Fine-scale Vertical Distribution and Diel Migrations of Pyrosoma atlanticum in the Northern California Current

Abstract

An unusual marine heatwave preceded anomalous blooms of the colonial pelagic tunicate Pyrosoma atlanticum in the Northern California Current (NCC) in 2014-2018. Although aggregations of pyrosomes have the potential to shape marine trophic dynamics through grazing and rapid reproduction, little is known about their vertical distribution patterns. In February and July 2018, we sampled P. atlanticum colonies in the NCC. Depth-stratified net tows provided volume-normalized abundance estimates that complemented fine-scale counts by a vertically-deployed camera system. Pyrosome distribution and size structure varied over space and time. Pyrosomes were distributed non-uniformly in the water column with peak numbers associated with vertical gradients in environmental parameters, notably density and fluorescence. Vertical distributions shifted over the 24-hour period, indicative of diel vertical migration. Understanding the distribution of these subtropical gelatinous grazers gives insight to their ecological role, particularly related to carbon transfer, in the NCC as conditions become more favorable for recurring blooms.