Salp-Mediated Export Processes in the Northeast Subarctic Pacific Ocean

Periodic blooms of salps (pelagic tunicates) give rise to dense populations. Combined with some remarkable features of salp ecology and physiology, these high abundances can result in significant export of organic matter, potentially leading to an ‘outsized’ role in the biological pump vs. other taxa. We examined the role of salp-mediated export processes in the biological pump as part of the EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign near Ocean Station Papa in the NE subarctic Pacific Ocean. During this Aug. 2019 cruise, we encountered a bloom of Salpa aspera. Depth-discrete, diel sampling using MOCNESS tows indicated salps were migrating from 300-750m during the day into the surface 0-100m at night, contributing to a >2-fold increase in total nighttime epipelagic zooplankton biomass due to diel vertical migration (DVM). Fecal pellet production experiments showed that when present, salps accounted for up to 72% of the POC in fecal pellets produced by the whole zooplankton community in the upper 100m. Sinking velocities of salp fecal pellets, measured in rolling tanks, were a function of size and ranged from 400-1200 m^-1 d^-1. Microbial respiration rates on pellets ranged from < 1 to 3 µmol O₂ pellet^-1h^-1and were a linear function of both pellet volume and C content. Salp fecal pellets caught in sediment gel traps deployed during our sampling period comprised a patchy but significant proportion of the total sinking POC. Salp active transport of C by DVM quantified using metabolic algorithms applied to salp biomass data, and potential C export from sinking of salp carcasses following the demise of this bloom, were of the same order of magnitude. We use an advection model to further describe the bloom, and discuss the unique and important effects salps have on ocean biogeochemistry.