Seasonal and Spatial Changes inTrichodesmiumAssociated With Physicochemical Properties in East China Sea and Southern Yellow Sea

Abstract Trichodesmiumis broadly distributed and occasionally blooms in the East China Sea (ECS) and southern Yellow Sea, where it contributes to local N and C budgets. However, its population structure, spatiotemporal distribution, controlling factors, and N2fixation remain poorly documented. Here we provide high spatial resolution data sets ofTrichodesmiumduring the four seasons of 2011–2012 using water‐ and net‐collected methods. The net‐collected method captures colonial trichomes ofTrichodesmiumeffectively but results in an underestimation of free trichomes. Colonies are rarely observed and occur only on the ECS shelf, which are easily missed in water‐collected samples. Depth‐integrated densities ofTrichodesmiumwere found to be significantly higher in warm seasons than in cold seasons. Maximum densities in the water column were generally found at depths of 10–50 m.Trichodesmiumthrives on the oligotrophic, warm, offshore ECS shelf (controlled by the Kuroshio and Taiwan Warm Current), but restrains in the cold southern Yellow Sea and the eutrophic, inshore ECS. Seasonal and spatial variations inTrichodesmiumare closely correlated with physicochemical properties (mainly temperature and P), which are primarily controlled by circulation alteration and water mass movement. The N2fixation rates ofTrichodesmiumin the ECS in summer and autumn (>20°C) are roughly estimated at 17.1 and 41.7 μmol N m−2 d−1under nonbloom conditions, which potentially contribute to 81% and 57% of biological N2fixation, respectively. Compared with historical data since the 1970s,Trichodesmiumdensities have increased considerably in all seasons, and the distribution boundary has shifted northward under regional warming and hydrological changes.