Phytoplankton production during a summer coastal upwelling in the East China Sea

Nitrate-based new production and primary production of phytoplankton communities in the continental shelf of the East China Sea (ECS) were studied in the summer (June–July) of 1998. Along the China coast, including the water mixed by the Changjiang (Yangtze) River discharge, upwelling caused diatom blooms. Primary production in the blooming water ranged between 1.47 and 4.50 g C m−2 d−1 and nitrate-based new production ranged between 0.86 and 1.38 g C m−2 d−1. In the adjacent non-blooming mid-shelf and shelf break where water was stratified and surface nitrate concentration was not detectable, primary production ranged between 0.62 and 1.10 g C m−2 d−1 and nitrate-based new production was between 0.06 and 0.46 g C m−2 d−1. Phytoplankton production generally aligns positively with surface phosphate and nitrate concentrations in the whole ECS. The southwest monsoon that prevails in summer acts together with the northward Taiwan Warm Current and China Coastal Current to cause the coastal upwelling. Phosphate from the upwelling partially eased the phosphorus deficiency that has been known to be a special feature of the ECS water mixed by the Changjiang discharge. Surface phosphate concentrations (0.03–0.70 μM) in the upwelling were higher than those of the non-upwelling water (0.02–0.07 μM). The excessive phosphorus from the upwelling stimulates phytoplankton growth and consequently consumes excessive nitrogen from the Changjiang discharge. These observations suggest that in summer phytoplankton production in the riverine coastal water is modulated not only by the nutrients from the upwelling but also by those from the Changjiang discharge; These enhanced productions are likely to fall away in the fall when ceasing of the upwelling stops phosphorus supply.