Size structure, vertical distribution, and trophic ecology of Pyrosoma atlanticum blooms in relation to particles in the California Current

The California Current Ecosystem (CCE) has experienced large, persistent blooms of the pelagic tunicate Pyrosoma atlanticum over the past decade. We collected depth-discrete samples of P. atlanticum and particles across the southern CCE on 2 cruises (August 2020, June 2021). We quantified P. atlanticum biomass, size structure, and vertical distribution in relation to environmental conditions, and used carbon and nitrogen stable isotopes to examine the trophic ecology of P. atlanticum and particles. In total, we collected 271091 P. atlanticum . Colonies were larger but less abundant in 2021 (mean length ± SD = 35.1 ± 23.6 mm) than in 2020 (5.5 ± 2.2 mm), resulting in total biomass that was 5 times larger in 2021, which coincided with higher surface fluorescence. Depth distribution was related to colony size, where a higher proportion of colonies was found in the 0-200 m depth stratum in 2020 (93.3%) than in 2021 (69.5%). Size decreased with increasing distance from shore, with a steeper nearshore-offshore gradient in 2021. Length-weight relationships and elemental composition differed across stations and years, indicating potential shifts in body condition and growth rates. P. atlanticum nitrogen isotope values were significantly lower than crustacean zooplankton, suggesting reliance on smaller prey and microbial loop pathways. In both years, P. atlanticum isotopic niches overlapped most with shallow particles (across all size classes), and less so with small particles from deep layers. These results suggest that P. atlanticum consumes surface-derived material, which highlights potential for their blooms to impact particle and food-web dynamics across space, time, and depth in the CCE.