Spatial and temporal variation of chlorophyll a, primary productivity and chemical hydrography in the southern East China Sea

Chemical hydrography, chlorophyll a distribution and primary productivity in the southern East China Sea from the inner shelf to the offshore region were examined for their temporal and spatial variability based on observations along a cross-shelf transect during 1991–1995. In summer, all surface waters were depleted in nutrients and low in chlorophyll, resulting in widespread subsurface chlorophyll a maxima. In other seasons, shelf waters were usually nutrient-laden, but phytoplankton biomass was limited by light availability in the turbid inner shelf water and the short exposure time of the upwelled water in the outer shelf. The main nitrogen source in the inner shelf was from river runoff. Nitrate deficit, defined as the negative deviation from the conservative mixing line, was well correlated with phytoplankton biomass. In spite of the seasonal and spatial variability of chlorophyll a distribution, the euphotic zone integrated value of chlorophyll a was correlated well with that of primary production in shelf waters. This relationship was used to convert chlorophyll a data from 28 cruises to primary production. The results showed that the lowest productivity on the shelf was in summer with little spatial variation. The elevated primary productivity in the inner shelves in spring was due to a mild spring bloom. On the other hand, the elevated primary productivity in the middle and outer shelves in autumn was due to the intensification of upwelling during the transition of the monsoon. The annual mean values of the euphotic zone integrated primary production in the shelf waters were quite uniform (513–576 mgC m−2 d−1) with an overall mean of 549±84 mgC m−2 d−1. The offshore Kuroshio Water showed a weak seasonal variation (276–309 mgC m−2 d−1) with an annual mean of 292±15 mgC m−2 d−1.