Biodegradability of dissolved and particulate organic matter in tributaries of contrasting land-use in the Upper Mississippi River

Abstract:

Recent estimates of significant CO₂ efflux from inland waters have spurred interest in respiration of organic matter (OM) as a contributor to regional carbon budgets. Dissolved organic carbon (DOC) has been the focus of many investigations, with numerous studies targeting the structural and environmental controls on degradation rates. Very little is known about the reactivity of riverine particulate OC (POC), which can be composed of a range of materials from freshly fixed photosynthetic organic matter to ancient sedimentary OC. This study examines OC reactivity in two watersheds with contrasting land-use, the highly forested Chippewa River basin in Wisconsin (45% forested, 19% agricultural) and the heavily agricultural Minnesota River basin in Minnesota (3% forested, 72% agricultural). The Minnesota and Chippewa rivers are the largest tributary sources of suspended sediment to the Mississippi River upstream of Iowa, and their distinct land-use features lead to a diversity in carbon sources and loads across a small geographic range. Respiration incubations were conducted on the DOC and POC collected at different seasonal and flow conditions from these two rivers. Optical oxygen measurements were used for non-destructive monitoring of incubations at high temporal frequency. Coupled with traditional DOC loss and CO₂ production approaches, these experiments allow for comparison of potential CO₂ production from DOC and POC, determination of oxygen:carbon respiratory quotients, and compositional changes in OM (e.g. DOM fluorescence, POM elemental composition). We observed potential CO₂ production from POC that was 1x and 3x greater than that of DOC at field concentrations in the Chippewa and Minnesota rivers, respectively, for incubation samples collected in spring of 2015. By linking OM respiration rates to metrics such as land use types and environmental variables, these results can help improve estimates of CO₂ efflux from rivers across seasonal and spatial gradients.