Examining Controls on Dissolved Organic Carbon Quantity and Quality in Large North American Rivers

Dissolved organic carbon (DOC) accounts for more than half of all organic carbon delivered to the ocean by world rivers and is a critical component of the global carbon cycle. Previous work has explored the potential for discharge and wetland-cover to predict bulk DOC flux, but little has been done to characterize its quality. We analyzed DOC quantity and quality at USGS NASQAN gauging stations near the mouths of 12 large rivers over a two year period from 2008 to 2009. We explored DOC quality estimates based on both specific ultraviolet absorbance (SUVA) and XAD resin fractionation data. These catchments were heterogeneous, with DOC flux ranging from 0.01 to 2.99 g/day*m2 and SUVA ranging from 1.35 to 4.23 L/mg*cm. As in some previous studies, we found a positive correlation between bulk DOC flux and both discharge (R2=0.43; p=0.028) and wetland-cover (R2=0.57; p=0.007). However, we also found that a basin’s wetland-cover controls variability in DOC quality among these large watersheds (R2=0.42; p=0.031). In addition, we found evidence that SUVA is a useful surrogate for the humic fraction of the bulk DOC load as measured by XAD (R2=0.83; p<0.0001). This relationship is important because SUVA measurements are far simpler than those made by XAD resin fractionation and records are already available at many USGS gauging stations and around the world. Because DOC quality influences and is influenced by ecosystem processes such as microbial metabolism, accounting for quality in watershed-scale studies will be critical for understanding continental-scale carbon dynamics.