Summary of measured O₂ consumption rates, method settings and statistics.

<p>The O₂ consumption rates (O₂ Rate) measured at different O₂ concentrations (O₂ Conc.) and with different sensors and sampling frequencies (F) are summarized. For each rate measurement, standard errors (SE) and root mean square of the residuals (RMS_RES) were calculated from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0089369#pone.0089369.e001" target="_blank">equations 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0089369#pone.0089369.e003" target="_blank">2</a>. The potential rate detection limit of an assumed 24 hour incubation (Pot.Det.Lim.) was calculated from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0089369#pone.0089369.e004" target="_blank">equation 3</a>. Please note, a low RMS_RES denotes a high precision, and potential rate detection limits are only based on precision and measuring frequency, but do not consider possible limitation from sensor drift or O₂ contamination.</p

Experiment 3: Time series of ^18-18O₂ concentrations in Exetainers.

<p>The O₂ consumption in 5 incubations with different initial ^18-18O₂ concentrations was measured. For each Exetainer, individual MIMS measurements are averaged (black dots) and presented with the respective standard deviation.</p

O₂ consumption rates versus O₂ concentrations.

<p>The O₂ consumption rates of Experiment 1, 2 and 3 are summarized and plotted over the initial O₂ concentrations of the respective incubation. Experiment 3: The dashed line represents diffusion limited rates estimated from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0089369#pone.0089369.e005" target="_blank">equation 4</a> assuming aggregate diameters (r₀) of 0.14 mm. Please note: rates in Experiment 1 also increase with incubation time and could as well reflect bacterial growth over time.</p

Experiment 1: Comparison of O₂ decrease over time.

<p>The O₂ consumption measured by optodes and STOX sensor after the adjustment to O₂ concentrations of 1.2 µmol L⁻¹ (A), 5.5 µmol L⁻¹ (B) and 14.3 µmol L⁻¹ (C), and after the addition of 20 ml ZnCl₂ (D). Linear fits are indicated by the straight lines. Please note, the aspect ratio of time and concentration axis (µmol L⁻¹ to hour) is the same for all figures to allow comparison of slopes.</p

Experiment 2: Comparing MIMS and STOX sensor.

<p>O₂ concentration and consumption rates measured after the addition of 1.2 µmol ^18-18O₂ L⁻¹ (arrow) (A). Individual MIMS measurements (grey dots) are averaged (black dots) and presented with the respective standard deviation. From the entire experimental run 3 hours of simultaneous measurements (B) were used for rate calculations.</p

Experiment 1: Comparing optodes (trace and full range) and STOX sensor.

<p>Degassed water was successively enriched with oxygen by adding known volumes of air saturated water (arrows). Consumption rates were calculated after each addition from the O₂ decrease over time (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0089369#pone-0089369-g004" target="_blank">Figure 4</a> A–C). ZnCl₂ was added after 30 hours to stop biological activity. The insert shows the initial adaptation of the optodes in the first few hours.</p

Locations of the sampled stations and distribution of dissolved O₂.

<p>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.</p

Rates of NH₃ oxidation, NO₃⁻ reduction and anammox measured at varying concentrations of dissolved O₂.

<p>*No addition of ¹⁴N-species.</p>§<p>In µmol L⁻¹.</p>‡<p>Adjusted concentrations of O₂, determined by μ-sensor measurements.</p>†<p>In nmol N L⁻¹ d⁻¹.</p

N-cycle processes in Namibian and Peruvian OMZ waters with respect to dissolved oxygen.

<p>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).</p

Physicochemical zonation and N-conversion rates at selected stations.

<p>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.</p