ICE-BRINE AND PLANKTONIC MICROHETEROTROPHS FROM SAROMA-KO LAGOON, HOKKAIDO (JAPAN) - QUANTITATIVE IMPORTANCE AND TROPHODYNAMICS

Biologists have rarely had the opportunity to investigate the communit y characteristics and dynamics of heterotrophic microorganisms in high ly productive first-year sea ice. In this study, sterile seawater was used as a salinity buffer to extract the ice-brine microheterotroph co mmunities (bacteria, flagellates and ciliates) from a coastal lagoon i n Japan (Saroma-ko, Hokkaido; 44 degrees N, 144 degrees E) during the late winter (February-March) of 1992. This procedure reduced osmotic s hock during the melting of ice cores and allowed the recovery of up to 323% more cells than the traditional melting method. Most of the orga nisms were concentrated in the bottom 3-4 cm of the ice, where abundan ces were up to 33 times higher than in the plankton. In ice and plankt on samples, heterotrophic flagellates were dominated by small species (<8 mu m, mainly choanoflagellates) and cryothecomonad-type cells whil e ciliates were dominated by a photosynthetic species, Mesodinium rubr um. In contrast to higher latitudes, increased snow cover appeared to favor the development of protozoa beneath the relatively thin 30-40 cm ice cover of Saroma-ko Lagoon. Temporally, a successional sequence wa s observed between protozoa and the bacterial compartment. Bacteria de creased in abundance throughout the sampling period while protozoa inc reased or attained their maximum number in late winter, toward the end of the sampling period. These observations support previous suggestio ns of the existence of a functional microbial food web within the sea- ice community. Heterotrophic flagellate biomass greatly exceeded bacte rial biomass in the sea ice (30-60 X). Coupled with similar potential growth rates, this suggests the utilization of additional (non-bacteri al) food items by ice-brine flagellates. Finally, the effects of salin ity variations (ranging between 15 and 120 psu) on potential microhete rotroph growth rates are discussed.