Seasonal phototransformation of dissolved organic matter to ammonium, dissolved inorganic carbon, and labile substrates supporting bacterial biomass across the Baltic Sea

• Published in: Journal of Geophysical Research: Biogeosciences 2012-03, Vol.117 (G1), p.n/a
• Main Authors: Aarnos, Hanna, Ylöstalo, Pasi, Vähätalo, Anssi V.
• Format: Article
• Language: English
• Subjects: Ammonium; Bacteria; bacterioplankton; Biological oceanography; Biomass; Carbon; Chemical oceanography; Coastal waters; Dissolved inorganic carbon; Dissolved organic carbon; Dissolved organic matter; DOC; DOM; DON; Earth sciences; Earth, ocean, space; Exact sciences and technology; Mineralization; Nitrogen; photoammonification; photochemical mineralization; Photochemicals; Photochemistry; Rivers; Salinity; Seasonal variations; Summer
• ISSN: 0148-0227; 2169-8953; 2156-2202; 2169-8961
• DOI: 10.1029/2010JG001633

We studied the photochemical transformation of dissolved organic matter (DOM) to dissolved inorganic carbon (DIC), ammonium (NH4+), and labile organic substrates supporting bacterial carbon biomass along a salinity gradient throughout the Baltic Sea during summer, autumn, and spring. The photoproduced DIC, NH4+, and labile DOM supporting bacterial biomass were related to the number of photons absorbed during the irradiations of biologically recalcitrant DOM to determine apparent quantum yields. The apparent quantum yields for the photoproduction of DIC and NH4+ lacked seasonal variation, but behaved differently along the salinity gradient; the photoproduction of DIC decreased, while photoammonification increased with increasing salinity. The apparent quantum yield for the photoproduction of labile DOM supporting bacterial biomass was highest in summer and unaffected by salinity. The annual photoammonification rate over the entire Baltic Sea ranged from 0.038 to 0.049 Tg N, equivalent to 13%–23% of the annual atmospheric deposition of inorganic N. The annual phototransformation of dissolved organic carbon (DOC), including the direct photomineralization and indirect bacterial mineralization of photoproduced labile DOM (total of 2.71–3.94 Tg C), exceeded the annual river loading of photoreactive DOC, assuming that half of the total river DOC input to the Baltic Sea is photoreactive. As the annual photomineralization of DOC exceeded the annual terrestrial input of photoreactive DOC to the Baltic Sea, the photochemical transformation is a major sink for terrestrial DOC in such coastal systems. Key Points Photochemistry is a major sink for terrestrial DOC in coastal waters Apparent quantum yields for the photoproduction of bacterial biomass No seasonality on the photomineralization of DOC or DON