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PSU Researcher Receives Prestigious Lindeman Award
PSU Researcher Receives Prestigious Lindeman Award

 

In the classic Walden, Thoreau wrote that ponds betray “the spirit that is in the air… continually receiving new life and motion from above,” they are intermediate in their “nature between land and sky.” If only Portland State University’s Dr. Meredith Holgerson had been around to tell Thoreau how right he was.

Dr. Holgerson, a David Smith Conservation Fellow, recently received the prestigious Raymond L. Lindeman Award from the Association for the Sciences of Limnology and Oceanography. The award honors young scientists for exceptional peer-reviewed research in the field of aquatic sciences. An aquatic ecologist, Holgerson studies small pond systems. She received the Lindeman award in recognition of the paper “Large contributions to inland water CO2 and CH4 emissions from very small ponds,” published in the journal Nature Geoscience. Holgerson co-authored the paper with Yale professor Peter Raymond while a doctoral student at the Yale School of Forestry & Environmental Studies.

The paper presents the findings of a study that compiled direct measurements of concentrations of the greenhouse gases CO2 (carbon dioxide) and CH4 (methane) from 427 lakes and ponds of all sizes gathered from sites around the world. In addition to analyzing emissions data from sample sites, Holgerson and colleagues estimated the number of small ponds around the world by extrapolating data from the literature, then assessed data on the number and surface area of larger lakes from existing studies to determine the global distribution of lakes and ponds relative to size. With that data, the team calculated the contributions of small ponds to overall greenhouse gas emissions emanating from all inland bodies of water.

The results were surprising. According to Holgerson, ponds smaller than a quarter acre in size account for roughly 8.6 percent of the surface area of all inland bodies of water. Small ponds, however, produce as much as 15.1 percent of all CO2 emissions and a whopping 40.6 percent of all CH4 emissions originating from lakes and ponds. The reason for the seemingly disproportionate figures, Holgerson noted, has to do with the biogeochemical processes that occur in small bodies of water.

“Proportionately, small ponds receive much more terrestrial carbon from leaf litter and other sources than do lakes,” Holgerson said. “So, relative to lakes, there’s more decomposition of organic matter in small ponds, and greenhouse gasses are the byproduct of decomposition.”

It’s bacteria driving that decomposition. Some bacteria in small ponds use oxygen in the water to break down organic matter in a process that depletes oxygen levels in the pond and produces CO2. When those bacteria have depleted the oxygen, other bacteria that thrive in anaerobic conditions rich in CO2 continue to break down the organic matter in a process that creates CH4. The CH4 and what CO2 remains after the bacteria have done their work then enter the atmosphere through gas exchange with the air.

A part of Holgerson’s dissertation research at Yale, the findings provide new insights into CO2 and CH4 flows on a global scale and shed light on the role small ponds play in greenhouse gas cycles. Researchers citing Holgerson’s work in journals including Nature, Ecological Applications, and Global Change Biology illustrate the value of the discovery.

At PSU, Holgerson’s research continues to focus on small ponds and the ecosystems they support. As a Smith Fellow, Holgerson joined Environmental Science and Management Professor Dr. Angela Strecker’s lab where she works with the Washington State Department of Fish and Wildlife and the US Geological Survey studying the impacts of altered hydrology and non-native species in Southwest Washington’s Chehalis River Basin. That research aims to improve restoration outcomes that seek to reduce flooding and improve the health of the river’s many ecosystems.

As Holgerson’s research shows, ponds like those Thoreau called “perennial springs in the midst of pine and oak woods,” really are intermediaries between land and sky, though just not in a sense Thoreau may have intended. Rather, small ponds care catalysts, transforming large volumes of carbon stored in organic matter into CO2 and CH4 and then releasing the gases into the atmosphere. With a new awareness of the contributions of small bodies of water to global greenhouse gas supplies, scientists like Holgerson can develop methods to evaluate their effect on global climate, which is critical to take into account as we continue to evolve policies meant to address climate change.