Effects of vertical mixing on phytoplankton blooms in Xiangxi Bay of Three Gorges Reservoir: implications for management

Since the initial filling of Three Gorges Reservoir (TGR), serious phytoplankton blooms have occurred in its tributary bays. Cyanobacteria blooms have been observed in a number of tributary bays and threaten the drinking water security of residents in the TGR region. To identify the key factors controlling phytoplankton blooms in tributary bays and propose an effective management strategy, a one-year water quality study (November 2009 to October 2010) was conducted in Xiangxi Bay (XXB) of TGR. The results show that a rapid decrease in mixing depth is associated with the spring bloom, fading of the fall bloom occurs with the rapid increase in mixing depth, and an extremely shallow mixing depth is associated with cyanobacteria blooms that predominate during the summer. Development of thermal stratification in XXB is the major cause of seasonal variation in mixing depth and density current intrusion from TGR is the major cause of short-term variation in mixing depth. The seasonal thermal stratification of XXB is disrupted by sufficiently large water level fluctuations in TGR. The density current is lifted from mid-depths to the surface and chlorophyll a concentrations rapidly decrease in response. Based on these findings, a conceptual model is proposed for a management strategy to control phytoplankton blooms in tributary bays via controlled releases from TGR.     

Establishment of an Alert Level Framework for cyanobacteria in drinking water resources by using the Algae Online Analyser for monitoring cyanobacterial chlorophyll a

The Algae Online Analyser (AOA) fluorometer simultaneously distinguishes four different phytoplankton groups by their specific fluorescence spectra and thus allows for real-time in-situ chlorophyll a measurements per algal group. This AOA was used for monitoring cyanobacterial chlorophyll a in the drinking water at the Bronis?awow Bay abstraction point in Sulejow Reservoir (Poland). The main goal of this research was to develop an early warning method for the detection of cyanobacterial biovolume in the source water, in order to establish an Alert Level Framework for the drinking water abstraction point in Sulejow Reservoir. A positive correlation between cyanobacterial biovolume, as determined by conventional methods, and cyanobacterial chlorophyll a, as measured by the AOA, was found (p < 0.05). The results of this study were used to determine threshold values for the Alert Level Framework, based on cyanobacterial chlorophyll a concentrations in the source water of Sulejow Reservoir. The presented threshold values are determined specifically for this abstraction point, but the principles can be applied to other locations.     

Driving factors of the phytoplankton functional groups in a deep Mediterranean reservoir

The control of phytoplankton growth is mainly related to the availability of light and nutrients. Both may select phytoplankton species, but only if they occur in limiting amounts. During the last decade, the functional groups approach, based on the physiological, morphological and ecological attributes of the species, has proved to be a more efficient way to analyze seasonal changes in phytoplankton biomass. We analysed the dynamics of the phytoplankton functional groups sensu Reynolds, recognising the driving forces (light, mixing regime, and nutrients) in the Sau Reservoir, based on a one-year cycle (monthly surface-water sampling). The Sau Reservoir is a Mediterranean water-supply reservoir with a canyon-shaped basin and a clear and mixed epilimnion layer. The long stratification period and high light availability led to high phytoplankton biomass (110.8 fresh-weight mg L⁻¹) in the epilimnion during summer. The reservoir showed P-limitation for phytoplankton growth in this period. All functional groups included one or more species (X2-Rhodomonas spp.; Y-Cryptomonas spp.; F-Oocystis lacustris; K-Aphanocapsa spp.) selected by resources, especially phosphorus. Species of Cryptomonas (group Y) dominated during the mixing period (winter season) in conditions of low light and relatively high availability of dissolved nutrients. Increases in water-column stability during spring stratification led to phytoplankton biomass increases due to the dominance of small flagellate functional groups (X2 and X3, chrysophyceans). The colonial chlorophycean O. lacustris (group F) peaked during the mid-summer stratification, when the mixed epilimnion was clearly depleted in nutrients, especially SRP. High temperature and increases in nutrient concentration during the end-summer and mid-autumn resulted in a decrease of green algae (group F) and increase of Aphanocapsa spp. (cyanobacteria, group K) and dinoflagellates (group L_o). The study also revealed the important role of physical processes in the seasonal gradient, in selecting phytoplankton functional groups, and consequently in the assessment of ecological status. The Q index (assemblage index) based on functional groups indicated the overall good ecological status of the Sau Reservoir, which varied as a function of the mixing regime. This is the first application of the Assemblage Index to a European water-supply reservoir.     

Nitrogen characteristics in sediments of Xiangxi Bay,China Three‐Gorge Reservoir

Spatial and temporal distribution characteristics of total nitrogen and ammonium nitrogen in the sediments, pore water and overlying water of Xiangxi Bay in China Three‐Gorge Reservoir were investigated. In surveys, the sampling was undertaken from six sites of Xiangxi Bay on three dates: 29 March 2009, 28 March 2010 and 17 August 2010. Mean values of total nitrogen and ammonium nitrogen contents in the sediments and pore water of Xiangxi Bay all increased with time, especially in summer. There were intimate relationships between internal nitrogen loading and water ecosystem. Correlation coefficients R were 0.7408 (between total nitrogen in pore water and total nitrogen in sediments) and 0.7483 (between total nitrogen in sediments and chlorophyll‐a in surface water), respectively. Total nitrogen concentration differences between pore water and overlying water were all positive and had good correlation with chlorophyll‐a concentration of Xiangxi Bay. The release of nitrogen in sediments had an important impact on phytoplankton growth in Xiangxi Bay.     

Phytoplankton community succession shaping bacterioplankton community composition in Lake Taihu, China

PCR-denaturing gradient gel electrophoresis (DGGE) and canonical correspondence analysis (CCA) were used to explore the relationship between succession of phytoplankton community and temporal variation of bacterioplankton community composition (BCC) in the eutrophic Lake Taihu. Serious Microcystis bloom was observed in July-December 2008 and Bacillariophyta and Cryptophyta dominated in January-June 2009. BCC was characterized by DGGE of 16S rRNA gene with subsequent sequencing. The DGGE banding patterns revealed a remarkable seasonality which was closely related to phytoplankton community succession. Variation trend of Shannon-Wiener diversity index in bacterioplankton community was similar to that of phytoplankton community. CCA revealed that temperature and phytoplankton played key roles in structuring BCC. Sequencing of DGGE bands suggested that the majority of the sequences were affiliated with common phylogenetic groups in freshwater: Alphaproteobacteria, Betaproteobacteria, Bacteroidetes and Actinobacteria. The cluster STA2-30 (affiliated with Actinobacteria) was found almost across the sampling time at the two study sites. We observed that the family Flavobacteriaceae (affiliated with Bacteroidetes) tightly coupled to diatom bloom and the cluster ML-5-51.2 (affiliated with Actinobacteria) dominated the bacterioplankton communities during Microcystis bloom. These results were quite similar at the two sampling sites, indicating that BCC changes were not random but with fixed pattern. Our study showed insights into relationships between phytoplankton and bacterioplankton communities at species level, facilitating a better understanding of microbial loop and ecosystem functioning in the lake.     

Occurrence of toxic cyanobacterial blooms for the first time in Lake Karaoun,Lebanon

The increasing occurrence of cyanobacterial bloom in freshwaters worldwide is of great importance because of public health risks. In addition, they are very likely to have negative impact on ecological and economic aspects. In this study, the seasonal succession of phytoplankton population in Lake Karaoun in Lebanon was monitored from May 2009 to June 2011. The physicochemical parameters of lake water were then monitored for 1 year, from June 2010 until June 2011, to correlate the physicochemical parameters with the phytoplankton population in the lake. Our results showed, for the first time in Lebanon, that the eutrophied Lake Karaoun has been under the invasion of toxic cyanobacterial blooms since May 2009. The cyanobacterial bloom was persistent from late spring (May) until late fall (December) for 2 consecutive years. The high water temperature in the summer season is the main factor that has been affecting the growth of the cyanobacteria. The most frequently encountered bloom‐forming species were Microcystis aeruginosa and Aphanizomenon ovalisporum, which were either present individually or coexistent. The obtained results showed that during the period of cyanobacterial bloom, a deterioration of water quality defined by low levels of dissolved oxygen, total dissolved solids, and electric conductivity was reported. During cyanobacterial bloom period, the concentration of the orthophosphate–P (PO4‐P) was very minimal. The measured high value of chlorophyll‐a concentration during cyanobacterial bloom period (48.6 μg/L) was attributed to high photosynthetic activity. Cyanobacterial blooms can cause a variety of water‐quality problems in Lake Karaoun in addition to human health risk.     

Using primary productivity as an index of coastal eutrophication: a case study in Daya Bay

Seasonal measurements of phytoplankton biomass [chlorophyll a (chl a)], primary production (PP) and nutrient were conducted to understand the trophic status of Daya Bay in 2008–2009. Both nitrate and phosphate concentrations in winter were higher than those in summer. Maximum chl a and PP records appeared in the summer, when a harmful algae bloom (HAB) occurred in the north‐western part of the bay. Microplankton dominated the phytoplankton community in nearshore waters, while pico‐ or nanophytoplankton was predominant in other area of the bay. High chl a and PP could often be found in the inner bay, such as the Dapeng'ao Cove and Aotou Cove. High PP values (> 20 mgC/m³/h) often appeared in waters with frequent outburst of HABs. The results suggested that PP was a suitable index of coastal eutrophication in Daya Bay.     

Design and application of a stratified sampling strategy to study the regional distribution of cyanobacteria (Ile-de-France, France)

This study describes the design and application of a stratified sampling strategy of waterbodies to assess and analyze the distribution of cyanobacteria at a regional scale (Ile-de-France, IDF). Ten groups of hydrographical zones were defined within the IDF on the basis of their anthropogenic and geomorphologic characteristics. Sampling effort (n = 50) was then randomly allocated according to the number of waterbodies in each group. This sampling strategy was tested in August 2006, using a field probe to estimate total phytoplankton as well as cyanobacteria biomasses. The sampled waterbodies exhibited a wide range of phytoplankton (<1-375 μg equiv. Chla L⁻¹) and cyanobacteria biomasses (<1-278 μg equiv. Chla L⁻¹). 72% of the waterbodies in the IDF were classified as eutrophic (42% hypereutrophic), and 24% of the sites studied were dominated by cyanobacteria. Waterbodies connected to hydrographical networks (n = 26) showed significantly higher total (p < 0.0001; 3.5 times greater) and cyanobacterial (p < 0.001, 3.2 times greater) biomasses than the isolated ones (n = 24). No significant overall relationship was found through contingency analysis between waterbody trophic status and global land use categories (urban, periurban, and rural) within their hydrographical zones. However, concerning the waterbodies linked to hydrographical networks, the percentage of land covered by forest appeared as a good indicator of phytoplankton and cyanobacterial biomasses. This observation may be a consequence of lower amounts of nutrients being discharged into waterbodies from highly forested hydrological zone than from urban and/or agricultural areas. Our results illustrate a successful means of selecting representative waterbodies to conduct a regional assessment of cyanobacteria distribution using accessible GIS analyses.     

Distribution of phytoplankton in the Three-Gorge Reservoir during rainy and dry seasons

After damming of the Yangtze River, in order to explore the impacts of the Three-Gorge Dam (TGD) on the aquatic ecosystem, phytoplankton composition, abundance and biomass spatial distribution were studied in the Three-Gorge Reservoir (TGR), and the closest upstream anabranch Xiangxi River, which is 38 km away from the Three-Gorge Dam (TGD) during August (rainy season) 2004 and April (dry season) 2005. In surveys, 6 transects (2 downstream and 4 cross-stream) and 25 stations have been investigated and 314 samples were collected from the surface to the river bed with water samplers. In TGR, 63 taxa and 60 taxa were identified in the rainy and dry seasons, respectively. In the Xiangxi River, 39 taxa were observed in the rainy and dry seasons. Algal blooms occurred in the Xiangxi River and at the influx region of the Yangtze and Xiangxi in both seasons, but had not occurred prior to damming. In the rainy season, the dominant species was Chroomonas acuta with 1.84x10(7) cells l-1, and in the dry season the dominant species were Asterionella formosa and Cryptomonas ovata with 1.34x10(7) cells l-1 and 1.79x10(6) cells.l-1, respectively. In the main channel of TGR, there were no significant correlations between phytoplankton abundance and the concentrations of the main soluble nutrients. In the Xiangxi River, significant negative correlations were observed between phytoplankton abundance and nitrate (Spearman, p<0.01, n=21), phosphate (Spearman, p<0.05, n=21) and silicate (Spearman, p<0.01, n=21) in the rainy season, and similar correlations were also observed with nitrate (Spearman, p<0.05, n=28) and silicate (Spearman, p<0.01, n=28), but not with phosphate in the dry season. Since the damming of the Yangtze River, eutrophication in the anabranch within the backwater has occurred and become severe, and the frequency of algal bloom within TGR and anabranches is expected to increase.     

Spring bloom in a hypereutrophic lake, lake Kasumigaura, Japan—I Succession of phytoplankters with different accessory pigments

Phytoplankters appearing during the spring bloom of 1980 in Lake Kasumigaura were classified into three groups characterized by different accessory pigments. Phytoplankters of the first group possess phycocyanin, those of the second group possess chlorophyll b, and those of the third group possess chlorophyll c. Only eucaryotic phytoplankters, belonging to the second or third group, were predominantly present during the bloom. Predominance of the second and third groups alternated during the bloom as individual phytoplankton pulses. The relationship between the population density of all phytoplankters and the abundance of phytoplankters possessing chlorophyll c could be expressed as a highly significant regression curve (r = 0.7997) throughout the spring bloom in Tsuchiura Harbor: Y = 102.9 e^−0.03138X + 29.04, where X is the concentration of chlorophyll a in all phytoplankters in lake water, and Y is the percentage of chlorophyll a possessed by phytoplankters possessing also chlorophyll c.     

Selective suppression of harmful cyanobacteria in an entire lake with hydrogen peroxide

Although harmful cyanobacteria form a major threat to water quality, few methods exist for the rapid suppression of cyanobacterial blooms. Since laboratory studies indicated that cyanobacteria are more sensitive to hydrogen peroxide (H₂O₂) than eukaryotic phytoplankton, we tested the application of H₂O₂ in natural waters. First, we exposed water samples from a recreational lake dominated by the toxic cyanobacterium Planktothrix agardhii to dilute H₂O₂. This reduced the photosynthetic vitality by more than 70% within a few hours. Next, we installed experimental enclosures in the lake, which revealed that H₂O₂ selectively killed the cyanobacteria without major impacts on eukaryotic phytoplankton, zooplankton, or macrofauna. Based on these tests, we introduced 2 mg L⁻¹ (60 μM) of H₂O₂ homogeneously into the entire water volume of the lake with a special dispersal device, called the water harrow. The cyanobacterial population as well as the microcystin concentration collapsed by 99% within a few days. Eukaryotic phytoplankton (including green algae, cryptophytes, chrysophytes and diatoms), zooplankton and macrofauna remained largely unaffected. Following the treatment, cyanobacterial abundances remained low for 7 weeks. Based on these results, we propose the use of dilute H₂O₂ for the selective elimination of harmful cyanobacteria from recreational lakes and drinking water reservoirs, especially when immediate action is urgent and/or cyanobacterial control by reduction of eutrophication is currently not feasible. A key advantage of this method is that the added H₂O₂ degrades to water and oxygen within a few days, and thus leaves no long-term chemical traces in the environment.     

Multi-scale strategies for the monitoring of freshwater cyanobacteria: reducing the sources of uncertainty

Cyanobacterial blooms are a frequent phenomenon in eutrophic freshwaters worldwide and are considered potential hazards to ecosystems and human health. Monitoring strategies based on conventional sampling often fail to cover the marked spatial and temporal variations in cyanobacterial distribution and fluctuating toxin concentrations inherent to cyanobacterial blooms. To deal with these problems, we employed a multi-scale approach for the study of a massive Microcystis bloom in Tajo River (Spain) utilizing 1) remote sensing techniques, 2) conventional water sampling and 3) analysis of chemotypical subpopulations. Tajo River at the study area is influenced by high temperatures waters diverted upstream from a nuclear power plant, the presence of a dam downstream and a high nutrient load, which provide optimal conditions for massive cyanobacterial proliferation. MERIS imagery revealed high Chl-a concentrations that rarely fell below 20 μg L⁻¹ and moderate spatiotemporal variations throughout the study period (March-November 2009). Although the phytoplanktonic community was generally dominated by Microcystis, sampling points highly differed in cyanobacterial abundance and community composition. Microcystin (MC) concentrations were highly heterogeneous, varying up to 3 orders of magnitude among sampling points, exceeding in some cases WHO guideline values for drinking and also for recreational waters. The analysis of single colonies by MALDI-TOF MS revealed differences in the proportion of MC-producing colonies among points. The proportion of toxic colonies showed a highly significant linear correlation with total MC: biovolume ratio (r² = 0.9; p < 0.001), evidencing that the variability in toxin concentrations can be efficiently addressed by simple analysis of subpopulations. We propose implementing a multi-scale monitoring strategy that allows covering the spatiotemporal heterogeneities in both cyanobacterial distribution (remote sensing) and MC concentrations (subpopulation analysis) and thereby reduce the main sources of uncertainty in the assessment of the risks associated to bloom events.     

Controlling harmful cyanobacterial blooms in a hyper-eutrophic lake (Lake Taihu, China): the need for a dual nutrient (N & P) management strategy

Harmful cyanobacterial blooms, reflecting advanced eutrophication, are spreading globally and threaten the sustainability of freshwater ecosystems. Increasingly, non-nitrogen (N₂)-fixing cyanobacteria (e.g., Microcystis) dominate such blooms, indicating that both excessive nitrogen (N) and phosphorus (P) loads may be responsible for their proliferation. Traditionally, watershed nutrient management efforts to control these blooms have focused on reducing P inputs. However, N loading has increased dramatically in many watersheds, promoting blooms of non-N₂ fixers, and altering lake nutrient budgets and cycling characteristics. We examined this proliferating water quality problem in Lake Taihu, China’s 3rd largest freshwater lake. This shallow, hyper-eutrophic lake has changed from bloom-free to bloom-plagued conditions over the past 3 decades. Toxic Microcystis spp. blooms threaten the use of the lake for drinking water, fisheries and recreational purposes. Nutrient addition bioassays indicated that the lake shifts from P limitation in winter-spring to N limitation in cyanobacteria-dominated summer and fall months. Combined N and P additions led to maximum stimulation of growth. Despite summer N limitation and P availability, non-N₂ fixing blooms prevailed. Nitrogen cycling studies, combined with N input estimates, indicate that Microcystis thrives on both newly supplied and previously-loaded N sources to maintain its dominance. Denitrification did not relieve the lake of excessive N inputs. Results point to the need to reduce both N and P inputs for long-term eutrophication and cyanobacterial bloom control in this hyper-eutrophic system.     

Seasonal periodicity of plankton and benthic fauna community structure and diversity in a small North African reservoir

Aquatic biota in terms of phytoplankton, zooplankton and benthic fauna were qualitatively and quantitatively followed seasonally during 2007 in a North African small reservoir, Aswan Reservoir. Simultaneously, seasonal variations in water quality were investigated in order to determine how far the aquatic biota could be altered by physicochemical and biotic factors. In phytoplankton of the investigated reservoir, 75 species belonging to four systematic groups were recorded, comprising 11 Cyanoprokaryota, 24 Bacillariophyceae, 2 Dinophyceae and 38 Chlorophyceae. Most numerous zooplankton were Rotifera (23 species). Cladocera and Copepoda were less numerous, with only seven and three species, respectively. Early development stages of copepods, nauplii and copepodites were sometimes recorded in considerable numbers. Twenty‐two invertebrate species related to five animal groups were exclusively benthic fauna: three Oligochaeta, nine Mollusca, seven larvae of Chironomidae, one Platyhelminthes, one Hirudinea and one Decapoda. Generally, the estimated values of phytoplankton species richness were relatively higher than those of zooplankton or benthic fauna. In particular, phytoplankton was influenced by physicochemical factors – water transparency, salinity and soluble reactive silica contents – as well as biotic factors – zooplankton and benthic fauna. Levels of pH value, Mg⁺², NO₃‐N and chlorophyll‐a contents appeared to be the most influential parameters dictating the pattern of seasonal periodicity of benthic fauna. Numerically, the phytoplankton was somewhat more abundant in winter and autumn than in spring or summer. The peak of zooplankton population densities was observed in spring season. Benthic fauna tended to be abundant in summer and autumn seasons, showing a reverse seasonal trend from that of zooplankton. Cyanoprokaryota and Bacillariophyceae were the most important groups that determined the seasonal fluctuation pattern of phytoplankton. Rotifera and Copepoda were the most predominant groups influencing the seasonal periodicity of zooplankton. Oligochaeta and Mollusca alternated the dominancy of benthic fauna community in Aswan Reservoir.     

Detection and estimation of potentially toxic cyanobacteria in raw water at the drinking water treatment plant by in vivo fluorescence method

In vivo fluorescence methods have been accepted as a quick, simple, and useful tool for quantification of phytoplankton organisms. In this paper, we present a case study in which fluorescence methods were employed for the selective detection of potentially toxic cyanobacteria in raw water at the drinking water treatment plant. The occurrence of cyanobacteria in the drinking water reservoir and in raw water was monitored by phycocyanin fluorescence measurements and by standard methods for phytoplankton quantification (cell counts, chlorophyll a). A special attention was paid to the most critical parts of the season -- spring recruitment of cyanobacteria from sediment to water column and autumn bloom collapse. All methods showed similar patterns within the season. Phycocyanin fluorescence was found to be a simple and sensitive indicator of cyanobacteria in water and can serve as a tool that can provide an early warning about the presence of potentially toxic cyanobacterial metabolites in water.     

佐洛塔廖夫法在某码头库岸塌岸预测中的应用

三峡库区三期地质灾害防治工程涉及到库岸的工程有44处之多,而塌岸预测是库岸整治设计的重要依据。在系统分析香溪汽渡北岸码头库岸谭家槽冲沟段岸坡工程地质条件的基础上,采用佐洛塔廖夫法对该岸坡段进行了塌岸预测。根据预测结果提出了谭家槽冲沟段岸坡的工程治理措施。     

MULTI-FUNCTIONAL WETLAND DESIGN ADAPTIVE TO ENVIRONMENTAL CHANGES — WETLAND ECOSYSTEM CONSTRUCTION OF FURONGBA BAY IN HANFENG LAKE OF THE THREE GORGES RESERVOIR AREA

Hanfeng Lake, an inland lake formed by the seasonal water fluctuations due to the water storage and sluice in the Three Gorges Reservoir, was faced with ecological challenges such as water pollution, aquatic biodiversity loss, and changes in land use pattern. This article takes the wetland ecosystem construction in Furongba Bay, Hanfeng Lake as an example to explore approaches to designing multi-functional wetlands which could adapt to hydro-fluctuation and other environmental changes, by drawing from the ecological wisdoms of water regulation, conservancy, and utilization developed in the agrarian age of China to support a dynamic, multi-layered landscape of mutualism and co-evolution.     

Hepatotoxic microcystin diversity in cyanobacterial blooms collected in portuguese freshwaters

Twelve toxic cyanobacterial bloom samples collected in natural lakes, reservoirs and rivers of Portugal were analysed. Toxicity was evaluated by mouse LD₅₀ bioassay of the lyophilised samples. The main bloom species present in the samples were Microcystis aeruginosa, Microcystis wesenbergii, Anabaena flos-aquae and Nostoc sp. Toxins were extracted, isolated by reverse phase HPLC and characterised by HPLC amino acid analysis and fast atom bombardment mass spectrometry. Two to seven microcystins were purified from each sample, and a total of seven different toxins were isolated and their structure assigned. MCYST-LR was the most common and its proportion in each sample ranged from 45.5% to 99.8% of the total microcystin contents. MCYST-RR, MCYST-YR and [D-Asp³]MCYST-LR were also identified in the samples. Three less common microcystins, MCYST-HilR, [L-MeSer⁷]MCYST-LR and [Dha⁷]MCYST-LR, were found in only one sample. Total MCYST concentration varied from 1.0 to 7.1 μg/mg dry weight of cyanobacteria. Significant relationships between LD₅₀-MCYST-LR-total MCYST content were found. The need for monitoring cyanobacteria and their toxins in eutrophic waters that are used for drinking and recreation purposes is discussed.     

Succession of phytoplankton functional groups regulated by monsoonal hydrology in a large canyon-shaped reservoir

Liuxihe reservoir is a deep, monomictic, oligo-mesotrophic canyon-reservoir in the subtropical monsoon climate region of southern China. Phytoplankton functional groups in the reservoir were investigated and a comparison made between the succession observed in 2008, an exceptionally wet year, and 2009, an average year. The reservoir shows strong annual fluctuations in water level caused by monsoon rains and artificial drawdown. Altogether 28 functional groups of phytoplankton were identified, including 79 genera. Twelve of the groups were analyzed in detail using redundancy analysis. Because of the oligo-mesotrophic and P-limited condition of the reservoir, the dominant functional groups were those tolerant of nutrient (phosphorus) deficiency. The predominant functional groups in the succession process were Groups A (Cyclotella with greatest axial linear dimension < 10 μm), B (Cyclotella with greatest axial linear dimension >10 μm), L_O (Peridinium), L_M (Ceratium and Microcystis), E (Dinobryon and Mallomonas), F (Botryococcus), X₁ (Ankistrodesmus, Ankyra, Chlorella and Monoraphidium) and X₂ (Chlamydomonas and Chroomonas). The development of groups A, B and L_O was remarkably seasonal. Group A was dominant during stratification, when characteristic small size and high surface/volume ratio morphology conferred an advantage. Group L_O was dominant during dry stratification, when motility was advantageous. Group B plankton exhibited a high relative biomass during periods of reduced euphotic depth and isothermy. Groups L_M, E, F, X₁ and X₂ occasionally exhibited high relative biomasses attributable to specific environmental events (e.g. drawdown, changes in zooplankton community). A greater diversity of phytoplankton functional groups was apparent during isothermy. This study underscores the usefulness of functional algal groups in studying succession in subtropical impoundments, in which phytoplankton succession can be significantly affected by external factors such as monsoonal hydrology and artificial drawdown, which alter variables such as retention time, mixing regime and thermal structure and influence light and nutrient availability.