Seasonal and interannual changes in a coastal Antarctic zooplankton community

Seasonal fluctuations are key features of high-latitude marine ecosystems, where zooplankton exhibit a wide array of adaptations within their life cycles. Repeated, sub-seasonal sampling of Antarctic zooplankton is rare, even along the West Antarctic Peninsula (WAP), where multidecadal changes in sea ice and phytoplankton are well documented. We quantified zooplankton biomass, size structure, and composition at 2 coastal time-series stations in the northern WAP over 3 field seasons (November-March) with different sea-ice, temperature, and phytoplankton conditions. Seasonal peaks in zooplankton biomass followed weeks after phytoplankton blooms. Biomass of mesozooplankton (0.2-2 mm) was consistent and low, while high biomass of macrozooplankton (>2 mm) occasionally resulted in a size distribution dominated by krill and salps, which appears to be a characteristic phenomenon of the Southern Ocean. Zooplankton composition and size changed between years and from spring to summer as the water column warmed after sea-ice breakup. Seasonal succession was apparent typically in decreasing zooplankton size and a shift to species that are less dependent upon phytoplankton. Mean central abundance dates varied by 54 d across 14 taxa, and specific feeding preferences and life-history traits explained the different seasonal abundance patterns. In all 3 yr, the dominant euphausiid species switched from Euphausia superba in spring to Thysanoessa macrura in late summer. Various taxa shifted their phenology between years in response to the timing of sea-ice breakup and the onset of phytoplankton productivity, a level of natural environmental variability to which they appear resilient. Nevertheless, the limits to this resilience in response to climate change remain uncertain.

Automated summary

Seasonal fluctuations in high-latitude marine ecosystems are influenced by the life cycles and adaptations of zooplankton. This study examined the biomass, size structure, and composition of zooplankton in the northern West Antarctic Peninsula over three field seasons. The results showed that zooplankton biomass peaked after phytoplankton blooms and varied in size and composition. The abundance and species of zooplankton changed between years and as the water warmed after sea-ice breakup. The results also highlighted the resilience of zooplankton to natural environmental variability but raised uncertainties about their ability to respond to climate change.