Contributions of Vertically Migrating Metazoans to Sinking and Suspended Particulate Matter Fuel N2 Production in the Eastern Tropical North Pacific Oxygen Deficient Zone

Oxygen Deficient Zones (ODZs) are the largest pelagic sinks of N containing nutrients in the ocean. The offshore Eastern Tropical North Pacific (ETNP) ODZ has been shown to be limited by organic matter. We propose zooplankton/forage fish as a key source of particulate and dissolved organic matter for N₂ production that has previously been ignored. We examined data sets from four cruises (April 2012, January 2017, April 2018, October 2019) at a station in the central ETNP. Backscattering data were used to determine zooplankton vertical migration depths (250–450 m, maximum at 270–280 m). Metazoan DNA concentrations, as measured by quantitative PCR, had a reproducible maximum at 270–280 m, confirming that these signals indicate the presence of zooplankton/forage fish. Additionally, a large maximum in sinking pteropod shells was found at 270 m, indicating that pteropods were part of the migrating community. While crustacean zooplankton have been shown to reduce respiration and excretion of ammonium under anoxia, we found intermittently measurable ammonium concentrations at 270 m. Here we show signatures consistent with organic matter of zooplankton/forage fish origin in the C:N and δ¹³C of suspended and sinking organic matter at the vertical migration depth that suggest transportation to these depths by migrating zooplankton/forage fish. Also coincident with the migration maximum was a reproducible-between-years maximum in the biological N₂ gas, and a repeatable shoulder on the nitrite maximum, which suggest that the migrating zooplankton partially fuels N loss. Thus, zooplankton/forage fish appear to be one source of organic matter which can fuel N₂ production in ODZs.