Yaoyang Xu

Daily Dynamics of Nutrients and Chlorophyll a during a Spring Phytoplankton Bloom in Xiangxi Bay of the Three Gorges Reservoir

ABSTRACT We studied the daily dynamics of nutrients (total phosphorus [TP], total nitrogen [TN], and dissolved silicate [SiO2]) and chlorophyll a (chl a) during a spring bloom in Xiangxi Bay of the Three Gorges Reservoir in year 2005. According to the daily dynamics of chl a, the bloom occurred in two stages (23 February–25 March and 26 March–28 April). The concentration of SiO2 decreased at different layers of the water column with the development of the bloom. However, the decrease of SiO2 in the layers with high concentration of chl a was more dramatic than in the layers with low concentration of chl a. The concentration of TP was lowest value a few days after the peak of chl a during the first bloom period, and the lowest value of TN was found a few days after the peak of chl a during the second bloom period. Correlative analyses indicated that SiO2 and TP were limiting factors in the first bloom period, and SiO2 and TN were limiting factors in the second bloom period.

Effects of Reservoir Mainstream on Longitudinal Zonation in Reservoir Bays

ABSTRACT Spatial longitudinal zonation in reservoir bays is poorly documented, and most published papers considered that longitudinal zonation in bays is similar to that in reservoirs. Our results from analyses of the benthic macroinvertebrate community in the bays of the Three-Gorges Reservoir, China, showed that a typical bay contains four distinct zones—one more zones than a reservoir. This newly distinguished zone lies along the mouth stretch of a reservoir bay, and we call it a mainstream zone because it is disturbed by the reservoir mainstream. The mainstream zone is characterized by a lower standing crop and a more unstable macroinvertebrate community than in the lacustrine zone. Longitudinal zonation of reservoir bays is related to their lengths, and lacustrine zones develop only where the bay is sufficiently long. Similar to reservoirs, longitudinal zonation in bays is also dynamic and is to some extent influenced by the ages of bays and seasons.

Spring Diatom Blooming Phases in a Representative Eutrophic Bay of the Three-Gorges Reservoir, China

ABSTRACT We investigated dynamics of the phytoplankton community and abiotic factors in Xiangxi Bay of the Three-Gorge Reservoir, China, by daily sampling a specific site during a spring algal bloom (February 23-April 28, 2005). Among the 76 taxa observed, Asterionella formosa and Cyclotella spp. were the dominants, accounting for 47.2% and 29.9% of the total abundance, respectively. We determined the five distinct developing phases of the bloom by analyzing the dissimilarity of physicochemical parameters. Simultaneously, six phytoplankton community groups were distinguished by TWINSPAN classifications. The pattern for algal community succession was similar to that for the bloom phase shift, and the structural complexity of communities significantly decreased over time. Water temperature and silicate were the main factors that related to the development of the bloom and the shifts of the phytoplankton community.

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.

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.