Feeding dynamics of Salpa thompsoni and Euphausia superba in the Western Antarctic Peninsula (WAP) region

Salpa thompsoni is the most abundant pelagic tunicate (salp) in the Southern Ocean (SO). Due to global warming, recent years have shown a southward expansion of S. thompsoni, which has increased its abundance in the SO. These southern regions were previously dominated by Antarctic krill (Euphausia superba) but seem to have reduced in abundance during the past decades. A shift from Antarctic krill to salps as the dominant zooplankton may have cascading effects on the SO food web and biogeochemistry. Salps are suggested to be non-selective, efficient grazers ingesting a large size-range of food particles while producing fast-settling faecal pellets. Hence, a dominance of salps may cause strong pelagic-benthic coupling resulting in a simple pelagic food-web, and also affect organisms of higher trophic levels that primarily feed on Antarctic krill. Thus, it is of great importance to understand the competition between salps and krill when they co-occur. Currently it is unclear to what degree their diet differs and how their co-occurrence will impact the carbon cycle in the SO. During an RV Polarstern cruise from March to May 2018 we collected krill and salps along the Western Antarctic Peninsula (WAP). We conducted next generation sequencing of 18S rDNA variable region V4 and measured the fatty acid (FA) composition and chlorophyll concentrations of the gut contents, as well as the FA composition of tissue samples to reveal diet patterns. Sequencing data showed distinct regional patterns in the diet of salps, which were primarily feeding on (dino-) flagellates and diatoms. The main difference between krill and salp diet was shown by the fatty acid markers suggesting that salps had significantly higher ingestion of copepods. FA markers on gut content revealed a higher share of diatoms and dinoflagellates in salps, however, long-term FA storage in the tissue showed a higher proportion of diatom markers in krill.

This suggests that while both species feed intensively on diatoms, krill are more efficient assimilators of diatoms, which may be explained by salps inability to physically break diatom frustules and thereby to access their soft tissue. Yet, salps seem to be more efficient in capturing and ingesting copepods, and this omnivorous feeding behaviour might be of advantage for salps.