Southern Ocean salp diel vertical migration and the significant influence of life cycle stage and body size

Diel vertical migration (DVM) is a widespread behavior among zooplankton and micronekton, playing a crucial role in carbon cycling by transporting organic matter to depth. The Southern Ocean salp, Salpa thompsoni , is an abundant gelatinous zooplankton species recognized for its significant contribution to vertical carbon export. It’s high grazing capacity, production of rapidly sinking fecal pellets, and extensive DVMs make it a key player in biogeochemical cycles. Despite its ecological importance, the influence of the life cycle stage and body size on S. thompsoni ’s migratory behavior has been understudied. Utilizing net data from six Southern Ocean locations collected between 1989 and 2018, we provide stage- and size-specific vertical distribution and migration distance estimates, supported by circumpolar predictions using a mixed-effects modeling approach. Our results reveal that small-bodied, early developmental salps migrate amplitudes up to three times greater than their larger, reproductive counterparts, which tend to remain stationary or undergo reverse DVM. Environmental factors, such as salinity and euphotic zone depth strongly influence S. thompsoni DVM patterns, with the potential to impose as primary controls on the main hypothesized driver, reproduction. This first Southern Ocean-wide analysis of S. thompsoni DVM underscores the critical importance of incorporating life cycle stage and body size into assessments of salp behavior. Understanding these dynamics is essential for improving the estimated vertical carbon export via the migrant pump and refining global biogeochemical models. • Small Salpa thompsoni migrate 2-3x farther than large individuals • Life cycle stage and body size shape S. thompsoni daily vertical migration behavior • Reverse diel vertical migration observed in large S. thompsoni , likely linked to reproduction • Statistical models predict stage- and size-specific S. thompsoni migration • Salinity and euphotic zone depth influence S. thompsoni residence depths