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Anthropogenic nutrient input stimulates surface eutrophication and algal blooms and thus exacerbates oxygen depletion and hypoxia in aquatic ecosystems. However, existing studies have paid little attention on the understanding of the feedback relationships among oxygen consumption, algal succession and greenhouse gas effect after river damming, particularly in Three Gorges Reservoir. This field work investigated the thermal stratification, water quality and phytoplankton in four tributaries (Pengxi Bay (PXB), Daning Bay (DNB), Shennong Bay (SNB), and Xiangxi Bay (XXB)) of the Three Gorges Reservoir (TGR) in spring (April 25-28, 2022), and explored the drivers of hypoxia and its feedback response to algal blooms and methane (CH4) emissions. The results showed that during the observation period, DO, thermal stratification, and algal blooms were more severe in PXB. The high Chemical Oxygen Demand (CODMn) (11.27 mg•L-1) and Dissolved Organic Carbon (DOC) content (7.56 mg•L-1) in PXB were the main causes of the progressive hypoxia. Furthermore, stronger density stratification in PXB inhibited the vertical supply of DO and accelerated bottom hypoxia. But the downslope density current replenished bottom DO in the upper reaches of the DNB, SNB, and XXB. The CH4 concentration was generally higher in hypoxia zone, especially in where cyanobacteria are the dominant species (eg., PX03 and PX04). Dam construction led to the succession of algal-dominant species, exerting a significant influence on the river carbon cycle. The expansion of cyanobacterial blooms and the consequence of metabolic substrates increase are gradually increasing oxygen deprivation, even hypoxia, and enhancing CH4 production in rivers. These findings suggest that the succession of algal-dominant species caused by river damming has a pronounced positive feedback effect on reservoir hypoxia processes and greenhouse effect. [ABSTRACT FROM AUTHOR]

Objective: Algal blooms occurred in some sections of Shennong bay after impounding of Three Gorges Reservoir., Methods: Related environmental and hydrodynamic factors were monitored during the period of algal blooming season in 2014 (March 20, April 13, May 23) in Shennong bay, Three Gorges Reservoir. To study succession of planktonic algae, water stable coefficient, euphotic depth and mixed layer depth were used to analyze stratification and hydrodynamic characteristics., Results: We identified 6 phyla, 38 species (genera) planktonic algae. The sensitive area of algal bloom was at SN05 (677.677×105 cells/L) and SN06 (716.761×105 cells/L), and the planktonic algae biomass during this period was significantly different (ANOVA, p<0.05). Moderate water temperature, adequate nutrients, weak stratification and poor mixing promoted the rapid growth and breakout of the diatom bloom with Cyclotella spp. as the dominant species in March. Further increase of water temperature, stronger stratification and decrease of dissolved silicate and mixing layer restricted the diatom growth. Chlorella spp. and Chlamydomonas spp. grew better in shallow mixed layer with rich nutrients and evident stratification. Then Chlorophycean bloom broke out with Chlorella spp. as the dominant species and Chlamydomonas spp. the next-dominant species. High biomass maintained in April. In May, algal bloom gradually vanished due to sharp fluctuation of water level and increase of velocity. Monitored maximum water velocity was 0.1141 m/s at 2 m depth, exceeded an optimal flow rate perfect for growth of planktonic algae., Conclusion: Stratification and hydrodynamic characteristics had important effect on planktonic algae under the condition of adequate nutrients. Velocity became the main factor that inhibited the growth of algae in Shennong bay in pre-flood falling stage of the Three Gorges Reservoir.