Seasonal dynamics of Trichodesmium in the northern East China Sea

• Trichodesmium showed a clear seasonal cycle in the study area.
• The horizontal distribution patterns of Trichodesmium were quite different among the four seasons.
• Seawater temperature was the determined factor for seasonality. The horizontal distribution of Trichodesmium may be affected jointly by temperature and salinity.
• The seasonal dynamic in this study area was similar to that in the southern East China Sea (ECS).
• Trichodesmium might be an important source of biological N2 fixation in the shelf area of ECS.

Trichodesmium abundance was measured in the northern East China Sea (ECS) from 2006 to 2007 including four seasons. Our results showed an obvious seasonal variation of Trichodesmium that the abundance was very low in spring (0.96×103 trichomes/m3) and increased quickly to the highest value in summer (35.7×103 trichomes/m3), then declined successively in autumn (16.26×103 trichomes/m3) and winter (3.17×103 trichomes/m3). The horizontal distribution patterns of Trichodesmium were quite different among the four seasons. The seasonal variation may be mainly affected by seawater temperature. The horizontal distribution of Trichodesmium may be affected by multiple environmental factors jointly, while salinity seemed to play a more important role in shaping a horizontal distribution pattern of Trichodesmium than water temperature. Furthermore, neither the N-nutrient nor the P-nutrient was considered as a limiting factor for Trichodesmium. Comparison with adjacent areas showed that seasonal variation in this study area was similar to that in the southern ECS and the South China Sea (SCS) but different from that in Kuroshio. An estimation of N2 fixation rate by Trichodesmium abundance indicated that Trichodesmium might be an important source of biological N2 fixation in the shelf area of ECS.