Seasonal variation of chlorophyll a concentration, primary production and environmental conditions in the subtropical East China Sea

Chlorophyll a concentration, primary production, and environmental conditions over the entire shelf of the subtropical East China Sea (ECS) were studied extensively during four seasonal cruises between December 1997 and October 1998. Nutrient concentrations in the northwestern half of the shelf were enriched all year-round, but primary production showed high seasonal variations. Intensive primary production was mostly observed in summer at about 939 mg C m−2 d−1. On average, the value in summer was about 3 times higher than that in other seasons. Annual primary production was 155 g C m−2 y−1. In the southeastern half of the shelf, on the other hand, nutrient concentrations were seasonally variable, but primary production showed only slight seasonal variations with a mean value of 395 mg C m−2 d−1. Annual primary production was 144 g C m−2 y−1. The annual variations in shelf-averaged primary production can be well described with a normal distribution curve. For the entire shelf of the ECS, annual primary production was 145 g C m−2 y−1. The rate of primary production was regulated by seawater temperature from winter to early spring. The rate of primary production was, in turn, regulated by the availability of nutrients, especially phosphate, from summer to autumn. In addition, turbidity might also play a role in the regulation of primary production in the waters of the inner shelf.