Important Role of Biogas and Secretions in the Formation of Biological Fluid Sediment under Cyanobacterial Bloom Biomass Degradation

In eutrophic shallow lakes, cyanobacterial blooms will occur frequently and then accumulate on sediments, leading to the formation of biological fluid sediment. In this study, the formation process of biological fluid sediment was investigated in microcosm experiments by monitoring the variations in gas, sediment physicochemical properties, and microorganisms during 30 days. It was found that variations in sediment height reached the maximum value on day 8 (0.53 cm) with the minimum sediment density (1.04 g cm–3). Meanwhile, the amount of carbon dioxide (CO2) produced through fermentation was larger than methane (CH4) during the initial 4 days. Then, the amount of CH4 drastically increased, resulting in the release of gas from sediments. In addition, a high amount of extracellular polymer substances (EPSs) was produced and led to sediment particle agglomeration, which benefited in maintaining the structure of biological fluid sediment. Through the microbial community analysis, it was found that the strong symbiotic structure of the microbial community in the early stage provided favorable conditions for the formation of CH4 in sediments. The results of this study can clarify the formation mechanism of biological fluid sediment and provide help for understanding the effect of biogas and secretions on the sediment properties in shallow lakes.