Figure 3. Sinking particle flux, measured from surface-tethered sediment traps (large symbols), at ETNP Station P2. Trap types are shown by the shape of the large points. Superimposed are estimates of particle flux from the UVP generated by fitting the sum of particle numbers all four profiles to the trap observed flux. The black line indicates flux predictions made by fitting UVP observations to the trap data. Black circles indicate regions on the black line corresponding to the trap observation depths. Horizontal blue lines indicate the top and bottom of the ODZ, while the horizontal green line indicates the base of the photic zone.

Particle abundance measurements vary with size and depth

In all profiles, particle abundances were highest at the surface, and highest among the smallest particles (Figure S4). Visual examination of the relationship between particle number and size suggested a power law relationship where the log of volume and bin-size normalized particle abundance was proportional to the log of the particles’ size (Figure S5). The exception to this pattern were particles larger than 10 mm (Figure S4, S5), which are rare enough that they are usually not detected by the UVP. Generalized linear models that assume a negative-binomial distribution of the data accounted for this under-sampling of large particles to estimate power law slopes, while considering rare occurrences of the large particles at each depth (Figure S5).
Total particle numbers were generally similar between different casts, regardless of which day or hour they were collected (Figure 4A). Particle numbers were highest in the surface and decreased within the oxic region, then remained relatively constant from 160 m to 500 m, and gradually decreased between 500 m and the lower oxycline (Figure 4A).
The particle size distribution slope generally steepened (became more negative) between the base of the photic zone (160 m) and 500 m, flattened (became less negative) between 500 m and 1000 m, and then steepened again below 1000 m (Figure 4B). Steeper, more negative, slopes indicate a higher proportion of small particles relative to large particles, while flatter, less negative, slopes indicate a more even particle size distribution. Flatter distributions still have a higherabsolute number of smaller particles than larger particles; however, they have a higher proportion of larger particles relative to other samples with steeper distributions.