|Other Abstract||The Three-Gorges Dam (TGD) is the largest water conservancy project ever built in China, and so in the world. With the normal pool level at 175 m, the total storage capacity of the reservoir is 39.3 billion m3. Damming of the Three-Gorges brought many benefits in controlling flood, producing hydroelectric power, navigating, irrigating the field and other aspects. However, it also exerted far-reaching and profound impacts on the ecological environment. For example, in the mainstream and tributaries of the Yangtze River, there were no algal blooms reported before the building of TGD. However, phytoplankton blooms occurred repeatedly in TGR every spring since water storage started. The excessive growth of phytoplankton deteriorated the water quality, decreased reservoir natural functions, damaged water ecology system, and even threatened human health. Therefore, it was very urgent to develop safe and efficient ways to control blooms occurring in TGR.
The present research aims to investigate spatial and temporal variability of phytoplankton, to explore the developmental discipline of algal blooms, to analyze the whole course of spring phytoplankton variability, to identify the main environmental factors which affected phytoplankton growth and algal blooms, and finally to provide the basis for controlling blooms in Three-Gorges Reservoir. The main results were as follows:
Before water storage, a total of 77 algal taxa belonging to 7 phyla was identified. Green algae (30 species) and diatoms (28 species) were the dominant species, representing 38.96% and 36.36% of the total taxa. After water storage, algal taxa increased dramatically and a total of 165 algal taxa belonging to 8 phyla was identified. Green algae (90 species) and diatoms (41 species) were the dominant species, representing 54.55% and 24.85% of the total taxa. Whether in mainstream or tributary, green algae and diatoms were clearly the dominant phytoplankton population. Before water storage, obvious dominant species and algal blooms had never appeared in the mainstream and tributaries of TGR. However, algal blooms appeared frequently since water storage started in the year of 2003. Especially in some bays and tributaries, algal blooms occurred repeatedly every spring. Additionally, the spatial extents and frequency of algal blooms exhibited an increasing trend in last five years. Algal blooms seemed to be caused by high nutrients, slow current speed and suitable water temperature.
The main taxa of algal blooms were Microcystis aeruginosa、Microcystis wesenbergii、Aphanizomenon flos-aquae、Stephanodiscus neoastraea、Asterionella formosa、Cyclotella sp.、Peridiniopsis niei、Peridiniopsis sp.、Pandorina morum、Eudorina elegans、Volvox aureus、Rhodomonas lacustris and Mallomonas elongata. Several bays and tributaries of TGR had suffered from algal blooms, where Peridiniopsis niei was the most serious dominant species. Algal blooms occurred mainly in spring and sometimes in other seasons in last five years. For example, the Xiangxi Bay suffered from serious algal blooms in summer and autumn of the year of 2008.
Chlorophyll a concentration and phytoplankton biomass were both correlated significantly negatively with nitrate concentration, and nitrate uptake was extremely high during the pre-bloom and bloom periods. This phenomenon indicated phytoplankton growth was closely related to nitrate consumption. Heavy rainfalls affected seriously phytoplankton growth and the environment to which the cells were subjected. A large number of soil and sand from surrounding land entered the water body and caused turbidity to increase abruptly. Most of these species were dead because they did not adapt the sharply environmental change. Therefore, heavy rainfalls caused phytoplankton chlorophyll a concentration, abundance and biomass to decrease sharply and algal blooms to disappear. Heavy rainfalls also caused nitrate to increase sharply and this provided sufficient nitrogen for algae blooms occurring again. After heavy rainfalls, strong environmental changes may be no longer suitable for the growth of previous dominant species and thus resulted in variation of dominant species.
The sampling sites in mainstream of TGR were co-limited by N and P or P-limited alone, and sites in Xiangxi bay were N-limited alone. Treatments that were enriched with Fe showed markedly increased growth in comparison to Fe not enriched treatments, but enrichment with Ca alone did not result in a significant increase in algal biomass. This showed Fe likely played an important role in determining the appearance and disappearance of algal blooms of TGR. Native algae, Selenastrum capricornutum and Cyclotella meneghinana had high growth potential in TGR. Maybe that was the reason that blooms of diatoms and green algae were sometimes detected in spring in TGR.
Nitrogen nutrition came mainly from non-point source pollution and rainfall was a very important way of nitrogen input. Therefore, it was difficult to control nitrogen input because we could not change rainfall. Compared with nitrogen, the phosphorus pollution in Xiangxi Bay was more serious. Serious phosphorus pollution came from the continuous discharge of a large number of phosphorous industry wastewater and domestic sewage. Additionally, enough phosphorus nutrition was one of important causes of bloom occurring in Xiangxi bay. Necessary measures must be taken to control the point source of pollutants of phosphorus in Xiangxi bay in order to prevent the occurrence of water bloom.
Peridiniopsis niei, the principal specie of water bloom, preferred standing water region and Peridiniopsis niei bloom also firstly occurred in standing water region every spring and then spread to the other regions of the Bay. It seemed that standing water regions was the source of Peridiniopsis niei bloom to diffuse in Xiangxi Bay. There were special geographical conditions for standing water regions which were shallow, almost static and closed. Therefore, the regions could be managed artificially easily. To control Peridiniopsis niei bloom in standing water regions maybe was an effective way to decrease blooms occurring in the whole bay. Based on our study, soil input caused by heavy rainfalls could trigger algal blooms disappear. Thus, it was maybe effective that using local soils to control Peridiniopsis niei bloom when it had just been found in standing water regions. Certainly, the local soils must be modified (for example, chitosan-modified) in order to avoid nutrient input and increase algae-removal efficiency.
In our view, eco-hydraulic regulation should be performed according to local meteorological conditions. Dam storage was performed when heavy rainfall occurred because heavy rainfall could trigger algal blooms to disappear. Dam discharge was performed when rainfall reduced and algal blooms became serious. The disgusting Peridiniopsis bloom maybe disappear because it was not suitable to grow in flowing water region.|