| The frequent algal blooms in the backwater areas of tributaries of the Yangtze River in the Three Gorges Reservoir (TGR) after its initiate impoundment have been regarded as one of the most serious environmental issues. Compared to lake system, the ecological incentives of diverse algal blooms in this extremely dynamic river-reservoir hybrid system due to the operation of the TGR are still unknown. To reveal the ecological process of algal blooms, a 2-year field monitoring program from May, 2007 to April, 2009 was put forward in the backwater area of the Xiaojiang watershed (XBA), located in the mid-part of TGR. The phytoplankton C-R-S growth strategies and functional classification was applied in the study, and the potential impact of habitat transition on the succession of phytoplankton assemblages was discussed in the following 3 aspects:(1). The seasonal transition of environmental variables1D hydrodynamic model of the XBA was modeled based on HEC-RAS, and daily hydrologic retention time (HRT) was calculated. HRT was dually impacted by both the operation of the TGR and the discharge of the Xiaojiang River, with a range from 157.0d to 3.8d. It showed a significant variation among seasons. According to the classification of reservoir type, 15.6% of the HRT data in the XBA indicated a through-flowing type (less than 20d), which was mainly in the flood season from June to August. 84.4% showed the intermediate type (20~300d) in the XBA. Backward flow from the Yangtze to the XBA might occur in during the impounding period from Sep. to Nov. The maximum backward flow occred in the 1st~3rd Nov, 2008, which has an impact length of about 2km.Mean values of Nitrogen (TN), Phosphorus (TP) and Dissolved Silicon (DSi) were 1623±30μg·L-1,75.9±2.5μg·L-1 and 7982±130μg·L-1 separately, indicated a mesotrophic status in the XBA. Nitrate was the major forms of nitrogen while that of phosphorus was particulate phosphorus. Seasonal variation of different forms of nitrogen and phosphorus was significantly impact by the operation of the TGR and the hydrology process in the Xiaojiang Watershed. Spearman correlation analysis showed that the decrease of HRT and the increase of river flow would increase the relative abundance of ammonia (NH4+/NO3- ratio). Nitrate might probably dominate in the nitrogen in the high-water-level operation stage. Rainfall and intensive river flow transported the phosphorus that adsorbed on the particulate inorganic sediments in the forms of PP into the XBA during the low-water-level flood season. While in the high-water-level drought season, sedimentation of PP and release of SRP was more evident due to the prolonged HRT in the XBA. The input of DSi might be the same with SRP. Generally, TN/TP evidently decreased during the study. The P-limiting condition was weak and N-limiting was detected frequently in the XBA, although it showed a general limitation of phosphorus.Solar radiation in the Xiaojiang Watershed was 4007.98MJ·m-2 and 3542.76MJ·m-2 in 2007 and 2008 sparately. The ratio of photosynthetically active radiation (PAR) in solar radiation was 0.522. Mean values of the light attenuation coefficient, euphotic depth, Secchi Transprancy and water temperature in the upper layer was 0.916±0.037m-1, 6.64±0.22m, 170±7cm and 20.52±0.39℃separately with considerable seasonal varaiton. Factor analysis and multivariate regression was applied to reveal the impact factors on the varation of euphotic depth. It was found that particulate inorganic matters influenced the underwater light transfer significantly in spring, summer and autumn. Phytoplankton biomasses impact the underwater light transfer more evidently in winter.(2). Traits of the seasonal succession of phytoplankton assemblagesThe mean values of phytoplankton biomass and cell dentisy were (59.285±7.853)×105cell·L-1,4589±652μg·L-1 sparately. Algal blooms was frequently detect in the following period: February to March, April to May, the end of August to September. The abundance of phytoplankton was relatively low in December. 320 species, 117 genera and 8 phyla of phytoplankton was identified in the XBA. Generally, Cyanophta and Dinophyta dominated in the XBA in spring and summer generally, while in autumn and winter, Bacillariophyta and Cryptophyta was in the domination.26 functional groups of phytoplankton were identified in the XBA. J/F/H1/P/LO/LM/B/Y/G/C was the representative groups. The seasonal succession of functional groups was different in the 2-Year research. Nevertheless, succession of C-R-S growth strategies was the same generally: S-CS type dominated in the spring, while in the summer flood season a co-occurrence of S-CS type and R-CR type was evident. From autumn, the succession from S-CS type to R-CR type which dominated in winter was remarkable. In the February to March, the shift from R-CR type in winter to S-CS type in the early spring was completed in a relatilve short time span. Functional diversity represented the diversity of habitat. It was anticipated that habitat diversity and transition might be the driven forces of the structure and succession of phytoplankton assemblages in the XBA. In a year's time scale, steady state of the phytoplankton assemblage was frequently accessed.(3). Response phytoplankton assemblages succession on habitat transitionThe significant negative correlation between inorganic nutrients, i.e. nitrate and SRP, and phytoplankton abudance indicated that the uptake of inorganic nutrients by the growth of phytoplankton was evident. Solar radiation and water temperature showed the significant impact on the phytoplankton abundance, while there showed no significant impact of total nitrogen, total phosphorus as well as their relative abundance on the phytoplankton abundance generally. An index of feasible energy representing the possiblitiy of phytoplankton entrained into the euphotic zone sustaining photosynthesis ( Ef * and E f*-SD) was put forward. The impact of phytoplankton biomass on the Ef * and E f*-SD index showed a quadric non-linear response. Its correlation with phytoplankton biomass was better than that of water temperature.Canonical Correspondence Analysis (CCA) was applied to study the response of phytoplankton structure on environmental parameters. Phytoplankton functional classification was verified, indicating a better and more powful explanatory ablity compared to the traditional biological classification. It was found that water level of the XBA, HRT, solar radiation and water temperature was the driven forces of succession of phytoplankton assemblages. In the low-water-level stage, the phytoplankton structure was impacted by total suspended matters and ammonia, while in the high-water-level stage, radiation, water temperature and river flow of XBA might be the driven forces on the succession of phytoplankton assemblages. No significant impact of nitrogen and phosphors as well as their relative abundance on the phytoplankton structure was found.Clustering analysis was applied to summarize the succession traits of phytoplankton and transition of their habitat in the whole year succession. 8 representative time intervals was divided, i.e. late April to may (or early June), June (or mid June) to late July (or early August), August to September, Early October, late October to early December, late December to early Feburary, late Feburary to early March, and finally late March to early April. Based on the functional traits of phytoplankton assemblages, a general succession patterns of phytoplankton assemblages with respect to habitat transition was put forward. It was dicussed that a deep lake type with a calm period condition was easily found in the XBA during the winter time, and during the flood season with low-water-level stage might be found close to the turbid rivers. The operation of the TGR differentiates the gap between both types and showed the different characteristic of phytoplankton.The original innovative aspects of this study were as follows:(1) Phytoplankton functional classification was firstly applied in the research of algal blooms in tributaries of the Three Gorges Reservoir, to investigate the phytoplankton community structure and succession patterns. The application fields of phytoplankton functional classification were expanded to such unstable variable environment. The cause-and-effective relationship between functional groups and environmental parameters was verified and validated by the multivariate statistical model.(2) The concept of phytoplankton feasible energy concept was originally put forward and simplified. It combined the ligh and temperature effects on the growth of phytoplankton in the lab-scale and the variation of hydraulic retention time in the field-scale, which provide the opportunities to elucidate the effect of hydraulic retention time on the growth of phytoplankton in the context of physical environment in the Three Gorges Reservoir.(3) The specific time period of habitat transition and phytoplankton succession in the Xiaojiang River Backwater Area during the operation of the Three Gorges Reservoir was originally sepreated. The general succession patterns of phytoplankton in such system were originally put forwad based on the phytoplankton functional classification.
|
PDF Full Text Request