Nitrate-based new production and its relationship to primary production and chemical hydrography in spring and fall in the East China Sea

The purposes of the present research were to identify the factors modulating the dynamics of nitrate-based new production (NP) and primary production (PP) in the East China Sea (ECS) during spring and fall, the transitional periods between winter and summer when phytoplankton production in the ECS is limited by light and nutrient, respectively. Two cruises were carried out in March (spring) and October (fall) of 1998, respectively. In the fall when solar irradiance was relatively high, phytoplankton productions in terms of NP and PP were related to light attenuation and the degree of stratification of water column, indicating that light availability for phytoplankton growth was decided by water property. But in the spring, when solar irradiance was very low and water column was transparent, production was related to solar irradiance. In addition to light, nitrate abundance modulated spring productivity, but not in the fall when upward diffusion of nutrients supported phytoplankton growth but the ambient nitrate concentrations were mostly not detectable. Thus, ambient nitrate concentration was not an effective indicator of nutrient availability in this transitional period. l and seasonal variations in production in the ECS were pronounced. While water-column-integrated PP (IPP) ranged between 0.14 and 1.64 g C m−2 d−1 in the fall and between 0.08 and 1.20 g C m−2 d−1 in the spring, integrated NP (INP) was 0.05–0.59 g C m−2 d−1 in the fall and 0.02 -0.42 g C m−2 d−1 in the spring. In the fall, both INP and IPP were higher in the mid-shelf than in the China coastal waters. In the spring, however, both productions were higher in the western half of the ECS shelf than in its eastern half. In the ECS, the coastal water adjacent to the Changjiang mouth showed the most significant seasonal variations in both phytoplankton productivity and f-ratio. INP and IPP in other China coastal waters were higher in spring and summer than in fall and winter, and opposite was true in the mid-shelf ECS waters. Year-round INP and IPP were positively regressed to integrated chlorophyll concentration. Year-round f-ratio was 0.32, indicating two-thirds of the primary production in the ECS is attributed to nitrogen sources other than nitrate.