Changes in sea-ice phagotrophic microprotists (20–200 μm) during the spring algal bloom, Canadian Arctic Archipelago

Heterotrophic microflagellates and ciliates (i.e., 20–200 μm size fraction) were examined for evidence of their response to the spring accumulation of algal biomass in the bottom of the annual sea ice in Resolute Passage (Canadian High Arctic, ∼ 74°N, 95°W). The most abundant heterotrophic microflagellates were dinoflagellates in the water column and cryothe-comonad-type cells in the ice. Ciliates were exclusively represented by typical planktonic species in the water column while the ice community was characterized by the occurrence of benthic-type species. This contrasts with observations in the Antarctic and at the southern limit of sea ice in the northern hemisphere, where annual sea ice seems to serve as a temporary habitat for planktonic communities. Protist biomasses in Resolute Passage were one to two orders of magnitude higher in the ice than in the plankton. In the ice, a seasonal increase in the biomass of phagotrophic microprotists as well as in the number of micrometazoa (from our microprotist samples) followed the spring algal bloom. These observations (1) support previous suggestions of the existence of a functional microbial food web within sea-ice communities and (2) indicate that micrograzers may represent one of the basic levels of the ice food web that responds to the seasonal accumulation of algal biomass. Heterotrophic microprotists growing in the ice accumulated about 4 mg C m −2 d −1, a net production rate that is two to four times higher than those reported for sea-ice bacteria (both Arctic and Antarctic), and represented 1–9% of the net production of ice algea in the early season at resolute. A carbon budget exercise indicated that the required energy for microprotozoan growth in the later season, when algal biomass was declining, corresponded to 1–8% of the net biomass loss from the ice algal populations. The specific growth rates of microprotozoan populations within the ice (0.04–0.18 d −1) appeared to increase significantly with decreasing algal productivity. This may be critical for the protracted heterotrophic food web in multi-year ice and to many consumers during the long polar winter.