Temporal changes of phytoplankton and nutrients under the fast ice in the Ongul Strait during the austral summer of 2004/05

Abstract

Time series oceanographic observation was carried out in Ongul Strait during the austral summer of 2004/05 in order to clarify the reason why dense chlorophyll a can exist under fast ice where the light condition is unfavorable for ordinary phytoplankton. Chlorophyll a concentration observed in this study was smaller than that previously reported, and the maximum was 7.6 mg/m3. However, the decrease of nitrate and phosphate concentration in surface water during the observation period was large, and primary production under the fast ice seemed to be active. At the same time, a northward surface current of 4.4 cm/s was observed under the fast ice. Therefore, the water mass in Ongul Strait is believed to have been about 30 km south of the strait 8 days earlier. The fast ice in the Sôya coastal area in southeastern Lützow-Holm Bay melts earlier than that around Ongul Strait. Underwater light intensity increases and a favorable condition for multiplication of phytoplankton exists in early summer. Consequently, it is believed that the high biomass of phytoplankton observed under fast ice in the Ongul Strait increased in open water and under thin ice in the southern area (off the Sôya Coast area) and was transported northward by the current.