Based on the field survey data of four cruises in 2011,all phytoplankton communities in the southern Yellow Sea (SYS) were investigated for the species composition,dominant species,abundance and diversity indices.A total of 379 species belonging to9 phyla were identified,of which the most abundant group was Bacillariophyta (60.9%) ,followed by Pyrrophyta (23.7%) and Haptophyta (6.9%) .The seasonal distribution of abundance was:summer (4137.1×103ind m 3) >spring (3940.4×103ind m 3) >winter (3010.6×103ind m 3) >autumn (340.8×103ind m 3) ,while the horizontal distribution showed a decreasing tendency from inshore to offshore regions.The dominant species of phytoplankton varied in different seasons.The dominant species were Thalassiosira pacifica,Skeletoema spp.and Chaetoceros cinctus in spring,Chaetoceros debbilis,Chaetoceros pseudocurvisetus and Chaetoceros curvisetus in summer,Thalassiosira curviseriata,Alexandrium catenella and Ceratium fusus in autumn,Paralia sulcata,Phaeocystis sp.and Bacillaria paradoxa in winter,respectively.In SYS,the group of temperate coastal species was the major ecotype,and the groups of the central SYS species and oceanic species were also important constituents.The average values of Shannon-Weaver diversity index (H’) and Pielou evenness index (J) were 2.37 and 0.65 respectively.The indices H’and J in the open sea were higher than those in coastal waters.Obvious co-variation tendencies between H’and J were observed in all but the summer cruise of this survey.
The eutrophication,hypoxia and coastal acidification are attracting more and more attention.In this study,inorganic carbon parameters,including dissolved inorganic carbon (DIC) ,total alkalinity (TA) and calculated partial pressure of CO2 (pCO2) ,obtained from a summer cruise in August,2009,were used to investigate their integrated response to biological processes accompanying the oxygen depletion in the areas off the Changjiang Estuary.According to the observations,the typical hypoxia occurred in the bottom water just outside the Changjiang Estuary with Dissolved Oxygen (DO) lower than 2.00 mg L 1.The biological uptake in the surface water and the decomposition of organic matter in the bottom water were fully coupled with each other.The high concentration of Chl_a (Chl_a=10.9μg L 1) and DO (9.25 mg L 1) ,profoundly decreased DIC concentration (1828μmol kg 1) and elevated pH (8.42) was observed in the surface water.The correspondingly increased DIC and depletion of oxygen were observed in the bottom water.The semi-quantitative analysis proved that the locally-produced phytoplankton,determined by primary productivity,was deposited to the bottom and contributed about 76%of total amount of the organic carbon decomposition in the bottom.However,in the bottom hypoxia (DO=2.05 mg L 1) area observed in the Southern Zhejiang coastal water,the responding patterns of inorganic carbon parameters deviated from the previous one.The expanding of Changjiang Diluted Water (CDW) ,the adding of Hangzhou Bay water (with high DIC concentration) and Coastal Current together modify the DIC background value in this area,and the local degeneration and upwelling process may also help to offset the local DIC removed by net biological uptake in surface water.In addition,when the mixing occurring in autumn,which may break the summer stratification,the excess release of high DIC in the bottom water to the subsurface water could have an important influence on coastal acidification and the CO2uptake capacity in this area.
The distributions of N utilizing bacteria (denitrifying bacteria and ammonifying bacteria) ,P utilizing bacteria (organic phosphobacteria and inorganic phosphobacteria) and heterotrophic bacteria in the Changjiang Estuary,and the roles of main environmental factors in distributing bacteria,are explored with observations from two cruises in June and August 2006.Comparisons between the two important periods of initial hypoxia phase (June) and developed hypoxia phase (August) show differences in both bacterial distributions and the associated main environmental factors.First,the primary group of ammonifying bacteria has larger magnitude with spatial maximum value in the hypoxic stations related to sediment in August.The phosphobacterial abundance and detection rates in August are much lower than those in June,but the denitrifying bacterial abundance becomes greater in August.However,the difference of heterotrophic bacterial abundance between June and August is not obvious.Second,main environmental factors influencing bacteria vary from initial hypoxia phase to developed hypoxia phase.Two parameters (salinity and NO3) in surface water and five environmental parameters (pH,salinity,PO43,NO3and temperature) in bottom water and surface sediment play major roles in the bacterial abundance in June,while different parameter combinations (salinity and PO43) in surface water and different parameter combinations (DO,DOC,NO3,PO43 and pH) in bottom water and surface sediment play major roles in August.Moreover,the bottom bacteria distributions in area south of 31 N are related to the position of the Taiwan Warm Current in June.The bacterial abundance and distribution may respond to the environmental change in the hypoxia processes of initial phase and developed phase.During the hypoxia processes,the whole structure of bacterial functional groups probably turns to different states,causing the recycling of nutrient regeneration and aggravating hypoxia regionally.
LIU JingjingDU PingZENG JiangningCHEN QuanzhenSHOU LuLIAO YiboJIANG Zhibing
