Validating Fluorescence As An Optical Proxy For Fluvial DOC Contributed By The Spring Freshet In The Sagavanirktok And Kuparuk Rivers (Northern Alaska)

On the basin scale, fluvial input to the Arctic Ocean is dominated by a few large river systems, but the contribution of smaller rivers becomes more important regionally along coastal Arctic margins. In Alaska, the coastal Arctic Ocean receives input from multiple smaller rivers, among them the Kuparuk and Sagavanirktok: two rivers which despite entering the coastal ocean within 30 km of each other, drain watersheds with very different geomorphologies. To explore how optical approaches might be used to characterize fluvial differences in organic carbon content in unattended applications, we examined the relationship between organic matter fluorescence and dissolved organic carbon (DOC) concentrations in these rivers during the spring freshet. We sampled both rivers during the May 2019 freshet, performing in situfluorescence measurements for organic matter and taking discrete samples for analyses of DOC concentration. Over an 8-day period during the freshet, we observed that the Kuparuk River, which drains a largely tundra-covered watershed, exhibited DOC concentrations more than twice that of the Sagavanirktok River whose watershed includes the Brooks Range to the south. Further, DOC concentrations in the Kuparuk increased materially during the freshet event by a factor of two-thirds before later diminishing. In situfluorescence correlated well with DOC concentrations (r²=0.98 across both rivers), and parallel measurements of in situoptical backscatter and spectrofluorescence data provide added dimensions for further discriminating spatial and temporal variability in these fluvial sources. This study provides important validation data to support the implementation of similar bio-optical sensors to measure proxies for DOC in adjacent regions of the coastal Arctic Ocean that receive such fluvial input during the spring freshet.