Photochemical production of ammonium from dissolved organic nitrogen in arctic surface waters

In sunlit freshwaters, dissolved organic nitrogen (DON) can be degraded photochemically to ammonium (NH₄⁺; photo-production of NH₄⁺). While this NH₄⁺ could be an important source of N to primary and secondary producers, there are too few studies to assess its relative importance in sunlit freshwaters. This study quantified rates and controls of photo-production of NH₄⁺ in ten stream and lake waters in the Alaskan Arctic. Wavelength-dependent apparent quantum yields of NH₄⁺ photo-production (φ_NH4+,λ) were quantified and DON photodegraded was characterized by fluorescence spectroscopy and ultra-high resolution mass spectrometry. The φ_NH4+,350 ranged from 2.4 ± 0.6 μmol NH₄⁺ (mol photon)^-1 in a headwater stream to 100.8 ± 18.2 μmol NH₄⁺ (mol photon)^-1 in a coastal plain lake. Primary amines associated with protein-like compounds are likely the DON substrates photodegraded to NH₄⁺. Photo-production of NH₄⁺ in streams and lakes was not limited by photo-labile DON. In-stream rates of NH₄⁺ photo-production were much lower than watershed inputs of NH₄⁺ to the stream. In lakes, NH₄⁺ photo-production contributed on average 17% of phytoplankton demand and may also supply NH₄⁺ to bacteria. In waters with high DON inputs and low NH₄⁺ inputs or concentrations, photo-production of NH₄⁺ may contribute substantially to primary and secondary production.