Seasonal and interannual changes in a coastal Antarctic zooplankton community

Abstract

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.

Long‐term patterns in ecosystem phenology near Palmer Station, Antarctica, from the perspective of the Adélie penguin

“…Recruitment and seasonal migrations (vertical and horizontal) are fundamental processes underlying zooplankton changes throughout the annual cycle in the Southern Ocean (Dietrich et al, 2021; Ward et al, 2012). As a result, zooplankton taxonomic and size composition change from spring to late summer as SST increases after sea ice retreat near Palmer Station (Figure 3; Conroy et al, n.d., unpublished manuscript). The seasonal ascent of Antarctic zooplankton is coupled to sea ice retreat and the onset of increased phytoplankton productivity (Atkinson & Shreeve, 1995; La et al, 2019).…”

Omnivorous summer feeding by juvenile Antarctic krill in coastal waters

“…B and E across the study years. Mesozooplankton (0.2–2 mm) biomass, an indicator of metazoan prey availability, was measured twice weekly using a 1‐m diameter, 200‐ μ m mesh ring net from 0 to 50 m (Conroy et al 2023). Mesozooplankton dry weight density (g m −3 ) was depth‐integrated to 50 m (g m −2 ).…”

“…mg DW −1 d −1 ) were log‐transformed for linear regression to test for relationships with standard length and log‐transformed Chl a concentration. Chl a concentration was measured via fluorometric analysis of bottle samples and integrated 0–50 m (Schofield et al 2017; Conroy et al 2023).…”

“…We collected krill southwest of Anvers Island, near Palmer Station (64 46 0 S, 64 03 0 W), as part of the Palmer Antarctica Long-Term Ecological Research (PAL LTER) program. Our sampling spanned November to March over three field seasons: 2017-2018, 2018-2019(Conroy et al 2023. We collected krill aboard a rigid-hulled inflatable boat using two net types: a 1-m square frame, 700-μm mesh Metro net and a 1-m diameter, 200-μm mesh ring net.…”

“…E ($ 5 km from Palmer Station, bottom depth $ 160 m) twice per week (Supplementary Fig. S1) (Conroy et al 2023) and less consistently at other fixed sampling locations or by targeting krill swarms identified by echosounder.…”