Dramatic temporal variations in methane levels in black bloom prone areas of a shallow eutrophic lake

Global lakes serve as a key natural source of methane (CH₄) and suffer from increasing hypoxia due to unprecedented anthropogenic activities and climate change. A black bloom is a temporary hypoxia triggered by a longstanding algal bloom, which facilitates CH₄ production by creating reducing conditions and abundant algae-sourced organic carbon. One-year investigations were conducted to examine temporal CH₄ dynamics in the water and sediment pore water in black bloom prone areas (BBPAs) in Lake Taihu, China, where there had been at least two recorded black bloom events. The CH₄ in the water changed significantly with time (p < 0.001), with the highest concentrations appearing in warm months when an abnormal lower dissolved oxygen content was observed at different sites, which were one to two orders of magnitude higher than other months. Compared with the control site, there were significantly higher CH₄ concentrations in BBPA waters (p < 0.001), which was consistent with the higher CH₄ in the sediment pore water. Methane dynamics in the water showed significant positive correlations with temperature, total phosphorus, total nitrogen, ammonia-N, and soluble reactive phosphorus (p < 0.05), but showed a significant inverse correlation with dissolved oxygen (p < 0.01). Redundancy analysis indicated dissolved oxygen made the largest contribution to CH₄ dynamics in the BBPAs. A significant increase in the CH₄ in water will turn BBPAs into temporary hot spots with substantial CH₄ emissions with the appearance of black blooms. The results provide new insights into understanding future CH₄ dynamics under globally prevailing algal blooms and climate change.