entire section of each river with the light measured at just one location in each river using stationary buoys, Gardner and his team were able to determine that organisms flowing downstream experienced three times more light variability than plants that were rooted in place on the river bottom.

“The depths of rivers are mysterious. Large rivers are deep, fast, and dark, so dark that
your hand disappears when dunked just an arm’s depth below the surface. This underwater world is a challenging environment for scientists studying the plants and organisms being swept downstream,” said Gardner.

“By using new technology, we were able to look at rivers through the eyes of the organisms that are flowing downstream and shed light on this fascinating but little understood environment,” he said.

Gardner and Doyle’s coauthors were Scott H. Ensign of the Stroud Water Research Center and Jeffrey N. Houser of the United States Geological Survey’s Upper Midwest Environmental Sciences Center.

Funding for the study came from the National Science Foundation (grant #DGE-10688715), North Carolina Sea Grant, and the Nicholas School.

CITATION: “Light Exposure along John R. Gardner, Scott H. Ensign, Jeffrey N. Houser and Martin W. Doyle. Limnology and Oceanography, July 26, 2019. DOI: 10.1002/lno.11256

Note to Editors: John Gardner is available for additional comment at johngardner87@gmail.com. Martin Doyle is available at martin.doyle@duke.edu.