Seasonal variation and cross-shelf distribution of the nitrogen-fixing cyanobacterium, Trichodesmium, in southern East China Sea

The abundance of a nitrogen-fixing cyanobacterium, Trichodesmium, in the southern East China Sea was surveyed along a cross-shelf transect from 1993 to 1996. Trichodesmium existed mostly in surface water between 0 and 50 m depth, with population densities ranging from 0 to 600 trichomes l−1. A clear seasonal cycle was observed in Trichodesmium standing crop which reached more than 107 trichomes m−2 in summer and became undetectable in winter. The cross-shelf distribution showed a spring-type pattern with higher abundance in the Kuroshio current and low abundance in the coastal zone. In fall months, a reversed distribution pattern appeared with high abundance in the coastal zone and decreasing concentrations toward the shelf break. These distribution patterns were difficult to be explained by temperature alone, and a coastal substance may be important in determining Trichodesmium distribution in shelf seas. The calculated nitrogen fixation rate of Trichodesmium ranged from 0.1 to 59 μmol N m−2 d−1 in the upwelling zone and Kuroshio under non-bloom conditions. This result suggested that Trichodesmium is one of the major contributors to new production in Kuroshio waters.