Towering conifers store much of the carbon in boreal forests, the vast wooded expanses in the Northern Hemisphere’s circumpolar region, but a significant proportion is processed and transported through the ecosystem by inland waterways. Carbon perseveres in these bodies of water as dissolved organic carbon (DOC), organic matter in the submicrometer range, and as larger flecks of matter known as particulate organic carbon (POC). Only half of the aquatic carbon in boreal waterways eventually spills into the oceans; the remainder is consumed or lost before it reaches the sea. Much of this lost carbon escapes to the atmosphere as carbon dioxide (CO2), resulting in significant aquatic emissions.
Although POC accounts for nearly half of the total flux of organic carbon in rivers around the world, the bulk of the research to date has focused on the role of DOC in the carbon cycle; questions remain as to how aquatic carbon degrades and transforms when it is in POC form. On the basis of its ubiquitous distribution in northern waters, however, larger-sized organic material may appear to be a vital cog in the boreal carbon cycle and an essential regulator of CO2 emissions.
To better understand carbon transformation and the role of POC in boreal waters, Attermeyer et al. analyzed how carbon degraded in different aquatic habitats in Sweden’s boreal forests. The researchers sampled 30 water bodies, ranging from peat surface water to rivers to lakes. The samples spanned environments and water retention times, a variable that can dictate organic matter decomposition.
The researchers found that across all aquatic ecosystems, the degradation rates of POC were approximately 15 times higher than those of DOC. Furthermore, the half-life of POC was only 17 days, a remarkably short decay in comparison to other forms of aquatic organic matter. In addition, the rapid deterioration of the particulate matter correlated with a shift in the ratios of carbon to nitrogen, whereas the ratio stayed the same for the DOC.
The rapid decay of particulate matter ran counter to the study’s findings that the sampled waters maintained consistent POC levels, which suggests a continuous replenishment of the POC pool. The authors believe that dissolved matter may glom together to create bulkier particles as water flows downstream, and these larger particles are later counted as particulate matter. The loss of DOC along the aquatic continuum supported this determination.
These results imply that particulate organic matter may play a more significant role in aquatic CO2 emissions than previously thought. The findings also suggest that biogeochemical researchers should turn their attention toward particulate organic carbon, which is currently understudied. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004500, 2018)
—Aaron Sidder, Freelance Writer