Spatio-Temporal Variations in Phytoplankton Communities in Sediment and Surface Water as Reservoir Drawdown—A Case Study of Pengxi River in Three Gorges Reservoir, China

Abstract

The resting stages of phytoplankton are usually regarded as the seed bank and source of harmful algal blooms because of the recruitment of phytoplankton from sediment to the water column under suitable environmental conditions. Information about resting stages of phytoplankton is abundant in shallow lakes and littoral sea; yet, studies on river–reservoir systems are rare. The river–reservoir continuum shows a unique structuring of longitudinal gradients of hydrological and hydrodynamic conditions. We hypothesized that the seed bank and algal blooms in reservoirs are influenced by the hydrodynamic conditions of each reservoir. We used Illumina Miseq sequencing to examine the spatio-temporal variation in the phytoplankton community in the sediment as reservoir drawdown and in surface water during algal blooms in Pengxi River, a tributary of China’s Three Gorges Reservoir. The results show that the cyanobacteria community in sediment is significantly influenced by temperature, total carbon, maximum flow velocity, and total phosphorous, the eukaryotic phytoplankton community in sediment is significantly influenced by total phosphorous, temperature, total carbon, maximum flow velocity, and total nitrogen. Additionally, the dominant species in sediment is significantly different from that in surface water during algal blooms. Our results suggest that the dominant species in surface water during algal blooms is more influenced by the environmental factors and hydrodynamic conditions in the water column than the seeds in the sediment. These findings are fundamental for further research on the influence of hydrodynamic conditions on algal blooms in artificially regulated river-reservoir systems.

Biogeochemical Processes of Dissolved Nitrogen in the Backwater Zone of a Tributary in Three Gorges Reservoir, China: Implications from the Hydrologic Processes and Isotopic Tracing

“…Up to about 77.3% of exogenous sources contributed to the DON concentration in sampling sites (S4 and S5) near the estuary, which may be the result of sediment release and organic nitrogen deposition. These sites were usually accompanied by lower flow velocity, which was affected by the hydrographic regulation of the TGD. , Previous studies also showed that nitrogen load in the bottom water was positively correlated with flow velocity . For another perspective, the average DON loss rates of the surface, middle, and bottom water were estimated to be 8.0, 12.2, and 18.6%, respectively.…”

“…These sites were usually accompanied by lower flow velocity, which was affected by the hydrographic regulation of the TGD. 35,41 Previous studies also showed that nitrogen load in the bottom water was positively correlated with flow velocity. 75 For another perspective, the average DON loss rates of the surface, middle, and bottom water were estimated to be 8.0, 12.2, and 18.6%, respectively.…”

“…The upper reach of the Xiaojiang River is dominated by a karst landform, and groundwater is concentrated along the dark river . The elevations of middle and lower reaches of the Xiaojiang River are between 150 and 1500 m. The study area belongs to a subtropical humid monsoon climate, and the annual temperature is between 10.8 and 18.5 °C . About 75% of the rainfall is concentrated in May–September with the range between 1100 and 1500 mm .…”

“…40 The elevations of middle and lower reaches of the Xiaojiang River are between 150 and 1500 m. The study area belongs to a subtropical humid monsoon climate, and the annual temperature is between 10.8 and 18.5 °C. 41 About 75% of the rainfall is concentrated in May−September with the range between 1100 and 1500 mm. 42 The land reclamation and utilization rate of the Xiaojiang River basin is high, in which the coverage rates of farm land, forest, and grassland are about 56, 32, and 12%, respectively.…”

Microbial community day-to-day dynamics during a spring algal bloom event in a tributary of Three Gorges Reservoir

“…After the rain fall event and water transparency would increase as a result of particulate sedimentation and advection, the left over seed algae have the potential to initiate another bloom (e.g. Michalak et al, 2017;Ouyang et al, 2021). Thus, both intrinsic dynamics within the primary trophic level as well as extrinsic events, such as the rainfall, have the potential to regulate the timing and duration of HAB events.…”