Fig. 1: Physico-chemical parameters, particle abundance and anammox rates from an exemplary onshore and offshore station in the Peruvian upwelling system in April 2017.

In situ particle abundances and anammox rates from a onshore station 423 (total water depth 241 m) and c offshore station 549 (total water depth 4350 m). b and d Depth profiles of oxygen (purple line), ammonium (open circles) and nitrite (open triangles) concentrations. Particles were sorted into four size classes with equivalent spherical diameters (ESD) of 128–256, 256–512, 512–1024, and 1024–2048 µm. Particle abundances in the smallest size class reached up to 398 particles L⁻¹ in the upper 20 m of the onshore station. Anammox rates were determined in time series incubations at six discrete depths. Significant rates are shown with closed circles, rates that are not significantly different from zero are shown with open circles (see Supplementary Table 1). Error bars represent the standard error of the slope. The base of the euphotic zone (where photosynthetically active radiation (PAR) dropped to <1%, based on Aqua-MODIS satellite data) and the oxic–anoxic interface (where O₂ dropped to <1.5 µM) are indicated.