Program     Harmful algal blooms: mechanisms, monitoring, and prevention in a rapidly changing world       Poster Phytoplankton Community in Xiamen Harbor and Its Relationship to Environmental Factors P-B1-14-S Xiaojie Chai* , Key Laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of Sciences/University of Chinese Academy of Sciences Qi Zhang, Key Laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of Sciences Zhe Lu, Key Laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of Sciences/University of Chinese Academy of Sciences Lirong Song, Key Laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of Sciences Presenter Email: lrsong@ihb.ac.cn During January to October in 2018, We investigated the monthly variation of phytoplankton composition and abundance in Tongan Bay, Haicang Bridge and Jiulong River Estuary of Xiamen Harbor. This was aiming to find out the correlation between dynamics of phytoplankton and environmental factors, and the saxitoxin-producing algae as well. The results indicated that the average densities of phytoplankton were 2.59×106，6.7×105 and 4.7×105 cells/L in the three stations, and the average concentrations of chlorophyll a were 6.2, 2.6 and 4.0 μg/L, respectively. The diatoms such as Chaetocero spp. and Skeletonema spp. were dominated in all three stations. The average concentrations of TN were 4.0, 3.6 and 3.5 mg/L , TP were 0.11, 0.11 and 0.14 mg/L, and N/P were 43, 33 and 30, respectively. The principal component analysis (PCA) of the environmental factors showed that the temperature, salinity, dissolved oxygen, oxidation-reduction potential, transparency, pH, total phosphorus, total nitrogen and N/P explained 59.4%, 59.4% and 60.7% in the three stations, respectively. Temperature, dissolved oxygen, transparency and oxidation-reduction potential correlated positively with the first component and negatively with salinity, pH, total phosphorus, total nitrogen and N/P in Tongan Bay and Haicang Bridge. Transparency, salinity, oxidation-reduction potential and N/P correlated positively with the first component and negatively with dissolved oxygen, temperature, total nitrogen, total phosphorus and pH in Jiulong River Estuary. Additionally, we performed sxtA-based qPCR to identify saxitoxin-producing harmful algae, which structured the standard working curve of qPCR detection with mixed DNA from Alexandrium tamarense, A. catenella and A. minitum. The saxitoxin-producing algae by qPCR detection were at concentrations of 4.9×103, 1.0×103 and 1.2×103 cells/L by average in Tongan Bay , Haicang Bridge and Jiulong River Estuary. The quantifications of potential saxitoxin producers by microscope were at concentrations of 9.5×103, 4.8×103 and 6.9×103 cells/L by average in the three stations, which dominated by Gymnodinium spp., Alexandrium spp. and their cysts. The quantifications of potential saxitoxin producers by microscope were comparable with qPCR detection in the three stations (R2=0.84). Our result suggested that there was a high risk of paralytic shellfish poisoning in Xiamen Harbor.

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