Photochemical mineralization of dissolved organic nitrogen to ammonium in the Baltic Sea

Solar-radiation-induced photochemistry can be considered as a new source of nutrients when photochemical reactions release bioavailable nitrogen from biologically nonreactive dissolved organic nitrogen (DON). Pretreatments of Baltic Sea waters in the dark indicated that > 72% of DON was recalcitrant to biological mineralization. When this DON (16-21.5 mu M) was exposed to simulated solar radiation, the concentration of NH4+ increased 0.5-2.5 mu M more in irradiated waters than in the dark controls. The photochemical production of NH4+ and the dose of absorbed photons were used to calculate the apparent quantum yield spectrum for photoammonification [mol NH4+ (mol photons)(-1) nm(-1)] at wavelengths (lambda) of 290-700 nm (phi NH4,lambda). The modeled mean rates of photoammonification based on phi NH4,lambda were 143 and 53 mu mol NH4+ m(-2) d(-1) at the surface and in the whole water column, respectively, of Baltic Sea stations during summer. The results of this study indicate that the rate of photoammonification approximately equals and periodically exceeds the rate of atmospheric deposition of reactive inorganic nitrogen to the northern Baltic Sea. Forthese stratified surface waters beyond riverine input of labile nitrogen, photoammonification can periodically be the largest source of new bioavailable nitrogen.