527 Journal of Oceanography, Vol. 56, pp. 527 to 538, 2000 Keywords: ⋅ Japan Sea, ⋅ pigment concentra- tion, ⋅ critical depth, ⋅ mixed layer depth, ⋅ seasonal variation. * Corresponding author. E-mail: swkim@risgw.ris.ac.jp * Present address: Faculty of Geo-Environmental Science, Rissho Uni- versity, 1700 Magechi, Kumagaya-City, Saitama 360-0194, Japan. † Present address: Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo, Nagasaki 852-8521, Japan. Copyright © The Oceanographic Society of Japan. Temporal and Spatial Variability of Phytoplankton Pigment Concentrations in the Japan Sea Derived from CZCS Images SANG-WOO KIM 1 *, SEI -ICHI SAITOH 1 , JOJI ISHIZAKA 2† , YUTAKA ISODA 1 and MOTOAKI KISHINO 3 1 Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1, Minato-cho, Hakodate 041-8611, Japan 2 National Institute for Resources and Environment, Tsukuba 305-8569, Japan 3 The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan (Received 13 April 1998; in revised form 3 April 2000; accepted 3 April 2000) Temporal and spatial variability of phytoplankton pigment concentrations in the Japan Sea are described, using monthly mean composite images of the Coastal Zone Color Scanner (CZCS). In order to describe the seasonal changes of pigment concen- tration from the results of the empirical orthogonal function (EOF) analysis, we se- lected four areas in the south Japan Sea. The pigment concentrations in these areas show remarkable seasonal variations. Two annual blooms appear in spring and fall. The spring bloom starts in the Japan Sea in February and March, when critical depth (CRD) becomes equal to mixed layer depth (MLD). The spring bloom in the southern areas (April) occurs one month in advance of that in the northern areas (May). This indicates that the pigment concentrations in the southern areas may increase rapidly in comparison with the northern areas since the water temperature increases faster in spring in the southern than in the northern areas. The fall bloom appears first in the southwest region, then in the southeast and northeast regions, finally appearing in the northwest region. Fall bloom appears in November and December when MLD becomes equal to CRD. The fall bloom can be explained by deepening of MLD in the Japan Sea. The pigment concentrations in winter are higher than those in summer. The low pigment concentrations dominate in summer. equal to the destruction of phytoplankton was the critical depth (hereinafter referred to as the CRD). Only when the depth of mixing is shallower than the CRD do posi- tive productions occur in the water column and the populations sustain net growth. Thus, the vertical extent of the mixed layer depth (MLD) in relation to the sur- rounding light field is of crucial importance. Satellite ocean color remote sensing has improved our capability to define the variability of pigment con- centrations over wide areas. The Nimbus-7 Coastal Zone Color Scanner (CZCS), which operated from November 1978 through June 1986, provided synoptic sea surface ocean color distribution at a spatial scale of 1 km and measurements of the spectral radiance back-scattered out of the ocean successfully provided estimates of the near- surface pigment concentrations (e.g., Gordon et al ., 1983). Yoder et al. (1993) and Banse and English (1994) have already analyzed the seasonality of pigment concentra- tions in the global ocean using CZCS imagery. Obata et 1. Introduction Phytoplankton dynamics in the temperate zone are generally dominated by the spring and fall bloom (Parsons et al., 1984); a period of rapid population growth that often begins after the water column becomes ther- mally stratified (spring bloom) or when the stratification is destroyed (fall bloom) in the surface layer. Sverdrup (1953) proposed a useful concept for understanding some aspects of the initiation of a spring bloom. He stated that the depth to which phytoplankton could be mixed and at which the total photosynthesis for the water column was