Shift from denitrification to anammox after inflow events in the central Baltic Sea
Hannig, M., G. Lavik, M. M. M. Kuypers, D. Woebken, W. Martens-Habbena, and K. Jürgens
Limnol. Oceanogr., 52(4), 2007, 1336–1345

Incubation experiments with ¹⁵N-labeled compounds (NO₃^- and NH₄⁺) were performed during three cruises (2002, 2004, and 2005) to study the loss of inorganic N as dinitrogen gas (N₂) via denitrification and anammox in the water column of the Gotland Deep (central Baltic Sea). ¹⁵N incubations did not provide evidence for direct conversion of NO₃^- reduction to N₂ (heterotrophic denitrification) in the suboxic (O₂ < 10 µmol L^-1) sulfide-free waters. Substantial denitrification rates (up to 2.7 µmol L^-1 d^-1 N₂) were measured in water samples collected from the NO₃^--H2S interface (redoxcline) in 2002 and in water from the sulfidic zone in 2004, which indicates chemolithotrophic denitrification as the dominant N-loss process in both years. Massive inflows of oxygenated North Sea water from 2002 to 2003 caused a complete ventilation of the Baltic Sea with high oxygen concentrations in the Gotland Deep bottom water. After the reestablishment of the redoxcline in 2004, a newly formed suboxic zone above sulfidic waters—with NO₃^-, NO₂^-, and NH₄⁺ at the detection limit—was observed in spring 2005. The development of this zone was associated with a several-fold increase in reduced and oxidized manganese and with a shift from denitrification to anammox as the main N-loss process. Fluorescence in situ hybridization analysis confirmed the presence of anammox bacteria and the number of anammox cells was consistent with the observed N₂ production rates in 2005.