Antarctic phytoplankton and krill diet, 1955–1963: observations from a poorly known era of Southern Ocean change

Abstract Understanding contemporary changes in Antarctic marine ecosystems requires quantitative observations from earlier periods that predate satellite remote sensing and modern monitoring programmes. Here, we present quantitative phytoplankton community data and Antarctic krill diet analyses collected across the Atlantic, Indian and Pacific sectors of the Southern Ocean (50–68°S) during 1955–1963 - a poorly documented period that itself encompassed substantial change, including the latter phase of large-scale whaling and the mid-twentieth-century reduction in Antarctic sea-ice extent. Analysis of 561 phytoplankton samples revealed 150 taxa dominated by large diatoms, which accounted for ~99.7% of total biomass. Phytoplankton biomass showed pronounced spatial variability, with elevated volumetric and depth-integrated values consistently associated with ice-edge transitional zones characterized by meltwater-induced stratification. Depth-integrated biomass for the upper 0–100 m commonly exceeded 100 g m −2 at ice-edge transitional zones. The most widespread species were Corethron pennatum (Grunow) Ostenfeld (87% occurrence), Rhizosolenia hebetata f. semispina (Hensen) Gran (83%) and Fragilariopsis kerguelensis (O’Meara) Hustedt (68%), reflecting sectoral differences linked to water mass characteristics and nutrient supply. Eighteen phytoplankton community complexes were identified based on biomass dominance: assemblages dominated by R. hebetata f. semispina prevailed in the Atlantic sector, whereas C. pennatum complexes were most common in the Indian and Pacific sectors. Gut content analysis of 425 Euphausia superba specimens demonstrated size-selective feeding, with preferential retention of small pennate diatoms (5–20 μm), whereas large diatoms appeared predominantly as mechanically fragmented frustules, consistent with mandibular processing during ingestion. Comparison with contemporary observations indicates that, although core diatom taxa persist, regional shifts towards smaller cells, altered bloom phenology and declining krill populations point to ongoing ecosystem reorganization. These observations provide a circumpolar reference for evaluating biological responses to rapid Southern Ocean change, including the episodes of change occurring today.