Large vertical migrations of Pyrosoma atlanticum play an important role in active carbon transport

Pyrosomes are efficient grazers that can form dense aggregations. Their clearance rates are among the highest of any zooplankton grazer, and they can rapidly repackage what they consume into thousands of fecal pellets per hour. In recent years, pyrosome swarms have been found outside of their natural geographical range; however, environmental drivers that promote these swarms are still unknown. During the austral spring of 2017 a Pyrosoma atlanticum swarm was sampled in the Tasman Sea. Depth‐stratified sampling during the day and night was used to examine the spatial and vertical distribution of P. atlanticum across three eddies. Respiration rate experiments were performed onboard to determine minimum feeding requirements for the pyrosome population. P. atlanticum was 2 orders of magnitude more abundant in the cold core eddy (CCE) compared to both warm core eddies, with maximum biomass of 360 mg WW·m⁻³, most likely driven by high chlorophyll a concentrations. P. atlanticum exhibited diel vertical migration and migrated to a maximum depth strata of 800–1,000 m. Active carbon transport in the CCE was 4 orders of magnitude higher than the warm core eddies. Fecal pellet production contributed to the majority (91%) of transport, and total downward carbon flux below the mixed layer was estimated at 11 mg C·m⁻²·d⁻¹. When abundant, P. atlanticum swarms have the potential to play a major role in active carbon transport, comparable to fluxes for zooplankton and micronekton communities.