Dec 7, 2000 - Pacific, copepodite V (C5) stages of three large calanoid copepods, Neocalanus .... M. Orlov and B. Sinclair (1999): The ecology, distribution,.
Journal of Oceanography, Vol. 57, pp. 361 to 375, 2001
Biomass, Abundance, and Vertical Distribution of Micronekton and Large Gelatinous Zooplankton in the Subarctic Pacific and the Bering Sea during the Summer of 1997 J UN NISHIKAWA*, S HUHEI NISHIDA, M ASATOSHI MOKU , KIYOTAKA H IDAKA and KOUICHI K AWAGUCHI Ocean Research Institute, The University of Tokyo, Minamidai, Nakano-ku, Tokyo 164-8639, Japan (Received 9 August 2000; in revised form 7 December 2000; accepted 7 December 2000)
The biomass, abundance, and vertical distribution of micronekton, including cnidarians, mysids, euphausiids, decapods, thaliaceans, and fishes, were studied on the basis of samples collected with an 8-m 2 opening-closing rectangular midwater trawl (RMT-8, mesh size: 4.5 mm) at three stations in the subarctic Pacific (the western subarctic gyre, the central Subarctic, and the Gulf of Alaska) and one station in the oceanic Bering Sea. The total biomass in the 0–1000 m water column ranged from 2.9 to 5.1 gDW m–2. Except for primary consumers that showed highly variable biomass (thaliaceans and euphausiids), biomass was highest in the oceanic Bering Sea followed by the central (boundary between eastern and western gyres), western gyre, and eastern Gulf of Alaska. The biomass compositions by higher taxa were basically similar between regions: fishes were most dominant, followed by cnidarians at all stations, except for the marked predominance of thaliaceans in the Gulf of Alaska. High biomasses of gelatinous animals (31% of overall dry weight), occasionally comparable to those of fishes and crustaceans, suggest their potential importance in the subarctic Pacific. Characteristics in vertical patterns of micronekton biomass common in all stations were: (1) a mesopelagic peak around 500–600 m both day and night, (2) a layer of low biomass in the cold intermediate water and/or in the upper mesopelagic zone, (3) a nighttime shift of biomass to upper layers, and (4) an highly variable biomass of epipelagic/interzonal migrants (euphausiids and thaliaceans).
Keywords: ⋅ Micronekton, ⋅ gelatinous zooplankton, ⋅ biomass, ⋅ abundance, ⋅ vertical distribution, ⋅ subarctic Pacific, ⋅ Bering Sea.
diversity is low, and at basin scale the variation in species diversity is remarkable on the basis of relative dominance rather than the presence-absence of major species (e.g., Johnson and Brinton, 1963; McGowan, 1971; Mackas and Tsuda, 1999). Four large calanoid copepods dominate the biomass in the most of regions, especially in spring and summer (e.g., Miller et al., 1984; Mackas et al., 1993; Goldblatt et al., 1999). Although these copepods do not control phytoplankton production through their grazing (Dagg, 1993), they play an important role in the transport of food energy through their feeding on microzooplankton (Gifford and Dagg, 1991; Gifford, 1993) and sinking particles (Dagg, 1993). The large biomass and production of these copepods are then transported to various higher trophic animals (e.g., micronektonic fishes, small squids, pelagic shrimps, gelatinous animals) as part of the food-web structure in the subarctic Pacific (e.g., Brodeur et al., 1999b). While rela-
1. Introduction The oceanic regions of the subarctic Pacific and the Bering Sea are basically characterized by similar seasonal patterns of phytoplankton dynamics in all basins: spring and summer blooms are absent throughout, and the entire region is a high nutrient (nitrate) and low chlorophyll regime, except off Hokkaido (reviewed by Banse and English, 1999). The phytoplankton community is dominated by very small cells (
