Role of heterotrophic dinoflagellates in the fate of diatoms released from fast ice in coastal water of Lützow-Holm Bay, East Antarctica

摘要

ABSTRACT: To understand the fate of ice algal diatoms released from fast ice, we investigated the abundance and sinking loss of diatoms and the grazing impact on diatoms by heterotrophic dinoflagellates (HD) under the ice near Syowa Station, Antarctica, during the austral summer of 2005–2006. After a rapid increase, diatoms showed a clear declining phase. Among the diatom assemblage, Porosira pseudodenticulata and Pseudo-nitzschia cf. turgiduloides were abundant in the water column but low in the sinking flux as they are able to maintain their position in the surface layer after release from the fast ice. Potential grazing impact by HD was calculated to reach 233 mg C m–2 d–1, equivalent to 48.7% d–1 of the diatom biomass being removed daily. Only 14.9 to 71.3 mg C m–2 d–1 (2.5 to 3.2% d–1) was attributable to diatom sinking loss. This suggests that a significant fraction of the diatoms was consumed in the surface layer and the sinking loss was comparatively small. HD often had ingested diatoms in their cells, although ciliates rarely did, and the abundance of HD fecal pellets peaked after the diatom peak. In bottle incubations at in situ temperature, the growth rates of HD ranged from 0 to 0.19 d–1, indicating almost positive growth of HD in the water column. This demonstrates that HD are major consumers of the diatoms released from the fast ice, forming a dominant trophic link between diatoms and HD in the Antarctic under-ice ecosystem.