Hydrological And Chemical Characteristics Of The Taiwan Warm Current In Summer And Its High Incidence Of Red Tide On The East China Sea Biogenic Elements Complement The Preliminary Study

In recent years, Harmful Alga Bloom (HAB) became one of the most serious problems that affected marine environment and fishery industries in East China Sea (ECS). Based on long term investigations, nutrient distributions in ECS and the nutrient import from YRDW to the High Frequency HAB Occurrence Areas of ECS was studied in detail. However, nutrient import from off shore areas (especially from Taiwan Warm Current) still remained unknown. In this text, based on the cruise carried out in summer 2006, the hydrological and chemical properties of TWC was studied, nutrient flux in the TWC original area and upwelling area were calculated, nutrient structure of TWC was analyzed and its hydrological affections to the High Frequency HAB Occurrence Areas of East China Sea was discussed. Conclusions were summarized as follows:(1)In summer, eleven water masses could be divided in the East China Sea. Taiwan Warm Current has two origins: Taiwan Warm Current Surface Water originates from Taiwan Strait Water which is low in phosphate, silicate and nitrate, but high in nitrite. While Taiwan Warm Current Deep Water originates from intruded Kuroshio Subsurface Water(KU), which is low in nitrite, but high in phosphate, silicate and nitrate. Nitrite is a good indicator for distinguishing TWCSW and TWCDW. Taiwan Warm Current has a two-layer structure. In its early period, the two-layer structure is vertical, but when it reaches the middle of Zhejiang coast, the structure becomes horizontal. In this area, suffered from extrusion, TWCSW moves in outer-shelf direction, sinks and diffuses into East China Sea Shelf Water. Meanwhile, TWCDW could still moves northward and goes across the Yangtze River mouth, with its boundary shrinks.(2)In summer, the Taiwan Strait Water originates from South China Sea Surface Water and part of the Kuroshio Water. Current speed is relatively high in Taiwan Strait In the north of Taiwan Strait, Taiwan Strait Water could intrud into Kuroshio region. Kuroshio Water could not intrude into the shelf in the surface, however it could reach the shelf below 50 meters and become a stronger intruder in 100 meter layer. Horizontal nutrient flux is calculated based on the current velocity and nutrient data of three sections: S10 in the north of Taiwan Strait and section a and b between Taiwan, Pengchiahsu island and Diaoyutai Island (Senkaku jima). Results show that in summer, nutrient flux of Kuroshio Subsurface Water is a little higher than Taiwan Strait Current. Due to lower velocity, the nutrient flux density of Kuroshio Subsurface Water is much lower than Taiwan Strait Current, but since KU could constitute TWCDW which could directly reach the High Frequency HAB Occurrence Areas of East China Sea, its contribution is still greater than Taiwan Strait Current.(3) N/P Ratio of both TWCSW and TWCDW are low. Thus the TWC does play an important role in phosphate complementarity. In the TWC led upwelling areas in Zhejiang coast, nutrient vertical flux that calculated from upwelling velocity and nutrient data also confirms in this viewpoint. At the same time, the importance of YRDW and CW are recognized again, without them the area will probably suffer from N limitation.(4) In summer, TWC provide favorable temperature for Harmful Alga. At the same time, the appearance of pycnocline could urge the phytoplankton conglomeration, which could further induce the HAB occurrence. However, the appearance of pycnocline could prevent particles that input from land runoffs from sedimentation. Thus reduces the thickness of euphotic zone, which further limit the growth of phytoplankton.