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these, since phytoplankton and zooplankton differ greatly in size, they were
classified into picoplankton, nanoplankton, microplankton, macroplankton
and five sizes of large animal plankton. Chlorophyll a and phaeopigment
were also incorporated into the investigation as parameters for
understanding primary production.
2.3. Survey strategy
2.3.1. Line-transect survey
The upper layer investigation occurred at 11 stations located in a
straight line, as shown in figure 2. The survey area was near 9 degrees
north latitude, 146° west longitude on the southeastern side of the
Intertropical Convergence Zone (ITCZ). The latitude of 9° N in the Central
Pacific Ocean lies at the boundary of the North Equatorial Current and the
Equatorial Counter Current. In this area, mixing occurs at the surface, with
upwelling from lower layers. With this in mind, an effort was made to follow
a north-south survey line centring on Japan’s western mining area, so as to
understand the complicated environment of the study area.
As an example of an observation result, figure 3 shows the spatial
distribution of water temperature.
The surface-water temperature decreased from north to south, and
thermoclines were well developed in the central area. At the central
sampling stations, the mixed layers were shallow and low water
temperatures appeared below them. Ogura 1 reported that surveys carried
out along the 155° W meridian 2 indicated a region bounded by the North
Equatorial and Equatorial Counter currents where water masses of relatively
low temperatures had been observed near the surface. Similarly, Nishibori
et al. 3 carried out fixed line observations along the meridian of 175° east
longitude, and results showed that there were two water masses with
relatively low temperatures in the surface areas near the equator.
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