Vertical occurrence of copepod carcasses in the Cosmonaut Sea during austral summer

Zooplankton carcasses and their contribution to carbon export are increasingly garnering attention in food web studies of polar oceans. We investigated the occurrence of carcasses of dominant copepod species at four depths (100–200 m, 200–500 m, 500–1000 m, 1000–1500 m) and their potential contribution to particulate organic carbon (POC) concentrations in the Cosmonaut Sea, Antarctica, during summer, 2019/2020. The abundance of small copepods (< 2 mm), either live or dead individuals, was 1-3 orders of magnitude higher than the four dominant large copepod species Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas at each depth. Most carcasses were observed at the surface (100–200 m), while the proportions of dead individuals increased with depth, from 22% in the 100–200 m layer to 70% of the 500–1500 m layer. The total number of passive sinkers (carcasses and moults) was dominated by carcasses of small copepods in each depth layer, while the carbon biomass of passive sinkers was dominated by carcasses of large copepods. Both abundance and carbon biomass of passive sinkers decreased with depth. The average contribution of passive sinkers to the POC concentration decreased from 0.27% in the 100–200 m depth stratum to 0.10% in the 500–1500 m layer. This study provides the first report of occurrence and contribution of carcasses from the surface to the mesopelagic layer of the Southern Ocean. This information can improve understanding of the role zooplankton play in downward carbon flux within the ocean’s twilight zone.