Latitudinal variations in Salpa thompsoni reproductive fitness

Abstract

AbstractSalpa thompsoni is a major grazer that plays a significant and disproportionate role in the Southern Ocean biogeochemical cycle. It occurs in higher numbers in the warmer mid‐latitude waters of the Southern Ocean. In recent decades, salp populations shifted significantly southward intruding into areas generally dominated by Antarctic krill. It is unclear whether salps can survive in high latitude waters because of their hypothesized negative reproductive response to low temperatures and scarcity of food. To explore how environmental conditions impact S. thompsoni reproductive fitness, we examined a long‐term (1993–2013), cross‐latitudinal dataset of S. thompsoni population demographics in the Atlantic sector of the Southern Ocean. Observations cover over 25° of latitude, from just south of the Subtropical Convergence to the Antarctic continent. S. thompsoni populations were strongly driven by temperature with higher abundances observed at temperatures between 3°C and 5°C. Occasionally, dense and mature populations of S. thompsoni occurred at high latitudes during summer, declining in density by autumn, which was accompanied by significant increases in the appearance of failed embryos. The proportion of failed embryos was significantly correlated with decreasing temperature and Chl a, indicating the importance of these parameters in autumn and high latitude population collapses. The successful development and release of an embryo is a critical factor determining the persistence of S. thompsoni populations. Blastozooid bud release occurred continuously from spring to autumn, and may be a way S. thompsoni populations mitigate against the sensitivity of the blastozooid's reproductive fitness to high‐Antarctic conditions.