Photoreactivity of chromophoric dissolved organic matter transported by the Mackenzie River to the Beaufort Sea

Abstract

The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf environments by rivers has only recently been studied and its quantitative role in Arctic shelf biogeochemistry has received little attention. Sunlight exposure experiments were performed on CDOM collected over a three year period (2002 to 2004) from river, estuary, shelf, and gulf regions of the Western Canadian Arctic. Decreases in CDOM absorption, synchronous fluorescence (SF), and dissolved organic carbon (DOC) concentration were followed after 3 days of exposure, and in two experiments, six optical cutoff filters were used to incrementally remove ultraviolet radiation incident on the samples. Apparent quantum yields for CDOM photobleaching (AQY ble) and for DOC photomineralization (AQY min) were computed, as were two AQY spectra (ϕ ble and ϕ min) for the Mackenzie River and a sample from the Mackenzie Shelf. The photoreactivity of Mackenzie River CDOM was highest after break-up and peak discharge and lowest in late summer. The half-lives of CDOM and DOC were estimated at 3.7 days and 4.8 days, respectively, when Mackenzie River water was exposed to full sunlight. Photobleaching of Mackenzie River CDOM fluorescence after most UV-B wavelengths were removed increased the correlation between the river and offshore waters in the Beaufort Sea. When light attenuation from particle- and CDOM-rich river water was considered for the Mackenzie Shelf, our photodegradation models estimated around 10% loss of absorption and < 1% DOC loss, suggesting that sunlight exposure does not substantially degrade CDOM on Arctic shelves.