Vertical Distribution of the Pyrosome Pyrosoma atlanticum off the US West Coast from Camera Profiles and Depth-Stratified Net Tows

Anomalous blooms of the colonial pelagic tunicate Pyrosoma atlanticum in 2014-2018 followed an unusual marine heatwave off the western coast of the US. Typically found in tropical or subtropical waters, pyrosomes are among the most efficient zooplankton grazers capable of rapid reproduction and, when numbers are high, consuming much of the local phytoplankton biomass. In this study, we used vertical camera tows and MOCNESS sampling to estimate the vertical distribution of P. atlanticum along transects off the coast of Oregon (45 N, 124 W) and northern California (41 N, 124 W) during the winter and summer of 2018. Pyrosomes underwent diel vertical migration and were distributed non-uniformly in the water column with peak numbers associated with the base of the surface mixed layer. Considerably more pyrosomes were observed in the winter than summer, and they were more common offshore of Oregon than northern California. Large blooms of P. atlanticum similar to those seen in 2018 could dramatically affect pelagic food webs of the NE Pacific due to high clearance rates and carbon export from fecal pellet production and mortality events. Understanding the distribution of these gelatinous grazers may give insight to their ecological role in the Northern California Current as conditions become more favorable for recurring blooms.