Xiangxi Bay Research Articles

Silica retention in the Three Gorges Reservoir

A mass balance of dissolved silica (DSi) based on daily measurements at the inflow and outflow of the Three Gorges Reservoir (TGR) in 2007 and a more precise budget, with inflow, outflow, primary production, biogenic silica (BSi) settlement, dissolution of BSi in the water column and flux of DSi at the sediment–water interface in the dry season (April) of 2007 were developed. We address the following question: How much does the Three Gorges Dam (TGD) affect silica transport in the TGR of the Changjiang River (Yangtze River)? The DSi varied from 71.1 to 141 μmol/l with an average of 108 μmol/l, and it ranged between 68.1 and 136 μmol/l, with an average of 107 μmol/l in inflow and outflow, respectively, in the TGR in 2007. The linear relationship of DSi between inflow and outflow water is significant (r = 0.87, n = 362, p < 0.01). Along the main stream of the TGR, the DSi concentration decreases with an average concentration of 84.0 μmol/l in the dry season. However, the stratification of DSi was not obvious in the main channel of the TGR in the dry season. The BSi is within the range of 0.04–5.00 μmol/l, with an average concentration of 2.1 μmol/l in the main channel of the TGR, while it is much higher in Xiangxi Bay (1.30–47.7 μmol/l, 13.1 μmol/l) than in the main stream of the TGR and the other bays. After the third filling of the TGR, approximately 3.8% of the DSi was retained by the TGR based on a 12-month monitoring scheme in 2007, which would slightly reduce nutrient fluxes of the Changjiang River to the East China Sea (2%). DSi was lost during January to June and November, whereas the additions of DSi were found during the other months in 2007. The budget results also indicate that there is a slight retention of DSi. The retention of DSi in the reservoir is approximately 2.9%, while BSi is approximately 44%. Compared with the total silica load, the retention of DSi and BSi in the reservoir is only 5.0% in the dry season. With its present storage capacity, the reservoir does not play an important role as a silica sink in the channel of the TGR. The DSi load is significantly related to discharge both in inflow and outflow waters (p < 0.01). DSi retention, to some extent, is the runoff change due to impoundment.

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.

Community structure characteristics of phytoplankton along a cascade of reservoirs in Xiangxi River Basin, Three Gorges Reservior

于2005年10月、2006年1、4、7月对三峡水库香溪河流域3座水库(古洞口一级水库、古洞口二级水库和香溪河库湾)组成的梯级水库的浮游植物种类组成、优势种、群落结构、密度和生物多样性指数进行了周年调查研究.共鉴定出浮游植物7门58属121种(含变种),以绿藻和硅藻种类最多,绿藻有26属49种,占40.50%;硅藻14属41种,占33.88%;其次是甲藻,3属11种,占9.09%;蓝藻5属7种,占5.79%;隐藻3属7种,占5.79%;其它藻类仅占4.96%.浮游植物在古洞口一级水库共有25属31种,古洞口二级水库29属40种,香溪河库湾46属81种.优势度分析显示:古洞口一级水库藻类优势类群为硅藻门、绿藻门,古洞口二级水库为硅藻门、隐藻门和甲藻门,香溪河库湾为绿藻门、硅藻门、甲藻门和隐藻门.3座水库浮游植物年均密度分别为1.110×10⁶、4.837×10⁵和1.734×10⁶ cells/L;其中,最高密度出现在香溪河库湾(4.87×10⁶ cells/L),最低密度出现在古洞口二级水库(5.76×10⁵ cells/L).运用主成分分析对梯级水库进行水质分析,表明沿着水库的梯度水质逐渐恶化.Shannon-Wiener多样性指数和Pielou均匀度指数在3座水库间无明显差异,而香溪河库湾Margalef丰富度指数显著大于古洞口一级、二级水库.前两个指数与浮游植物优势种的评价结果显示,香溪河流域梯级水库处于中污染状态.;Xiangxi River originates from the Shennongjia Forest region and is the largest tributary in Hubei Province of Three Gorges Reservoir. Three reservoirs(Gudongkou Reservoir 1, Gudongkou Reservoir 2 and Xiangxi Bay of Three Gorges Reservoir) in Xiangxi River Basin were selected to investigate phytoplankton ecology in such a reservoir cascade system. In this study, phytoplankton composition, dominant species, community structure, abundance and biodiversity index were investigated and analyzed from October 2005 to July 2006. A total of 121 species, belonging to 58 genera in 7 phyla were identified, among which Chlorophyta(49 species of 26 genera) is the most important compontent, accounting for 40.50% of the total; followed by Bacillariophyta (41 species of 14 genera) for 33.88%, and Pyrrophyta(11 species of 3 genera) for 9.09%, Cyanophyta(7 species of 5 genera) for 5.79%, Cryptophyta(7 species of 3 genera) for 5.79% and other species for 4.96%. 31 species of 25 genera, 40 species of 29 genera and 81 species of 46 genera were found in Gudongkou Reservoir 1, 2, and Xiangxi Bay, respectively. The dominant species in Gudongkou Reservoir 1 belonged to Bacillariophyta, Chlorophyta, while Bacillariophyta, Cryptophyta and Pyrrophyta dominated in Gudongkou Reservoir 2. Chlorophyta, Bacillariophyta, Pyrrophyta, Cryptophyta also played an important role in Xiangxi Bay. The annual abundance of phytoplankton community in the three reservoirs was 1.110×10⁶, 4.837×10⁵ and 1.734×10⁶ cells/L, respectively. It had highest density in Xiangxi Bay(4.87×10⁶ cells/L) and lowest density in Gudongkou Reservoir 2(5.76×10⁵ cells/L). Principal Component Analysis was applied to analyze the water quality of the three reservoirs. The result implies that the water quality was degrading along the cascade of reservoirs. There were no significant differences in diversity index(H) and evenness(J) among the three reservoirs, while the Margalef index of Xiangxi Bay was significantly larger than that of Gudongkou Reservoir 1 and Gudongkou Reservoir 2. The valuation results by the first two indices and dominant phytoplankton species showed that the cascade reservoirs in Xiangxi River Basin were of intermediate pollution state.

Support Vector Regression Model of Chlorophyll-&amp;lt;i&amp;gt;a&amp;lt;/i&amp;gt; during Spring Algal Bloom in Xiangxi Bay of Three Gorges Reservoir, China

To study the relationship between chlorophyll-a and environmental variables during spring algal bloom in Xiangxi Bay of Three Gorges Reservoir, the support vector regression (SVR) model was established. In surveys, 11 stations have been investigated and 264 samples were collected weekly from March 4 to May 13 in 2007 and February 16 to May 10 in 2008. The parameters in SVR model were optimized by leave one out cross validation. The squared correlation coefficient R2 and the cross validated squared correlation coefficient Q2 of the optimal SVR model are 0.8202 and 0.7301, respectively. Compared with stepwise multiple linear regression and back propagation artificial neural network models using external validation, the SVR model has been shown to perform well for regression with the predictive squared correlation coefficient R2pred value of 0.7842 for the test set.

Longitudinal Differences of Phytoplankton Community during a Period of Small Water Level Fluctuations in a Subtropical Reservoir Bay (Xiangxi Bay, Three Gorges Reservoir, China)

AbstractThe Three Gorges Reservoir (TGR) had a great inflow discharge with small water level fluctuations in the flood season of the year 2008, when a large‐scale cyanobacterial bloom broke out in Xiangxi Bay (a tributary bay behaving like a lake) for the first time after the construction of the TGR impoundment. To compare spatiotemporal longitudinal differences of phytoplankton community structure during this period, weekly surveys were performed in Xiangxi Bay. The cyanobacterial bloom lasted from June 6 to July 18, 2008, with Microcystis aeruginosa as the dominant species. During this bloom the species diversity, evenness and community change rate was relatively low. The probable causes for the interruption of the bloom were precipitation and water temperature. In the non‐cyanobacterial bloom period (July 25 to September 26, 2008) many other species dominated the community including Stephanodiscus hantzschii and Cryptomonas ovata, with higher values of species diversity, evenness and community change rate. As for the longitudinal differences, the community structure in the riverine zone was different from that in other zones, indicating the important effect of inflow from the upstream of Xiangxi Bay. The management actions in Xiangxi Bay should prevent blooms in the mainstream, lacustrine and transitional zone. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Seasonal variation in functional phytoplankton groups in Xiangxi Bay, Three Gorges Reservoir

We describe the phytoplankton dynamics and structure in Xiangxi Bay, Three Gorges Reservoir. Samples were collected monthly in the surface waters between August 2007 and July 2008. We identified 10 principle functional groups. C-strategists and S/R-strategists with a wide range of tolerance dominated the phytoplankton assemblage. Seasonal variation was related to water column stability because of changes in hydraulic operation in October, January, and May. Functional group C (Asterionella formosa) and P (Aulacoseria granulata) dominated in August and September, whereas group Lo (Peridiniopsis niei) was the most abundant between February and April, forming a dinoflagellate bloom. Group B (Stephanodiscus hantzschii), X2 (Komma acudata), and Y (Cryptomonas erosa) were present throughout most of the year but were most abundant in late spring. A cyanobacterial bloom occurred from June to July, during which group M (Microcystis aeruginosa, M. wesenbergii) and H1 (Anabaena flos-aquae) were dominant. Green algae, characterized by group G (Eudorina sp., Pandorina sp., Pyramidomonas sp.) and J (Pediastrum spp., Coelastrum spp., Scenedesums spp.), were abundant after the bloom degraded. This sequence was corroborated by canonical correspondence analysis (CCA). The summary sequence of functional groups resulting from CCA was: C/P → Lo → H1/M/J/G. The dynamics of the phytoplankton community may be explained by the stability of water column, irradiance, water temperature, and nutrient structure.

Factors influencing thermal structure in a tributary bay of Three Gorges Reservoir

To better understand the factors influencing the thermal structure of tributaries in the Three Gorges Reservoir (TGR), a well validated three-dimensional hydrodynamic and water temperature model was proposed to simulate the water temperature distribution in the Xiangxi Bay, a representative tributary of TGR. The numerical results show that water temperature stratification seasonally occurred in the Xiangxi Bay, with stable vertical temperature profiles. It is found from the numerical experiments that three key factors are responsible for the formation of water temperature structure: (1) very often, the locations of thermocline are mainly determined by wind speeds, and the higher the wind speed is, the deeper the thermocline is located beneath the water surface, which could be expressed by a fitted exponential function, (2) the thermal structure is affected by static stability of water column, and the thermocline becomes closer to the water surface and its thickness increases with the increase of temperature, (3) due to the effect of the thermal density inflow, the water temperature of the hypolimnion tends to be uniform, however, even under the condition of larger inflow discharge, the influence of the inflow on the epilimnion and the thermocline is not significant.

Phytoplankton development and ecological status during a cyanobacterial bloom in a tributary bay of the Three Gorges Reservoir, China

Reservoirs can provide suitable conditions for cyanobacterial bloom development, which may impact on water quality and biological communities. Weekly surveys in a cyanobacterial bloom process were carried out in the mainstream and Xiangxi Bay of the Three Gorges Reservoir (China), from June 6 to July 18 in 2008. By application of the phytoplankton functional group approach, the spatiotemporal pattern, impact factors, and the ecological status based on Q index (assemblage index) were analyzed. The depth of euphotic layer was apparently the key factor driving the phytoplankton functional group variations. Longitudinal patterns of phytoplankton distribution were detected during this bloom: in the beginning phase, groups D (mainly Stephanodiscus hantzschii) and B ( Cyclotella stelligera) dominated in the mainstream of the Three Gorges Reservoir and the mouth area of Xiangxi Bay, group Y ( Cryptomonas species) dominated in the upper area, while groups J (mainly Pediastrum duplex), F (mainly Sphaerocystis schroeteri) and G ( Pandorina morum and Eudorina elegans) were important in other areas; in the mid phase, group M became absolutely dominant in the whole region; and in the ending phase, besides groups M and Y, groups X2 ( Chroomonas acuta, Pyramimonas nanella, etc.) and Lo ( Ceratium hirundinella) became more important in the lower and upper area respectively. Generally the ecological status was bad, temporally varied with the bloom process. No spatial difference of ecological status was found in the mainstream, while longitudinal patterns in Xiangxi Bay were detected for different phases: firstly a few sites had relatively better status than the others, then nearly all the sites were in the bad condition, and at last the status in the downstream was better than that in the upstream. The longitudinal patterns of ecological status were related to phytoplankton distribution, disturbed by jacking from the downstream and flood from the upstream of Xiangxi Bay.

Patterns of asynchrony for phytoplankton fluctuations from reservoir mainstream to a tributary bay in a giant dendritic reservoir (Three Gorges Reservoir, China)

Seasonal fluctuations of phytoplankton have often been regarded as one of the important cyclic events in aquatic ecosystems, and have even been emphasized as an important sign of regional climatic variability in limnology. However, few attempts have been made to examine synchrony for phytoplankton fluctuations among different habitats in a single reservoir system. The present study employed Spearman rank correlation analysis and the Mantel test to assess levels of synchrony for phytoplankton abundance and taxonomic composition from reservoir mainstream to a tributary bay (Xiangxi Bay) in a giant dendritic reservoir, China (Three Gorges Reservoir, TGR). At the selected scale, asynchronous patterns of phytoplankton were found when looking at total abundance and taxonomic composition, suggesting that regional drivers were not strong enough to synchronize phytoplankton fluctuations, and local regulators were predominant. As a riverine system, the mainstream of TGR had high levels of algal synchrony, while asynchronous patterns of phytoplankton were detected within Xiangxi Bay, which is characterized as a lacustrine system. The present study further confirmed that external hydrological disturbances strongly homogenize habitat conditions and synchronize phytoplankton fluctuations within the mainstream, while spatial divergence of phytoplankton succession depend on local habitat conditions within Xiangxi Bay. Moreover, low synchrony of phytoplankton between the mainstream and Xiangxi Bay is probably caused by spatial divergence of phytoplankton succession within Xiangxi Bay and the lack of succession in the rapidly flushed mainstream. Independently of these mechanisms possibly explaining phytoplankton fluctuations, the present study has also an applied perspective to the improvement of a long-term observation program in which phytoplankton trends at the mainstream scale could be compiled from the data set in a single site, while a set of sampling sites should be required at the Xiangxi Bay scale.

Daily and vertical dynamics of rotifers under the impact of diatom blooms in the Three Gorges Reservoir, China

The Three Gorges Dam was built in 2005 with a storage capacity of 39.3 billion m3, ranking 22nd in the world. However, since the impoundment of the reservoir, serious blooms of phytoplankton have occurred. Rotifers, having a key role in the freshwater aquatic food web, are important grazers of phytoplankton and an essential food resource to higher trophic consumers. To explore the impacts of phytoplankton blooms on the rotifer community, daily and vertical surveys of rotifers were conducted in a bay of the Three Gorges Reservoir (Xiangxi Bay). Altogether 46 rotifer species were registered, and Synchaeta tremula, Polyarthra vulgaris, and Brachionus calyciflorus were the most abundant species accounting for 36, 26, and 16% of the mean rotifer densities, respectively. Although these dominant species always prevailed in the rotifer community, their proportions changed significantly from non-bloom phase to bloom phase, e.g., the significance of S. tremula deceased from 46.8 to 33.2%, while P. vulgaris and B. calyciflorus increased from 23.9 and 13.9% to 26.2 and 16.2%, respectively. In the vertical water column, all the rotifer following phytoplankton displayed an aggregated distribution, concentrating at the upper layers (0.5–5 m), especially during the bloom phase. From the non-bloom phase to the bloom phase, rotifer densities, the dominant rotifers, Shannon–Wiener and Margalef’s diversity increased significantly, while the evenness displayed the opposite trend. Non-metric multidimensional scaling analysis (NMDS) revealed that the samples in the non-bloom phase were well separated from those in the bloom phase. This means that the outbreak of the diatom bloom in the Xiangxi Bay had significant impacts on the rotifer community. Further investigations are needed to address the impacts of the changes of rotifer community on higher trophic levels.

Spring phytoplankton blooms in Xiangxi Bay of Three-Gorges Reservoir: spatiotemporal dynamics across sharp nutrient gradients

After the impoundment of Three-Gorges Reservoir, the specific hydrodynamics created sharp nutrient gradients in Xiangxi Bay. To discover how spring blooms in Xiangxi Bay respond to sharp nutrient gradients, we investigated the phytoplankton community composition and dynamics along nutrient gradients and analyzed the strategies of different types of phytoplankton in filling their niches. We observed a total of 148 taxa of 59 genera belonging to seven phyla. Phytoplankton concentration ranged from 0.03 × 107 to 3.74×107 cell L−1 with two pulses during the spring bloom period. Bacillariophyta was the predominant group, which accounted for 75.4% of total abundance. Canonical correlation analysis (CCA) revealed that phytoplankton occurred in optimal niches for their growth across the nutrient gradients. Specifically, species of Bacillariophyta were found in the region with Si:DIN > 1, and species of Cryptophyta, Chrysophyta, Euglenphyta, Pyrrophyta, and some of Chlorophyta were most abundant in the region with DIN:PO4P < 16. High abundances of Cyanophyta and some Chlorophyta were observed in the region where DIN:PO4P > 16. The CCA also indicated a potential nutrient limitation of Si on Bacillariophyta, of DIN on Cyanophyta and some Chlorophyta, and of PO4P on Cryptophyta, Euglenphyta, Pyrrophyta, and some Chlorophyta. Moreover, as a complement for the traditional Redfield ratio of 16N:1P in determining whether N or P limits a system, our study revealed that the stoichiometric ratio of 1Si:1DIN is an important criterion to determine limitation for diatoms.

Changes in water types under the regulated mode of water level in Three Gorges Reservoir, China

The creation of a reservoir is often the most serious impact of damming on upstream river ecosystems, which may cause the dramatic change of water types. A giant subtropical reservoir (Three Gorges Reservoir, TGR) formed by the construction of Three Gorges Dam (TGD) has experienced three impoundment stages. According to the combined classification criterion of reservoir water quality management, a study of monthly change of water types was conducted in the mainstream and Xiangxi Bay of TGR from Jun. 2003 to Dec. 2009. Due to the rise of water level from the first stage to the last stage of impoundment, dramatic changes of water types were found during non-flood season for the mainstream, and during most months of the year for Xiangxi Bay. Under the planned mode of water operation, the mainstream would still be characterized by full mixing during the season of algal growth, which prevents the development of algal blooms, even though it experiences extreme drought events. However, Xiangxi Bay is characterized as a stable system similar to natural lakes (e.g., thermal stratification), which may contribute to the development of algal blooms, especially in years of extreme drought.

Influence of Thermal Density Flow on Hydrodynamics of Xiangxi Bay in Three Georges Reservoir, China

Hydrodynamic conditions play an important role in formation and disappearance of spring algal blooms in the Xiangxi Bay (XXB), a typical tributary water body of the Three Georges Reservoir. As it is difficult to obtain sufficient hydrodynamic information through expensive field monitoring, in this paper, a three-dimensional hydrodynamic model is developed to simulate the flow field of XXB in spring. In order to study the influence of thermal density flow on the hydrodynamics of this tributary, two typical scenarios for the spring of 2005 are considered here: with and without the temperature sources. Our modeling results show that the flow around the mouth of XXB is mainly controlled by the mainstream of the Yangtze River, and: if the temperature effect is considered, the computed maximum influence range of the Yangtze River can reach up to about 12 km upstream from the mouth of XXB. However, if the temperature effect is ignored, the influence range is only about 1 km. In reality, due to the vertical temperature difference, the stratified flow is formed and the mainstream flow can reach up to 12 km upstream from the mouth. The modeled results show that, the flow of Yangtze River enters into XXB in the upper layer, while the water of XXB flows out through the bottom layer. A huge vertical circulation was formed in the XXB, which enhances the exchange capacity between the Yangtze River and the XXB. The field survey also found that. It is included that the water temperature must be included in hydrodynamic numerical simulation for XXB in spring. The study is a useful supplement to field survey, and can provide technical support for eutrophication control in other tributary of Three Gorges Reservoir.

Diel vertical migration and distribution of phytoplankton during spring blooms in Xiangxi Bay, Three Gorges Reservoir

三峡水库蓄水后,香溪河库湾每年春季均会暴发硅、甲藻水华.为了揭示香溪河库湾水华的暴发规律,于2008年春季对水华期间优势浮游植物的昼夜垂直分布格局及其影响因子进行了研究.利用时间深度等值图模拟浮游植物在水柱中的昼夜垂直分布格局,并用Morisita指数对其进行定量检验,采用修正的K-S test来检验浮游植物是否存在昼夜垂直迁移现象.结果表明,在甲藻水华发生的吴家湾样点,倪氏拟多甲藻(Peridiniopsis niei)、膝曲裸藻(Euglena geniculata)和具尾逗隐藻(Komma caudata)为优势种类,相对密度分别为53.47%、14.36%、21.94%.倪氏拟多甲藻具有明显的昼夜垂直迁移现象,白天主要聚集分布于水体近表层,晚间在水柱中趋于均匀分布;膝曲裸藻和具尾逗隐藻没有明显的昼夜垂直迁移活动,它们主要聚集分布于水体亚表层,但随着白天光照强度的增强均表现出一定的趋光性.在硅藻水华发生的平邑口样点,优势种类为汉斯冠盘藻(Stephanodiscus hantzschii)和塔胞藻(Phramidomonas sp.)(相对密度分别为89.53%和7.96%).汉斯冠盘藻昼夜垂直分布类型无显著差异,主要聚集分布于4m以上水体;塔胞藻具有昼夜垂直迁移现象.Sperman相关分析表明,光照强度的变化是引起倪氏拟多甲藻和塔胞藻昼夜垂直迁移行为的主要环境因素,而其他藻类的昼夜垂直分布格局与光照强度的变化无显著相关.膝曲裸藻和具尾逗隐藻的昼夜垂直分布格局可能受光照的变化及水体扰动的共同影响.汉斯冠盘藻不能自由游动,其昼夜分布格局可能主要受水体扰动的影响.;Diatoms and dinoflagellate blooms have always occurred in spring in Xiangxi Bay since the impoundment of Three Gorges Reservoir.Vertical distribution and migration of dominant phytoplankton were studied during two diel periods in Xiangxi Bay in spring 2008, which may be fundamental to understand the outbreak of bloom.In the present study, Peridiniopsis niei, Euglena geniculata and Komma caudata dominated in WJW, accounted for 53.47%, 14.36% and 21.94% of total abundunee.respec. tively.While in PYK, Stephanodiscus hantzschii and Phramidomonas sp. were dominant, and the relative abundance was 89.53% and 7.96%, respectively.Mapping method was used to describe the time-depth distribution of phytoplankton individuals.The dispersion patterns for phytoplankton population were tested with Morisita index, and a modified two sample Komogomv-Smirnov was used to test differences in vertical distribution. Both P. niei and Ph. sp.aggregated in the water upper layer in daytime and was close to a random distribution at night, performing diel vertical migration(DVM). E geniculata and K. caudata did not display DVM.They accumulated in upper water layers and showed phototaxis. S. hantzwhii inhabited the upper water layer.There was significant difference in day and night distribution.Sperman correlation analysis revealed that the diel vertical distribution of both P. niei and ph. sp. significantly correlated with Usht availability, but the other algae didn't.The vertical distribution pattern of E. geniculata and K. caudata may be influenced by dymamic of light availability and water column disturbance caused by wind.The vertical distribution of S. hantzschii which was non-motile may mainly affected by water column disturbance.

Temporal Asynchrony of Trophic Status Between Mainstream and Tributary Bay Within a Giant Dendritic Reservoir: The Role of Local-Scale Regulators

Limnologists have regarded temporal coherence (synchrony) as a powerful tool for identifying the relative importance of local-scale regulators and regional climatic drivers on lake ecosystems. Limnological studies on Asian reservoirs have emphasized that climate and hydrology under the influences of monsoon are dominant factors regulating seasonal patterns of lake trophic status; yet, little is known of synchrony or asynchrony of trophic status in the single reservoir ecosystem. Based on monthly monitoring data of chlorophyll a, transparency, nutrients, and nonvolatile suspended solids (NVSS) during 1-year period, the present study evaluated temporal coherence to test whether local-scale regulators disturb the seasonal dynamics of trophic state indices (TSI) in a giant dendritic reservoir, China (Three Gorges Reservoir, TGR). Reservoir-wide coherences for TSICHL, TSISD, and TSITP showed dramatic variations over spatial scale, indicating temporal asynchrony of trophic status. Following the concept of TSI differences, algal productivity in the mainstream of TGR and Xiangxi Bay except the upstream of the bay were always limited by nonalgal turbidity (TSICHL−TSISD <0) rather than nitrogen and phosphorus (TSICHL−TSITN <0 and TSICHL−TSITP <0). The coherence analysis for TSI differences showed that local processes of Xiangxi Bay were the main responsible for local asynchrony of nonalgal turbidity limitation levels. Regression analysis further proved that local temporal asynchrony for TSISD and nonalgal turbidity limitation levels were regulated by local dynamics of NVSS, rather than geographical distance. The implications of the present study are to emphasize that the results of trophic status obtained from a single environment (reservoir mainstream) cannot be extrapolated to other environments (tributary bay) in a way that would allow its use as a sentinel site.

Asynchrony of spring phytoplankton response to temperature driver within a spatial heterogeneity bay of Three-Gorges Reservoir, China

The increased air temperature is expected to have important driver on spring phytoplankton dynamics. To test whether spatial heterogeneity modifies the synchronous responses of phytoplankton to regional temperature driver, we evaluate temporal coherences for physical factors (temperature, water stability and non-algal light extinction), nutrients (nitrogen, phosphorus and silicon), and biomass and density of phytoplankton by Pearson correlation analysis and synchrony for phytoplankton community dynamics by Mantel test and nonmetric multi-dimensional scaling (NMS), during spring bloom (February 23–April 28, 2005) within Xiangxi Bay, a high spatial gradient bay of Three-Gorges Reservoir (China). The high level of temporal coherences for surface water temperature ( r = 0.946, p < 0.01) and relative water column stability ( r = 0.750, p < 0.01) were found between pair sites (A and B), in which the increase trends occurred with increase in regional air temperature during the study period. However, the low synchrony for phytoplankton dynamics were indeed observed between Site A and B, especially for the density of common dominant taxa ( Cyclotella spp.: r = 0.155, p = 0.388) and community structure (Mantel test: r = 0.351). Moreover, the local habitat characteristics such as nutrient (nitrogen and phosphorus) and non-algal light extinction showed low levels of temporal coherence. It indicated that local community of phytoplankton varies rather independently within the single lentic bay with high spatial heterogeneity and that dispersal of algal organisms among locations cannot overwhelm out these local dynamics. Contrary to many studies, the present results argued that, in a small geographic area (i.e., a single reservoir bay of approximately 24 km length), spatial gradients also may influence spring phytoplankton response to regional temperature driver.

Weekly dynamics of phytoplankton functional groups under high water level fluctuations in a subtropical reservoir-bay

In reservoirs, water level fluctuations strongly influence phytoplankton development. However, studies on the response of phytoplankton in the reservoir-bay to water level fluctuations are very scarce, especially in the highly dynamic reservoir system, for instance, the Three Gorges Reservoir (TGR) on the Yangtze River in China. Therefore, we carried out weekly monitoring in a typical tributary bay-Xiangxi Bay of the TGR from March 2008 to March 2009, to analyze the dynamics of phytoplankton functional groups, as well as their response to the water level fluctuations and other environmental conditions. The phytoplankton functional groups G (short, nutrient-rich water columns with high light and without nutrient deficiency), M (dielly mixed layers of small eutrophic, low latitude with high insolation and without flushing and low total light) and Lo (summer epilimnia in mesotrophic lakes with segregated nutrients and without prolonged or deep mixing) were the most important in biomass, mainly represented by Pandorina morum and Eudorina elegans, Microcystis aeruginosa, Peridiniopsis niei and Ceratium hirundinella, respectively. The dominant functional groups had close relationships with the water level fluctuations, light and nutrient, etc. Principal components analysis and redundancy analysis indicated that phytoplankton functional groups in Xiangxi Bay were restricted by the mixing regime and other abiotic variables under the influences of the mixing regime. In Xiangxi Bay, the water level fluctuation showed significant correlations with many physicochemical variables, including the mixing depth (r = 0.97, p < 0.001) and the relative water column stability (r = -0.80, p < 0.001). The study implied that water level fluctuations had complex influence on environmental changes and selecting for phytoplankton functional groups in a highly dynamic reservoir-bay. The important characteristics of the dominant phytoplankton functional groups in Xiangxi Bay were also discussed.

Longitudinal patterns of phytoplankton distribution in a tributary bay under reservoir operation

To investigate longitudinal patterns of phytoplankton distribution influenced by reservoir operation, weekly surveys were carried out in the mainstream of the Yangtze River and Xiangxi Bay of the TGR from February, 2008 to February, 2009. Using cluster analysis based on chlorophyll a, transparency, and relative water column stability, the longitudinal patterns in different water level fluctuations periods were identified. The mainstream zone, lacustrine zone, transitional zone and riverine zone could be successively distinguished from the mouth to the end of Xiangxi bay, but distinct differences existed in different periods, including the location and extent of each zone. In Periods I (spring) and II (summer and early autumn, flood season), the mainstream zone was relatively short, while in Periods III and IV (autumn and winter seasons), it became longer. At the end of Xiangxi Bay, it presented characteristics of a transitional zone in Periods I, III, and IV, while in Period II it could be recognized as the riverine zone. The correlations among chlorophyll a, transparency, and relative water column stability differed from the Yangtze River to longitudinal zones of Xiangxi Bay, influenced by various features of these regions.

Phytoplankton variation and its relationship with the environment in Xiangxi Bay in spring after damming of the Three-Gorges, China

Following the completion of the Three-Gorges Dam, there was a strong spring phytoplankton bloom in Xiangxi Bay of Three-Gorges Reservoir. However, our knowledge of relationship between spring phytoplankton bloom and environmental factors was still limited. In this study, phytoplankton species composition, biomass, chlorophyll a concentration and environmental factors at two sampling sites in Xiangxi Bay were investigated during 25 March to 18 May 2007. The Xiangxi Bay was eutrophic with the lowest values of total nitrogen and total phosphorus being 0.80 and 0.07 mg/L, respectively. A total of 66 algal taxa belonging to seven phyla and 45 genera were identified. Peridiniopsis niei Liu was the most abundant species which preferred standing water. Canonical correspondence analysis and correlation analysis revealed that nitrate was significantly associated with phytoplankton growth. The phytoplankton chlorophyll a concentration was correlated significantly negatively with nitrate concentration, and nitrate concentration was very low during bloom periods. Heavy rainfall was the main reason of phytoplankton chlorophyll a concentration and biomass decreasing and blooms disappearing. In addition, heavy rainfall also brought more nitrate into the Bay which provided sufficient nitrogen source for blooms occurring again.

Influence of the impounding process of the Three Gorges Reservoir up to water level 172.5 m on water eutrophication in the Xiangxi Bay

The water level of the Three Gorges Reservoir (TGR) rose to 172.5 m in two stages from September to November, 2008. The hydrodynamic parameters and water quality parameters (e.g. flow velocity, temperature, turbidity and nutrient content) had been continuously monitored to reveal the influence of the impounding process of the TGR on the water eutrophication in the Xiangxi Bay (XXB). The maximal daily rise of the water level of the TGR was about 2.38 m. The water exchange between the mainstream (the Changjiang River) and the XXB of the TGR was enhanced by density current, and its main characteristics were a density-stratified flow in different directions. The water exchange reduced the differences of the water quality parameters (for example, temperature, turbidity, nutrient level, pH, etc.) between the mainstream and the XXB during the impounding process. Meanwhile, the water stability index in the mainstream remained small but it was lowered in the XXB. The main causes included the dilution by water of lower concentration of the chlorophyll a (Chl. a) entering from the mainstream, the decline of the water stability, and the increase of suspended silt, which induced the Chl.a to decrease in the XXB during the impounding process. The grade of the water eutrophication changed from middle eutrophic level to mesotrophic state in the process. However, as available nutrients were imported into the XXB from the mainstream during the impounding process the risk of water eutrophication increases in the XXB in the future.