Figure 1.
Locations of the sampled stations and distribution of dissolved O2.
Maps show sampling locations on the A) Namibian shelf and in the B) OMZ off Peru during cruises M76-2 and M77-3, respectively. Water samples were collected by pump-CTD (max. sampling depth: ∼375 m). The oxygen sensitivities of anammox and coupled N-cycling processes were investigated at sampling stations indicated by numbers (red circles). Vertical distributions of dissolved O2 are plotted along blue lines.
Table 1.
Concentrations of O2, NH4+, NO2− and N-conversion rates in 15N-labeling experiments in the OMZs off Namibia and Peru.
Figure 2.
Physicochemical zonation and N-conversion rates at selected stations.
Stations are plotted for cruises M76-2 and M77-3 to the OMZs off A) Namibia and B) Peru, respectively. Water depths were 111 m at St. 252 and 2845 m at St. 36. N* was calculated from the fixed inorganic N- and PO43− concentrations (data not shown). Anammox rates were determined in 15NH4+ (St. 206) and 15NO2++14NH4+ incubations (St. 36). All rates are net rates corrected for the percentage of 15N in the pool of the respective N-species. Error bars for rates are standard errors calculated from linear regression.
Figure 3.
N-cycle processes in Namibian and Peruvian OMZ waters with respect to dissolved oxygen.
A) Anammox measured as 15N14N production in 15NO2− (+14NH4+) and 15NH4++14NO2− incubations. B) NO3− reduction measured as 15NO2− production in 15NO3−+14NO2− incubations. C) NH3 oxidation measured as 15NO2− production in 15NH4++14NO2− incubations. N-conversion rates are given as percentages of the highest rate observed ( = 100%) for the different O2 treatments at each incubation depth. Adjusted O2 concentrations were verified by micro-sensor measurements. Parentheses in figure legend indicate the corresponding sampling depths at each station shown. Station numbers with double digits and triple digits represent the Peruvian and Namibian stations, respectively. Shelf and open ocean stations are represented by red and blue symbols, respectively. The O2 sensitivity assays indicate an upper O2 limit for N-loss due to anammox of ∼20 µmol L−1 (grey shading).
Table 2.
Rates of NH3 oxidation, NO3− reduction and anammox measured at varying concentrations of dissolved O2.
Table 3.
Overview of the response of NH3 oxidation, NO3− reduction and anammox to changes in dissolved O2.