Distribution and downward flux of Trichodesmium in the South China Sea as influenced by the transport from the Kuroshio Curren

The South China Sea (SCS) is an oligotrophic marginal sea in the tropical-subtropical western North Pacific. Its warm, stratified and nitrate depleted water is an ideal habitat for the nitrogen-(N) fixing cyanobacteria Trichodesmium. Its abundant presence was not revealed, however, by our results, compiled from 7 cruises conducted between 2000 and 2002 in the northern SCS covering 2 springs (March), 3 summers (June to July) and 2 falls (October). Averaged Trichodesmium density in the surface water was 77 10 3 trichomes m -3 with great spatial and temporal variations. No definite spatial distribution pattern was recognizable. Its depth-(0 to 60 m) integrated abundance was much lower in the springs (57 to 1360 10 3 trichomes m -2 ) than either in the summers (480 to 19 229 10 3 trichomes m -2 ) or falls (667 to 21 308 10 3 trichomes m -2 ). This might be attributed to the relatively shallower nitracline and lower water temperature in the spring than the warm seasons. The year-round abundance was positively related to (p < 0.05) both nitracline depth and surface water temperature. In the warm seasons when the surface temperature ranged between 27.1 and 31.3C, only the nitracline depth was significantly related to the Trichodesmium abundance. Nitracline depth that was greater in the Kuroshio water compared to the SCS water also plays a decisive role in controlling the Trichodesmium abundance in the 2 regions, being greater in the Kuroshio than in the SCS. A sediment trap system deployed near the Luzon Strait in the SCS collected Trichodesmium samples as deep as 374 m. It showed a maximum downward flux of Trichodesmium at 24.69 10 3 trichomes m -2 d -1 in September and decreased to <1 10 3 trichomes m -2 d -1 from December to March. The sinking rate, when expressed as percentage of the integrated abundance, was 0.1 to 0.3 and 0.05% for the warm and the cold seasons, respectively. Despite its surface dwelling nature, Trichodesmium showed a downward flux when it was transported from the Kuroshio to the SCS where it became dissipated.