The use of swimmers from sediment traps to measure summer community structure of Southern Ocean pteropods

In the Southern Ocean, pteropods play an important role in biogeochemical cycling, and sediment traps are a valuable tool for investigating this role through the collection of passively sinking matter from productive surface waters to deep sea layers. Observations of ‘swimmers’ (e.g. organisms that actively swim into traps) can also prove valuable for studying zooplankton community structure. In this study, we used two separate sediment trap studies during the 2016–2017 summer to study pteropod population structure over time scales of 24 h and 28 days. In both studies, highest densities were measured for veliger-stage Limacina helicina antarctica (0.09–0.3 mm) relative to all species and age classes. Increases in shell diameters of veligers in all traps over time enabled the calculation of an intraseasonal potential growth rate of 0.0068 mm d⁻¹. Swimmer flux rates ranged from 121 to 2674 ind. m⁻² d⁻¹ at 53 m depth, and the 24-h vertical flux study measured 960 ind. m⁻² d⁻¹ at 57 m depth and 6692 m⁻² d⁻¹ at 90 m depth. Among a suite of environmental and biological covariates tested, fluorescence and sinking particulate organic and inorganic carbon (POC and PIC) possessed the most predictive power to explain abundances of near-surface pteropod age class and species composition. Gymnosome abundances were largely influenced by increasing adult L. helicina antarctica counts. Changes to pteropod population and community dynamics in response to climate change will have cascading effects throughout Antarctic epipelagic food webs, and these results provide a regional snapshot of patterns in structure and sedimentation from an under-surveyed region of the Southern Ocean.