Fluorescence as a tracer of the susceptibility of dissolved organic matter to photodegradation in the Arctic Ocean

The Arctic Ocean exports a large amount of terrestrial dissolved organic matter (DOM) to the Nordic seas. With climate change, the supply of terrestrial DOM from the Arctic Ocean will increase while less sea ice might lead to an increased loss of terrestrial DOM due to photodegradation. Here, we aimed to predict DOM photosensitivity based on fluorescence properties across the Eurasian Arctic. Exposures of seawater samples equaling approximately three d of surface irradiance at 85°N (above sea ice) were simulated. On average, 51% of fluorescence and 29% of DOM absorbance was lost while no significant photomineralization was observed. A N‐way partial least squares model was trained to predict the average loss of DOM fluorescence relative to the start value (“photosensitivity index”) from the unexposed fluorescence landscapes. The prediction root mean squared error equaled 7.8% of the average predicted value. We applied the model to > 1500 samples spanning from the Lena River plume, across central Arctic, through Fram Strait, to Denmark Strait. For terrestrial DOM in polar water, the photosensitivity index notably decreased on the Siberian and East Greenland shelf, whereas local, distinct decreases in photosensitivity index were confined to the summer mixed layer in the Central Arctic Ocean. The qualitative photosensitivity index can indicate the occurrence and extent of photodegradation and could be used to monitor the effects of sea ice retreat and thinning in the Arctic Ocean.