A pond full of decaying oak leaves soon turns as brown as tea. Eventually, much of that rotting organic matter is released into the atmosphere as carbon dioxide. Now, a new study could improve scientists’ ability to track such emissions by improving how satellites detect dissolved organic carbon (DOC) in freshwater.
Worldwide, inland waters such as rivers and lakes release about 1 billion tons of carbon as carbon dioxide (CO2) each year. By comparison, humans burning fossil fuels produced 9 billion tons of carbon as CO2 in 2010, about 20 times the weight of the world’s population at the time. Most of the emissions from freshwater bodies are produced by bacteria, which eat dissolved, microscopic specks of organic matter, digest them, and release greenhouse gas as a waste product.
Scientists are keen to track DOC using satellite imaging. Current methods use a colored component of dissolved organic matter (DOM). This metric, CDOM, can be used as a proxy because it can be visualized from space. However, it’s not clear how accurate this method is across different types of ecosystems, like pine forests and cornfields. In the new study, Li et al. ran a mesocosm experiment—an outdoor experimental system that studies the natural environment under controlled conditions—in parallel with sampling from 14 river outlets to see how reliable the ratio of DOC to CDOM really was.
The mesocosm experiment was conducted on Beaver Island, Lake Michigan. First, the authors filled six tanks with 500 gallons of clean lake water each and then put different types of leaf litter—corn, pine needles, and red maple—in mesh bags and tossed them into the tanks. They let the bags soak for 11 days, sampling the water for DOC and CDOM levels each day. To get a range of samples from the natural environment, the team also visited 14 river mouths across the Connecticut and Chippewa river watersheds. These were located in agricultural, deciduous, evergreen, and mixed ecosystems, in which different types of leaf litter found their way into the water.
Different types of leaf litter of the same biomass produced varying levels of DOC: Red maple leaves produced twice as much organic matter as the corn leaves did, for example. However, the ratio between DOC and CDOM for each type of vegetation litter stayed the same, increasing at a linear rate.
The finding fits with past studies showing that satellite CDOM measurements provide a reliable estimate of DOC, but only when a single type of vegetation dominates the watershed. In the Yukon River, where pine forests dominate, for example, the CDOM/DOM ratio remains steady as DOC increases. The method has not worked as well when used across large watersheds that include many different types of vegetation. The results of the new study indicate that scientists need to include the density and biomass of different types of vegetation in their models, according to the authors. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG004179, 2018)
—Emily Underwood, Freelance Writer