Phytoplankton Community And Its Related Carbon Sinking In The Changjiang (Yangtze River) Estuary And Adjacent Waters

In recent years,the Changjiang Estuary(CE)and its adjacent waters has been suffering from serious eutrophication and seasonal hypoxia.Studies have shown that hypoxia in CE and its adjacent waters is closely associated with stratification of the water column and the sinking and decomposition of organic matter from surface phytoplankton.There have been various snapshots,annual or seasonal observations of phytoplankton community,however,long-term continuous record of phytoplankton community is in need to study its correlation with dissolved oxygen(DO)in the hypoxic zone of the CE and its adjacent waters.Accordingly,based on the multi-disciplinary investigations in 2013 and 2015,the distribution and seasonal variations of phytoplankton carbon flux and the relationship with formation of the hypoxia were studied.The major findings were listed as following:1.Influenced by the Changjiang Diluted Water(CDW),the Chl a concentration was higher in August,with an average of 5.18 ?g/L and the highest of 32.05 ?g/L.Compared to historical data,the Chl a concentrations showed a fluctuate growth trend in the flood period.The phytoplankton assemblage was influenced by the salinity and turbidity during the survey.In March,80 taxa belonging to 41 genera and 3 phyla were identified.Paralia sulcate was the most dominant specie.While,135 taxa belonging to 67 genera and 4 phyla were identified in August,the main dominant species were Prorocentrum donghaiense and Skeletonema sp..The N/P ratio,instead of Si/N ratio,played a significant role in regulating the phytoplankton community structure.With the increase of N/P ratio,the diatom became the dominant phytoplankton assemblage.2.The Chl a concentration varied among ten months in 2015,with the values ranging from 0.17 to 20.44 ?g/L.The Chl a concentration was relatively higher in April,July and October.In total,265 taxa were identified during ten cruises,belonging to 5 phyla and 94 genera.Diatoms(accounted for 63.78 %)and dinoflagellates(accounted for 33.21 %)were the dominant groups,most of which were eurythermal species.Due to the influence of TWC,some warm-water species were found.3.The SETCOL method,with the technologically simple operation and reliable determined results,has been widely used to measure phytoplankton sinking rates in field.The sinking rates of the phytoplankton community varied clearly among months in the survey area.Phytoplankton sinking rates were relatively higher in September and October.There was no significant correlation between the sinking rates of the phytoplankton community and its ambient environmental parameters,especially the major nutrient components.While,a significant correlation between phytoplankton sinking rate and the phytoplankton community structure was found.The diatom-dominated community had higher sinking rates than the dinoflagellate-dominated community.4.Low DO(< 3 mg/L)and hypoxia(< 2 mg/L)were observed at the near-bottom waters in August of 2013 and July to October of 2015.Compared to the historical records,the hypoxic zone expanded and moved northward,with the minimum DO value decreased significantly.The lower DO zone in near-bottom water was in accordance with the surface water with higher Chl a.The statistical analysis showed that there was significantly negative correlation between phytoplankton carbon flux and DO in the near-bottom water.Thus,the station with higher phytoplankton carbon flux tended to the hypoxia.Therefore,the flourishing of phytoplankton in the surface water may contribute directly to the formation of low-oxygen zone in near-bottom water.In conclusion,based on the study of variation of phytoplankton and its carbon flux,and quantitative contribution of phytoplankton carbon sinking to the formation of hypoxia,the inherent relationship among environmental parameters,phytoplankton sinking rates,and the formation of hypoxia in the CE and its adjacent waters were revealed.The major findings of the dissertation will help further understand on the role of phytoplankton in the formation of hypoxia in the CE and its adjacent waters.