Comparisons of primary productivity and phytoplankton size structure in the marginal regions of southern East China Sea

The dynamics of size-fractionated (<3, 3–20 and >20 μm) chlorophyll a and primary productivity measured using the stable isotope tracer technique at three water regimes were compared in the southern East China Sea (the Kuroshio, the upwelling and the continental shelf). The Kuroshio water registered the lowest primary productivity at 0.18–0.61 gC m−2 d−1, the upwelling water the highest (1.01–3.95 gC m−2 d−1), while shelf water had an intermediate value of 0.40–1.10 gC m−2 d−1. Primary productivity was significantly related to the ambient surface nitrate concentration. Kuroshio water was dominated (67–89% of total) by picoplankton (<3 μm) in terms of chlorophyll a concentration and primary productivity (54–83% of total). Phytoplankton in the Kuroshio water were consistently smaller in size than those in the shelf and the upwelling. Although nano- (3–20 μm) and micro-plankton (>20 μm) contributed more to production or biomass in the upwelling and the shelf than in the Kuroshio water, picoplankton was still important and contributed to 29–86% of the chlorophyll a concentration and 19–72% of the primary productivity in the upwelling and the shelf waters. Phytoplankton from the upwelling and the shelf varied greatly in size spectrum. Among the three size fractions, microplankton accounted for most of the variability of chlorophyll a and primary productivity. Microplankton was a more dominate contributor in the upwelling water than it was in the shelf or in the Kuroshio waters, as based on the results of the microplankton/picoplankton ratios both for integrated chlorophyll a concentration and for primary productivity.