Zooplankton and Picocyanobacteria Contribute Characteristic Size Ranges of Organic Particles to Different Layers of the Anoxic Water Column

Abstract Oxygen deficient zones (ODZs) are key areas of nitrogen loss, a process dependent on organic matter. Understanding the sources of organic matter in the ODZ is necessary to predict how biogeochemical cycles will respond to ocean changes. Size fractionated (5–20, 20–53, 53–180, 180–500, >500 μm) particulate organic C and N (POM) concentration and isotopic composition depth profiles from three stations in the offshore Eastern Tropical North Pacific (ETNP) ODZ were used to gain insights into the origins of POM in the ODZ. Since the within‐ODZ Prochlorococcus cells assimilate nitrite, we used the resulting highly depleted δ 15 N signal to trace organic matter of cyanobacterial origin to medium sized particles at the secondary chlorophyll maximum, and to >500 μm particles directly below the secondary chlorophyll maximum. This organic nitrogen was consumed in the upper ODZ. Other POM maxima were seen at the depths of the zooplankton vertical migration maxima with the increase in POM marked in the 5–20 μm fraction. In the deep ODZ, below the zooplankton migration depth, POM concentrations in the 5–20 μm fraction were unusually small, the C:N ratios were extremely high (>20), and δ 15 N was enriched (8–12‰), indicating degraded material. In deep samples, δ 13 C was more depleted in larger particles and correlated with enriched δ 15 N, indicating increased degradation in 53–500 μm particles. This trend suggests an additional source of small particles, such as from in situ production, rather than just the fragmentation of large particles.