Early spring population dynamics of Salpa thompsoni linked to physical oceanography in the Atlantic sector of the Southern Ocean

The pelagic tunicate Salpa thompsoni accounts for a large portion of the zooplankton in the Southern Ocean. While its functional role in pelagic ecosystems is well recognised, the species’ life cycle during winter and spring remains poorly understood. To uncover the effects of environmental drivers on S. thompsoni population dynamics, we collected oceanographic and salp population development data in the Atlantic sector of the Southern Ocean during the austral spring of 2022. Salp abundances were generally low (< 5 ind. m⁻²) in the top 600 m layer. The most advanced stages of development, including a diverse stage composition and a bimodal size distribution, were observed in the sub-Antarctic Zone north of the Polar Front (PF), indicating ongoing salp reproduction that began at least one month before our voyage. Stations located in the Antarctic Zone south of the PF were characterised by even lower salp abundances (generally < 2 ind. m⁻²) and early developmental stages. Population distribution and development were strongly influenced by the regional physical oceanography, with the region’s strong currents and dynamic frontal systems, including eddies, emerging as major drivers. The findings from this study, combined with published sources, illustrate that salp populations north of the PF likely spawn year-round, while populations in regions influenced by Antarctic waters exhibit strong seasonal development, with reproduction likely ceasing during the austral winter.