Kellogg Biological Station
Flipping the carbon switch on ponds
In aerial photos, the Experimental Pond Facility at KBS looks a bit like an empty egg carton: a three-by-six grid of near-perfect circles, each about 30 meters across and two meters deep.
The ponds’ uniformity provides control for experiments, and their artificiality lets scientists alter them without disrupting natural systems. “They’re easily manipulable, unlike a lake system,” said Stuart Jones, a postdoctoral researcher at the pond lab.
Jones and microbial ecologist Jay Lennon are beginning a National Science Foundation-funded round of manipulations of the ponds to see how freshwater ecosystems respond to different amounts of dissolved organic carbon from land plants.
Anyone who has visited the Upper Peninsula’s Tahquamenon Falls or a backwoods bog pond and remarked on the tea-colored water has seen dissolved organic carbon – decomposed plant matter that stains water bodies. Some is normal in lakes, ponds and streams, but over the past decade or so, concentrations have been increasing around the world. “People have noticed on a global scale that lakes are becoming browner,” Lennon said.
As microbes munch on the carbon dissolved in water bodies, they convert it into, among other things, carbon dioxide that’s released to the atmosphere, contributing to climate change. The researchers want to find out at what point inputs of dissolved organic carbon can turn freshwater systems from sinks that absorb carbon to sources of carbon emissions.
To find out, they’ll dose the ponds with different amounts of rich, molasses-like, organic goo that Lennon said is “on its way to becoming coal,” while sensors monitor the ponds’ oxygen concentrations. As algae photosynthesize, they release oxygen, but as microbes use organic material, they consume oxygen, so the sensors will indicate whether carbon-sequestering photosynthesis or carbon-emitting microbial respiration dominates each pond.
Relatively speaking, Lennon said, lakes are fairly small sinks – and, potentially, sources – of carbon, but the same processes he and Jones observe in the experimental ponds occur in wetlands and estuaries, so their findings could have broad implications for biogeochemical cycles.
“There’s still a lot of basic science that needs to be done,” he said.