Surface biomass of net-zooplankton in the Indian Ocean sector of the Southern Ocean during the icebreaker Shirase cruises of JARE-39, -40

Abstract

Wet weight of the net zooplankton biomass (>315μm, collecting NGG54 netting) in the surface water of the Indian Ocean sector of the Southern Ocean was analyzed using continuously pumped-up water from ca. 8m depth during the two cruises of the 39th (4 Dec.-13 Dec. 1997 and 15 Feb.-19 Mar. 1998) and 40th (3 Dec.-20 Dec. 1998 and 24 Feb.-19 Mar. 1999) Japanese Antarctic Research Expeditions (JARE). The biomass varied widely; even samples collected from the same water body (difference of distance among the samples were several-100 miles away) sometimes had differences of 2-3 orders of magnitude. The biomass obtained around ocean frontal regions such as the Sub-tropical Front, Sub-Antarctic Front and Polar Front, an ocean ridge area (Kerguelen plateau) and off Prydz Bay tended to show higher biomass than other areas, often exceeding 500mg/m^3. The mean biomasses among the 4 water bodies separated by the ocean fronts, were 50,68,76 and 137mg/m^3 in the Sub-tropical, Sub-Antarctic, Polar Frontal and Antarctic Zones, respectively. In the Antarctic Zone, the biomass tended to decrease from north toward south, and the west and eastward distribution showed high biomass over a relatively extensive area east of Prydz Bay and slightly high abundance offshore of Amundsen Bay. According to previous JAREs and other studies carried out in the Indian Ocean sector, areas which showed constantly higher zooplankton biomass seem to agree well with areas reported as showing higher phytoplankton production or biomass. In spite of the methodological difference, the present results obtained by continuous sampling during the two cruises were very similar to the previous results of K. Takahashi et al. (Mem. Natl Inst. Polar Res., Spec. Issue, 52,209,1998), which was summarized over 22 years data of NORPAC net sampling, and we were able to confirm the surface biomass and distribution of net zooplankton in the Indian Ocean Sector of the Southern Ocean during austral summer. The method applied in the present study is one good way for long-term monitoring of the surface zooplankton biomass along the JARE cruise track, because the sampling requires non-ship time and gives a good estimation of the biomass.