Results

Physical and Chemical Data

The ODZ, characterized in this study by oxygen levels less than 1 μM, as measured by the CTD, extends from 90 m to 900 m depth, with a sharp upper oxycline and a gradual lower oxycline (Figure 1B-C). This station has been previously proven to be anoxic with a STOX sensor (Tiano et al., 2014). The upper oxycline tracks a sharp pycnocline (Figure 1B-1D), set by the high salinity of the 13CW water mass (Figure S2), and is characterized by an abrupt drop in temperature below the mixed layer and an increase in salinity (Figure 1B). Water mass analysis indicated that water in the top part of the ODZ is dominated by the 13CW water mass, while water between 275 and 500 m is primarily from the NEPIW, with water from the AAIW dominating in the lower 500 m (Figure S2) (Evans et al., 2020). The site is characterized by two fluorescence maxima (Figure 1C). The larger, shallower fluorescence peak is positioned just above the oxycline, with fluorescence from this peak and oxygen attenuating together. The smaller, lower peak is inside of the ODZ. Turbidity tracks the two chlorophyll peaks in the surface and has a tertiary maximum at the lower oxycline (Figure 1D). The cyanobacteria at the secondary chlorophyll maximum are known to be photosynthesizing and producing organic matter in the ODZ (Fuchsman et al., 2019; Garcia-Robledo et al., 2017). To avoid complication due to this source of organic matter production, we focus our further analysis below 160 m.
For the purposes of this study, we define the photic zone as ending at the base of this deeper fluorescence layer (160m). This photic zone base corresponds with photosynthetically active radiation (PAR) < 10-5 of surface PAR levels (Figure 1C). We note that this photic zone depth is deeper than conventional definitions, in which the base of the photic zone corresponds with 10-2(90 m) or 10-3 (120 m) of surface PAR.