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

Spatial gradient and seasonal variation of trophic status in a large water supply reservoir for the South-to-North Water Diversion Project, China

Carlson trophic state indices (TSIs) have been frequently employed in the water quality literature. The current study reports the spatial distribution and seasonal variation of trophic status in Danjiangkou Reservoir (DJKR), which is a large water supply reservoir of the South-to-North Water Transfer Project in central China. The calculation of TSICHL indicated a mesotrophic state (40 <= TSICHL < 50) for most regions of the reservoir, whereas a eutrophic state was indicated if the assessment was based on either TSITP (calculated from total phosphorus) or TSISD (calculated from Secchi depth). As indicated by Carlson's two-dimensional deviations, factors other than phosphorus limited algal growth in January and May when non-algal particles affected light attenuation. However, phosphorus was the controlling factor of large algae blooms in July and November in 2007. TSISD had positive correlations with TSICHL and TSITP. It was found that total phosphorus, SiO2-Si and SD were the major factors regulating the trophic state (TSICHL) of DJKR. Light attenuation and SD were mainly affected by non-algal particles