Estimates of IPP in Tosa Bay and ECS continental shelf waters were about three times higher than estimates for Kuroshio waters with a similar temperature regime (14-30°C). Nutrients may prove to be promising parameters for improvement of algorithms for primary production estimation (Berger et al., 1989; Behrenfeld et al., 2002). Differences in IPP may result from the level of nutrient supply, which may be influenced, for example, by the
Changjiang River, by resuspension on the continental shelf, and by local upwelling in the shelf break under the Kuroshio front (Yanagi et al., 1998). High productivity in Tosa Bay seems to have been induced by a new nutrient supply associated with upwelling into the subsurface layers, due to the influence of the Kuroshio meander, the intrusion of warm water, and coastal upwelling during the summer when the water column is stratified (Ichikawa and Hirota, 2004). On the other hand, because primary production in Oyashio-transition waters was concentrated in the surface layers during spring and summer, without a subsurface maximum, satellite observation at the sea surface is an effective means to estimate primary production integrated over the euphotic zone (IPP) (Kasai et al., 1998). However, iron-limited offshore areas east of Oyashio waters (Tsuda et al., 2003) will require different algorithms to estimate primary production (Behrenfeld et al., 2002).
Asanuma (2006) established a depth- and time-resolved primary productivity model using SST, sea surface chlorophyll a, and daily PAR derived from satellite observations, and recorded good agreement when the model was validated with this GCMAPS dataset. Maps of annual primary production from 1998 to 2002 (Fig. 8), produced according to Asanuma (2006), show high-productivity areas around the mouth of the Changjiang River and an intrusion of Oyashio waters, with low-productivity areas in the Kuroshio meander and in a warm core ring east of mainland Japan. Ning et al. (1995) reported that annual primary production (gC/m2/y) was 104 in the Bohai Sea, 155 in the Yellow Sea, and 252 off the mouth of the Changjiang River. The annual primary production figures that Ning et al. (1995) recorded for these coastal waters were lower than those measured for ECS continental shelf waters during the present study, possibly due to the light limitation by suspended matter in the coastal waters. However, algorithms estimating primary production in Chinese coastal waters as typical Case II should be validated using in situ primary production data, associated with chlorophyll a, suspended matter, and colored dissolved organic matter (C-DOM).
Furthermore, year-to-year variations in annual primary production in highly productive waters (Fig. 8) such as Oyashio waters and ECS continental shelf waters should contribute to global variations in primary production. In the GCMAPS project, we constructed a dataset of up to 176 measurements of primary production around Japan (Table 1) for validation of the algorithm that converts SeaWiFS data to primary production. Through validation and improvement of the algorithm with in situ data, satellite ocean color observation will become a more effective tool to monitor primary production, with fine spatial and temporal resolution on global scales. Satellite-derived estimates of organic production in food webs and carbon fixation in the surface layer may contribute to better understanding of variations in fisheries production and global-warming processes.
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