Study On Size-fractionated Phytoplankton And Impact Of Cascading Dams On It In The Lower Reaches Of The Pearl River

Spatio-temporal patterns in chlorophyll a(Chl-a)from size-fractionated phytoplankton(Micro-(> 20 ?m),Nano-(2-20 ?m),pico-(0.2-2 ?m))were examined in the West River and the North River with cascading dams in the Pearl River Basin in China.The concentration of total Chl-a was a good indicator of spatial patterns,since it increased along a nutrient gradient.A nonmetric multidimensional scaling(NMDS)model clearly showed spatial patterns in the distribution of size-fractionated Chl-a,dividing the study area into three groups of sampling sites.The three groups represented sites at the West River(G1),the central Pearl River Delta(G2)and the Guangzhou City center(G3).Temporal changes in physicochemical factors among the three groups were not obvious.And Linear Discriminant Analysis(LDA)model showed G1 and G2 were mostly explained by physical factors,G3 was closely associated with chemical factors.The potential of using size-fractionated Chl-a data as an indicator of water quality was compared with using two other size-based methods;the morphological traits of the algae Aulacoseira granulata,and using functional groups combined with trait-based(algae volume)classifications of benthic diatoms.Our study indicated that size-based methods of phytoplankton research could make a useful contribution to determining spatio-temporal patterns in the studied area,providing a greater insight into phytoplankton dynamics and enabling better managements of water quality.In the flood period of Beijiang River with cascading dams,the relative percentage of Micro-Chl-a increased from upstream to downstream,and the downstream reaches tended to be stable,accounting for more than 50 % of the total Chl-a concentration.The change of Nano-Chl-a from upstream to downstream was opposite to the change of Micro-Chl-a,the relative ratio of Micro-Chl-a decreased from upstream to downwards.In the period of low-waterflow,the relative percentage of Micro-Chl-a decreased upstream to downstream,and proportion of Micro-Chl-a in the upstream was higher than 50 %,and the relative proportion of Nano-Chl-a in the downstream was higher than that in the upstream.The change of Pico-Chl-a had no obvious regularity.Our results of Redundancy analysis(RDA)showed that total Chl-a and Micro-Chl-a concentration had positive correlation with ammonia nitrogen and water temperature,and had negative correlation with total phosphorus and phosphate.However,there was no obvious correlation between Nano-and Pico-Chl-a with existing environmental factors.Redundancy analysis of the relative percentage of different size-fractionated Chl-a and the environmental factors showed that the Micro-Chl-a had positive correlation with transparency,total nitrogen and ammonia-nitrogen,had negative correlation with the p H,but Nano-Chl-a was opposite.The relative percentage of the Pico-Chl-a did not have a significant association with the existing environmental factors.This study affirmed the indicating function of size-fractionated phytoplankton Chl-a in river ecology research.By comparing Chl-a concentration of the Xijiang River,which have no dams(G1)and the Beijiang River with cascading dams,it is considered that the construction of cascading dams was important factors affecting spatial and temporal distribution of size-fractionated phytoplankton Chl-a and the nutrient status,through increasing residence time of water body and changing the content of nutrient factors in the river,and also the natural life history of phytoplankton.