Variations Of Macrozoobenthic Community In Adjacent Areas Of The Yangtze River Estuary And The Influence Of Mild Oxygen Deficiency

Marine Macrozoobenthos is an important part of the marine ecosystem.Adjacent areas of the Yangtze River Estuary are under combined influence of the complex natural environment and intense human activities,which are ecologically sensitive areas.Studying variations of the macrozoobenthic community in this region has important implications for both protecting marine biodiversity and maintaining the stability of the marine ecosystem.In addition,the seasonal oxygen deficiency often occurs in adjacent areas of the Yangtze River Estuary,but few systematic studies were done on the influence of mild oxygen deficiency on the macrozoobenthic community in these areas.Taking macrozoobenthic community as the main research object,the present paper studied the temporal and spatial variations of the macrozoobenthic community and the influence of environmental variables in adjacent areas of the Yangtze River Estuary.Based on the bottom dissolved oxygen(DO)data in the study area,the influence of mild oxygen deficiency on seasonal variation of the macrozoobenthic community was studied.The benthic ecological environment of the study area was evaluated by macrozoobenthic evaluation indices.The present paper mainly includes the following four parts of work.Based on the research data of macrozoobenthos and bottom seawater environmental variables in four periods including 1959,2000-2001,2011-2012 and2014-2015,the long-term variation of the macrozoobenthic community and its relationship with environmental variables were studied in adjacent areas of the Yangtze River Estuary.A total of 624 macrozoobenthic species were collected in the four periods,and Polychaeta dominated in the main taxonomic groups and tended to increase over time.The results of cluster analysis,PCO(principal coordinates analysis)and PERMANOVA(permutational multivariate analysis of variance)showed that significant differences existed in the macrozoobenthic community among the four periods.The results of CCA(canonical correspondence analysis)and RDA(redundancy analysis)showed that water temperature,salinity and depth of the bottom seawater significantly influenced the long-term variation of the macrozoobenthic community(the first two axes of CCA and RDA separately explained 89.6% and94.3% of the species-environment relationship variance).The result of K-dominance curves(the relative elevation of the curves increased with period),the result of ABC(abundance/biomass comparison)curves(the W value decreased with period,which was 0.311 in 1959,0.305 in 2000-2001,0.222 in 2011-2012,and 0.167 in 2014-2015)and the result of Shannon-Wiener index(H',log 2)(the range of H' was 2.29-5.03 in1959,2.86-4.55 in 2000-2001,2.28-4.56 in 2011-2012,and 1.79-4.43 in 2014-2015,which showed a decreasing trend with period)all showed that the benthic ecological status of the study area tended to deteriorate with time.Based on the research data of macrozoobenthos and bottom seawater environmental variables in ten months from February 2015 to January 2016(except August and December of 2015),the north-south difference of the macrozoobenthic community and its major environmental driving factors were studied in adjacent areas of the Yangtze River Estuary.The results of PCA(principal component analysis)and PERMANOVA showed that significant seasonal and north-south differences existed in the bottom seawater environmental variables in the study area.The results of PCO and cluster analysis revealed the significant north-south difference in the macrozoobenthic community in the study area,and a phenomenon of patchy distribution was indicated for the macrozoobenthos in this area.BIOENV analysis and CCA were used to reveal the relationship between the difference of macrozoobenthic community and the environmental variables,which indicated that depth,salinity and turbidity of the bottom seawater were major environmental variables related to the north-south difference of the macrozoobenthic community,and depth contributed most to the difference.The CDW(Changjiang Diluted Water)was an important driving factor for the north-south differences of the bottom seawater environmental variables and the macrozoobenthic community.The result of ABC curves showed that the macrozoobenthos in the northern part of adjacent areas of the Yangtze River Estuary suffered more disturbance than those in the southern part,which could be because the northern part suffered more disturbance from the Yangtze River.Based on the research data of macrozoobenthos and bottom seawater environmental variables in August 2015 and December 2015,the seasonal difference of the macrozoobenthic community between summer and winter and the influence of mild oxygen deficiency on the difference were studied in adjacent areas of the Yangtze River Estuary.The results of PCA and PERMANOVA showed that significant difference existed in the bottom seawater environmental variables between summer and winter in the study area.The dissolved oxygen in the bottom seawater was in mild deficiency in summer but normoxic in winter.The result of PERMANOVA showed that significant seasonal difference existed in the macrozoobenthic community between summer and winter.The relationship between the bottom seawater environmental variables(especially dissolved oxygen)and the seasonal variation of the macrozoobenthic community was revealed by multiple regression and CCA,and the relationship between macrozoobenthic diversity indices and mild oxygen deficiency was tried to portray with linear regression models.The results showed that the mild seasonal oxygen deficiency in adjacent areas of the Yangtze River Estuary was significantly correlated with the seasonal variation of the macrozoobenthic community,but it would not cause great damage to the community,and in contrast,we speculated that it could reduce the number of natural predators of the macrozoobenthos,reducing the survival pressure of the macrozoobenthos to some extent,in this way causing the seasonal fluctuation of the macrozoobenthic community structure.Dissolved oxygen accounted for only part of the seasonal variation in the macrozoobenthic community,which was also strongly influenced by other environmental variables,particularly depth,phosphate and turbidity.Based on the research data of macrozoobenthos and bottom seawater environmental variables in ten months from February 2015 to January 2016(except August and December of 2015),the benthic ecological status of adjacent areas of the Yangtze River Estuary was assessed.Three biotic indices including AMBI(AZTI Marine Biotic Index),M-AMBI(multivariate-AZTI Marine Biotic Index)and BOPA(benthic opportunistic polychaetes amphipods)were used in the assessment.Combined with the assessment results of the three indices and the results of Pearson correlate analysis between the indices and four bottom seawater environmental variables(chlorophyll a,dissolved inorganic nitrogen,phosphate and silicate)related to eutrophication,M-AMBI was considered to be most suitable for the benthic ecological environment assessment in the study area.The four environmental variables related to eutrophication and the benthic ecological status showed similar spatial distribution patterns related to the distance to the estuary and the coasts,indicating that the study area could be under the environmental pressure of eutrophication.According to the result of M-AMBI,two major disturbed regions existed in the study area: one was located to the east of the Yangtze River Estuary,and the other was located to the east of Zhejiang Province.It was found that the formation of these two disturbed regions could be mainly related to the CDW.The result of M-AMBI showed that no significant seasonal variation occurred in the benthic ecological status of the study area during the year.The main findings and innovation points of this paper includes that: significant variation happened in the macrozoobenthic community in adjacent areas of the Yangtze River Estuary over the past more than half a century,and the biodiversity tended to decrease and the benthic ecological environment tended to deteriorate;significant north-south difference existed in the macrozoobenthic community in the study area,and the CDW was a key factor that caused the difference;the mild seasonal oxygen deficiency was significantly related to the seasonal variation of the macrozoobenthic community in the study area,which would reduce the number of natural predators of the macrozoobenthos,benefitting the survival of some macrozoobenthos to some extent,in this way causing the seasonal fluctuation of the macrozoobenthic community structure;The dynamic changes of the macrozoobenthic community could well reflect the long-term changes of the ecological environment,and M-AMBI was suitable for the benthic ecological environment assessment in the study area.