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 salp’s functional role in pelagic ecosystems is recognised, their life cycle during winter and spring remains poorly understood. To uncover the effects of environmental drivers on S. thompsoni population dynamics, we collected physical oceanography and salp population development data in the Atlantic sector of the Southern Ocean during the spring of 2022. Salp abundances were generally < 5 ind. m ^− 2 in the 0–150 m and 0–600 m layers. The S. thompsoni population was at the most advanced stages of development, including a diverse stage composition and a bimodal size distribution, in the sub-Antarctic Zone north of the Polar Front (PF). This indicated ongoing salp reproduction that started at least one month prior to our voyage. Stations located in the Antarctic Zone south of the PF were characterised by low salp densities (generally < 2 ind. m ^− 2 ) and early developmental stages. The distribution of biologically diverse salp populations and their development were strongly influenced by the regional physical oceanography. The region’s strong currents and dynamic frontal systems, including eddies emerged as a major driver of salp population dynamics. The findings from this study illustrate that salps north of the PF likely spawn year-round, while their populations in regions influenced by Antarctic waters show strong seasonal development and their reproduction may seize during the austral winter.