Biogeographic Pattern Of Phytoplankton And Its Effectiveness As An Indicator Of Water Quality In The Yellow River Mainstream

As the second largest river in China and the mother river of the Chinese nation,the Yellow River traverses the Qinghai-Tibet Plateau,the Loess Plateau,the Inner Mongolia Plateau,and the North China Plain,which are the most ecologically fragile in China.Due to the peculiarity of high sediment concentration,serious siltation,and variable wandering,the ecological environment of this watershed has strong sensitivity and fragility.At the same time,due to the continuous intensification of human activities,the natural conditions of the Yellow River are complicated and changeable,the utilization rate of water resources in the basin is high,the habitat fragmentation is aggravated,and the biodiversity is seriously damaged.Under the requirement of multi-objective synchronous development in the current river basin,it is very important to select phytoplankton as typical aquatic organisms that can intuitively and comprehensively reflect the state of the river,to clarify the biogeographic pattern of phytoplankton in the main stream of the Yellow River under the action of human activities and the indicator role of water environment.Therefore,in the spring and autumn of 2019,this study systematically investigated the phytoplankton community in the whole reach of the Yellow River trunk stream from the source area to the estuary,and studied the spatial and temporal distribution characteristics of phytoplankton community,such as species composition,density and biomass,dominant species,and diversity index.In combination with human disturbance,geographical climate and other factors,the driving mechanism of phytoplankton biogeographic pattern in the Yellow River was clarified.The research results provide data support for the construction of a comprehensive and systematic water ecological background value of the Yellow River,which is of great significance for the rational development and utilization of water resources and the protection and restoration of water ecological environment in the Yellow River basin.The main contents and results of this study are as follows:（1）A total of 350 phytoplankton species,which belong to 8 phyla and 130 genera,were identified.The number of species in spring（229 species）and autumn（307 species）accounted for 65.43%and 87.71%of the total species,respectively.Bacillariophyta and Chlorophyta were dominated.The average phytoplankton density was 162.39× 10⁴ cells L^-1 for spring and was 141.12× 10⁴ cells L^-1 for autumn.The average biomass was 2.53 mg L^-1 and 3.04 mg L^-1 in spring and autumn,respectively.The phytoplankton density and biomass of the section influenced by dams were greater than the adjacent natural section.The phytoplankton density and biomass decreased from upstream to downstream in the reservoirs regulated every year or for several years.The opposite phenomenon was investigated in the reservoirs not fully regulated.The values from upstream to downstream were not different significantly in the reservoirs regulated daily.（2）Based on the cluster analysis of phytoplankton community composition,the study area was divided into three regions,which are completely consistent with the changing trend of the river slope rate.Although the mainstream of the Yellow River is dominated by Bacillariophyta,the proportion of Chlorophyta（Chlamydomonas）in region II and Cyanophyta（Phormidium）)in region III has also increased substantially.The results of the variation partitioning showed that the contribution of pure spatial dispersal to the phytoplankton community structure was more than four times that of the combined contribution of pure water quality and climate component.Compared with the natural river,many Dinophyta,Chlorophyta,and Cyanophyta were growth in the reservoir.By comparing the biomass and composition of phytoplankton in different regions,this study revealed that phytoplankton is largely affected by the availability of light in the water,resulting in limited productivity,especially in arenose rivers like the Yellow River.（3）We explored the driving mechanisms of a diversity,β diversity and its components（turnover and nestedness）in different dimensions（species,functional groups,and phylogenetic diversity）from a macro perspective.The results of multivariate analysis showed that the α and βdiversity of different dimensions were mostly affected by geographical space factors,and the βdiversity of each dimension and its components were more responsive to land-use factors than a diversity.For example,the diversity of functional groups α and β were mainly affected by deterministic environmental filtering,which was consistent with the hypothesis of habitat template theory.In addition,both species and phylogenetic β diversity were dominated by the corresponding turnover components（the average contribution was about 70%）,while the proportion of turnover and nestedness components in functional group β diversity is not different.（4）Biological evaluation of water quality indicated that water quality in autumn was better than that in spring,and the pollution level from the source region to the estuary increased gradually.This result is basically consistent with the result of WQI water quality assessment.However,further analysis showed no significant correlation between biodiversity index and WQI,and the actual relationship between them did not fit well with the standard curves of water quality classification based on the respective biodiversity indices and WQI.Correlation analysis,multiple stepwise regression,distance-based redundancy analysis,and regression modeling were used to explore the biogeographical patterns and drivers of phytoplankton diversity.The results of regression analysis showed that water quality（such as sediment content and nutrient concentration）had significant effects on phytoplankton species diversity in the Yellow River channel,while the water surface slope,average annual precipitation and average annual temperature also contributed significantly to the variation of the diversity index.Therefore,in sediment-laden rivers with a large geographical span and complex environment,phytoplankton diversity cannot be used as a suitable water quality indicator,albeit it can reflect habitat changes to a certain extent.（5）The phytoplankton from the two surveys were divided into 31 functional groups.Groups C,MP,and D,which are well adapted to strong water disturbances and turbid habitats,showed distinct advantages over other groups.Despite no significant differences in many environmental variables between the river and reservoir sections,these variables（especially nitrogen nutrients）had remarkable effects on the phytoplankton community structure.The phytoplankton functional groups were sensitive to environmental changes even under sediment interference,although geoclimatic variables also exhibited non-trivial effects.The mean niche breadth of the abundant taxa（river:11.16;reservoir:7.93）was higher than that of the rare taxa（river:5.64;reservoir:4.86）in different water bodies.Thus,growth and diffusion of the abundant taxa played paramount roles in maintaining ecosystem stability.The results indicate that,in a large-scale sediment-laden river,phytoplankton functional groups can effectively indicate changes in the aquatic environment of either a free-flowing river or a man-made reservoir.