Biodegradation of Yukon River delta dissolved organic matter is marginally enhanced by nutrient enrichment

Blake Clark, NASA Goddard Space Flight Center, Ocean Ecology Laboratory, Greenbelt, MD, United States; Universities Space Research Association Greenbelt, Greenbelt, MD, United States and Antonio Mannino, NASA Goddard Space Flight Center, Ocean Ecology Laboratory, Greenbelt, United States
Abstract:
The Yukon River exports a large amount of dissolved organic carbon (DOC), much of which is colored, absorbing both visible and ultraviolet light. Yukon River water typically has low total dissolved nitrogen (TDN), with an average spring DOC:TDN ratio of 21.8 (mol C mol N-1) ~200 km upstream from the coast at Pilot Station, AK. Low TDN has been strongly correlated to the bioavailability of DOC, but a direct estimate of the dependence of DOC reactivity on nutrient availability has not been conducted in the Yukon River delta. The biological reactivity of DOC and colored dissolved organic matter (CDOM) was assessed along an environmental gradient in the Yukon River delta. To evaluate the importance of nutrient availability to the degradation of CDOM and DOC, samples were incubated in the dark at 20°C and half of the incubations were amended with ~80 μmol L-1 NH4+ and NO3- and 10 μmol L-1 PO43- . CDOM absorption and fluorescence and DOC concentration were measured in 0.2 μm filtered samples at five times through 27 days. Initial DOC concentration was 530-820 μmol C L-1 ­and CDOM absorption at 355 nm (a355) ranged from 14-33 m-1 from the river mouth to a high CDOM lake. DOC was relatively unreactive, with 4-5% of total DOC degrading across sample locations, and little difference between nutrient amended and control incubations. a355 decreased 3-5% percent in the control, while nutrient amended treatments saw a 3-12% decline in a355 over 27 days. Specific UV absorption at 254 nm (SUVA; m2 gC-1) declined 6-8% in the riverine samples. Nutrient enrichment enhanced the biological degradation of CDOM, and CDOM degradation was relatively greater than DOC loss. Overall, DOC and CDOM in the Yukon River delta and river plume was resistant to biodegradation in both nutrient amended and ambient conditions. The results here will be used to inform and parameterize a new organic carbon cycle model for the Yukon River delta and Norton Sound.