Continuous monitoring of dissolved organic matter fluxes in contrasting glacial and non-glacial watersheds in coastal Southeast Alaska

Riverine sources of dissolved organic matter (DOM) can play an important role in supporting primary and heterotrophic productivity in estuarine and coastal systems. In Southeast Alaska, much of the riverine DOM input to the marine environment originates from glacial ecosystems. Organic matter derived from glaciers in this region has been shown to have a high percentage of bioavailable carbon, much of which is likely exported into marine systems due to tight coupling between land and ocean in conjunction with transport in cold, turbid glacial rivers. Glacial rivers contrast to wetland and forest dominated rivers that contain greater DOM concentrations, but also greater percentages of high molecular weight aromatic material. The goal of this research is to quantify and understand the role of glaciated watersheds in supplying labile DOM to the marine environment, as many of the glaciers in the region are thinning at very fast rates, leading to a change in the watershed landscapes and riverine carbon export. Traditionally, fluxes for both total and bioavailable riverine carbon are calculated based on concentration discharge relationships where point measurements are made on either daily, weekly or monthly intervals. Flux estimates can be refined using continuous monitoring of DOC over diurnal timescales and during storm events. Storm events and/or significant diurnal variability in DOC fluxes can strongly influence yearly stream carbon flux calculations. In this study we used continuous in-situ sensors for fluorescent dissolved organic matter (FDOM), along with grab samples for DOC concentrations and 3D fluorescence scans with PARAFAC modeling to better understand the variability of riverine DOC over very small timescales. Sampling campaigns were undertaken during spring snowmelt, high glacial melt in the summer, and the fall rainy season to capture the dominant features of the seasonal hydrograph. Our work shows variability in DOC concentrations and thus fluxes from a glacial and wetland/forest dominated watershed, both diurnally and during storm events. This work not only has implications for examining carbon export into the coastal waters of Southeast Alaska, but also other stream parameters of concern associated with DOC, such as mercury.