The Impact of Salps on the Antarctic Krill Population - An Individual-based Modelling Study

Abstract

Background Krill and salps are key macro zooplankton grazers in the Southern Ocean ecosystem but due to differing habitat requirements, there used to be only little spatial overlap of both species. With ongoing climate change-induced seawater temperature increase and regional loss of sea ice, salps are now able to extend their spatial distribution into areas that are historically krill dominated and capable to increase rapidly due to asexual reproduction when environmental conditions are favorable. It is crucial to understand potential effects on krill since krill is a species of exceptional trophic significance of the Southern Ocean food web and negative impacts on krill could trigger cascading effects on its predators and prey. To address this question, we combined two individual-based models on salp and krill, which describe the whole life cycle of salp individuals as well as the dynamic energy budget of individual krill. The resulting new model PEKRIS (PErformance of KRIll vs. Salps) is used to simulate a krill population for 100 years under varying chlorophyll a concentration in the presence or absence of salps. Results The investigated krill population properties (dynamics of krill abundance, mean length and yearly number of released eggs) were impacted but not significant by the presence of salps. On the other hand, salp abundance was significantly reduced if krill was present. The medians of krill and salp population properties deviated when the other species was introduced by <1% (150 individuals) for krill abundance, -25% (-417,934 eggs) for krill eggs, 2% (0.49 mm) for mean length of krill and -38% (-427 individuals) for maximum seasonal salp abundance. Conclusions While no significant impact of salp presence on the krill population was detectable, a considerable reduction for eggs released by krill prevailed. In the future, the model could serve as a tool to investigate the possible effect of a climate change related increase in water temperature and the associated physiological effects on both species and, consequently, their population trends.