Dissolved organic nitrogen dynamics in the Arctic Ocean

a r t i c l e i n f o Little is known about the distribution and dynamics of dissolved organic nitrogen (DON) within Arctic Ocean surface waters, though seasonal inputs from both rivers and marine phytoplankton production are likely important. Here we combine multiple datasets to provide the first quasi-synoptic view of DON concentrations in the summertime Arctic Ocean Polar Surface Layer (PSL) and western Arctic shelves, and infer controlling processes. Distributions indicate inputs of DON by Arctic rivers followed by net consumption of up to 70% of this terrigenous material across the resulting salinity gradients over time scales of years. Coupling of DON losses to PSL residence times indicates first order decay constants (λ) of 0.15 ± 0.07 yr −1 and 0.08 ± 0.01 yr −1 for terrigenous DON in the eastern and western Arctic systems, respectively. Riverine delivery of both inorganic and organic nitrogen have only a minor (b15%) impact on Arctic shelf export production. In the Pacific Ocean-influenced Chukchi Sea, net biological production of ~ 2 μM DON, representing ~ 8% of net community production, is observed in surface waters following seasonal sea-ice retreat. These findings highlight the contrasting roles and locations of terrigenous versus marine derived DON in the cycling of nitrogen in the surface Arctic Ocean. Dissolved organic nitrogen (DON) is an important source of reac-tive nitrogen in the surface ocean, and the dominant pool of fixed N when inorganic nutrients are depleted. Marine DON ultimately results from primary production in the surface ocean, subsequently providing N as substrate for heterotrophic growth (Azam and Hodson, 1977; Azam and Cho, 1987) via direct uptake of smaller molecules and extracellular hydrolysis of the bulk pool. In addition, moieties such as urea and amino acids are directly bioavailable for autotrophic growth (Bronk et al., 2007). However, to date, analytical uncertainties in the measurement of marine DON (Sharp, 2002; Sharp et al., 2002) have made it difficult to adequately resolve spatio-temporal gradients within the open ocean from which DON dynamics can be inferred. The Arctic Ocean provides a unique system in which to study the dynamics of DON. Inflow shelves [e.g., Barents and Chukchi Seas; (Carmack and Wassmann, 2006)] are highly influenced by Atlantic and Pacific Ocean inflow of nutrient-rich waters and can be highly productive seasonally [e., while there are large inputs of terrigenous organic material from the many rivers that drain into the interior shelves …