Impacts Of Kuroshio Intrusion And Its Variation On Harmful Algal Blooms In The Sea Area Adjacent To The Changjiang River Estuary

The sea area adjacent to the Changjiang River estuary is one of the most notable redtide zone in the coastal water of China.Harmful algal blooms(HABs)occurred in this area are not only affected by the input of terrestrial materials,but also the hydrological conditions and hydrodynamical processes.Previous studies suggested that Nearshore Kuroshio Branch Current(NKBC)will influence the sea area adjacent to the Changjiang River estuary and hydrological,physical and chemical conditions,which will further affect the distribution and dynamics of phytoplankton communities and harmful algal blooms(HABs)in this area.However,the knowledge of Kuroshio intrusion on HABs is still quite limited.In this study,four seasonal cruises were conducted in the East China Sea(ECS)and the Yellow Sea(YS)from 2014 to 2016 to investigate distribution and composition of the phytoplankton in this area,and monthly cruises were organized at the same period in the sea area adjacent to Changjiang River estuary to study the impacts of NKBC variation on the occurrence of HABs.The major findings are as follows.1.The abundance,distribution pattern and seasonal variation of pico-sized and nano-sized phytoplankton assemblages in the ECS and the YS were analyzed.Distributions of major HABs species were revealed,and the significance of Prochlorococcus as the indicator of NKBC was addressed.Four major pico-sized and nano-sized phytoplankton assemblages(diameter less than 20 ?m),which were analyzed by flow cytometry,exhibited different distribution patterns in the ECS and YS.Photosynthetic nanoeukaryotes mainly distributed in the coastal waters.High abundance of picoeukaryotes were found in both coastal waters and subsurface offshore waters,which were composed of different groups of pico-sized microalgae.Cyanobacterium Synechococcus distributed in both coastal waters and offshore subsurface waters,resulting from at least two subgroups(ecotypes)of Synechococcus with different adaptive strategies.The distribution of Prochlorococcus was mainly confined in the ECS,and the abundance was the highest in the main stream of Kuroshio.Phytoplankton communities in the ECS and the YS showed distinct difference based on the results of cluster analysis,particularly in the summer.Cells of Prorocentrum donghaiense,Karenia mikimotoi and other typical causative species of HABs were mainly discovered in the sea area adjacent to Changjiang River estuary,and formed intensive HABs mainly in spring.The distribution pattern of Prochlorococcus in the ECS during investigation in spring well depicted the intrusion route of NKBC.High abundance of Prochlorococcus cells presented in the coastal waters in spring reflected the significance of NKBC in transporting the oceanic microalgae into the coastal waters adjacent to the Changjiang River estuary,one of the most significant red-tide zone in China.2.Seasonal variation of NKBC and pico-sized and nano-sized phytoplankton assemblages were investigated in the sea area adjacent to Changjiang River estuary,and spring was considered as the most important season for NKBC to influence this area.Investigations were conducted every month from February,2015 to January,2016 in the sea area adjacent to Changjiang River estuary.Hydrological parameters indicated that high-salinity seawater at the bottom of the sea area adjacent to the Changjiang River estuary representing NKBC appeared in spring(March),peaked in summer(June),and almost disappeared in late autumn at the end of October.The dynamics of Prochlorococcus and the “dim” type Synechococcus abundance correlated well with the variation of NKBC.Water mass analysis further illustrated that spring was the critical season for NKBC to influence phytoplankton communities in the sea area adjacent to the Changjiang River estuary.The results further suggested that NKBC intrusion will impact the dynamics of HABs in this area.3.Dynamics of HABs in the sea area adjacent to Changjiang River estuary were analyzed,and its relationship with variation of NKBC was discussed.In 2015,the dinoflagellate bloom of Prorocentrum donghaiense occurred in the sea area adjacent to Changjiang River estuary in mid-April,accompanied with the diatom bloom of Thalassiosira spp..The intensity of the P.donghaiense bloom occurred much earlier than previous years,and the intensity of the bloom was also smaller.Abundance of dinoflagellates Karenia mikimotoi and Alexandrium spp.were fairly low and did not develop into large scale HABs.The temperature of seawater during the bloom of P.donghaiense was in a range of 14–18?,which was significantly lower than the seawater temperature in other years during the bloom of P.donghaiense.The early occurrence of P.donghaiense was supposed to be relevant to the inter-annual variation of NKBC.This study revealed the potential impacts of Kuroshio intrusion on the sea area adjacent to Changjiang River estuary,as well as the seasonal variation pattern of its impacts.Cyanobacterium Pro can be considered as an indicator for the intrusion of NKBC.The results also revealed the potential impacts of NKBC on dinoflagellate blooms in the sea area adjacent to Changjiang River estuary and the mechanisms.The results will offer strong evidence for a better understanding on the mechanisms of HABs in the sea area adjacent to Changjiang River estuary and for the development of sound prediction and forecasting strategies.