Shift from denitrification to anammox after inflow events in the central Baltic Sea

Incubation experiments with N-15- labeled compounds (NO3- and NH4+) were performed during three cruises (2002, 2004, and 2005) to study the loss of inorganic N as dinitrogen gas (N-2) via denitrification and anammox in the water column of the Gotland Deep (central Baltic Sea). N-15 incubations did not provide evidence for direct conversion of NO3- reduction to N-2 (heterotrophic denitrification) in the suboxic (O-2 < 10 mu mol L-1) sulfide-free waters. Substantial denitrification rates (up to 2.7 mmol L-1 d(-1) N-2) 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.