Using temporal coherence to determine the responses of water clarity to hydrological processes in a giant subtropical canyon-shaped reservoir (China)

Investigating whether different locations of a large lake exhibit synchronous or asynchronous fluctuations in limnological characteristics is essential to lake management. If synchronous fluctuations (termed as “temporal coherence”) among sites are found to prevail, regional factors are likely to be the drivers; otherwise, local factors may be dominant. Here, this idea is tested at two spatial scales: the large scale being the mainstream of Three-Gorges reservoir (TGR, China) and the small scale being Xiangxi Bay of TGR. Data of transparency and chlorophyll a were gathered monthly at 26 sampling sites from July 2003 to July 2006 to evaluate temporal coherence for water clarity. High spatial and temporal variation in transparency (4-510 cm) and chlorophyll a (0.1-190.2 μ g/L) mark the system. For the mainstream of TGR, significant high temporal coherence of water clarity among sites (correlation analysis: 0.872 ≤ r ≤ 0.991, mean r = 0.960) was detected and the among-site correlations showed no significant relationship with geographical distance (Mantel test: r = −0.186, p > 0.05) or chlorophyll a (Mantel test: r = 0.249, p > 0.05). By contrast within Xiangxi Bay, lower synchronous fluctuations (correlation analysis: 0.396 ≤ r ≤ 0.971, mean r = 0.785) occurred, and the among-site correlations decreased with an increase in geographical distance (Mantel test: r = −0.715, p < 0.01) and a decrease in temporal coherence of chlorophyll a (Mantel test: r = 0.893, p < 0.01). Seasonal dynamics of water clarity were very well explained by inflow discharges (regression analysis: 0.771 ≤ R2 ≤ 0.906) and water residence time (0.782 ≤ R2 ≤ 0.918) in the mainstream, but not in Xiangxi Bay. The high coherency of the dynamics of water clarity in TGR mainstream is likely caused by regional hydrological processes driven by subtropical monsoon climate. For Xiangxi Bay, local processes such as phytoplankton dynamics may override the regional effects of hydrological processes, because the water clarity of this bay is strongly determined by phytoplankton bloom dynamics.