Mechanical harvesting of cyanobacterial blooms shifts greenhouse gas potential to post-harvest sludge in a hypereutrophic lake

Abstract

Cyanobacterial blooms are increasingly managed through large-scale mechanical harvesting, but greenhouse gas (GHG) production from moisture-rich cyanobacterial sludge (CS) remains poorly constrained. We conducted two controlled experiments using natural cyanobacterial biomass and harvested CS from Lake Dianchi, a hypereutrophic lake in southwestern China, to assess GHG accumulation in bloom-affected lake water and post-harvest CS. In the biomass-gradient experiment simulating natural cyanobacterial concentrations, dissolved CH 4 , CO 2 and N 2 O increased significantly with cyanobacterial biomass ( p < 0.001). Maximum GHG concentrations in the highest-biomass treatment were 1.91-, 2.95-, and 1.94-fold higher than those in the lowest-biomass treatment, respectively. Dissolved GHG concentrations were positively associated with chlorophyll a (Chl- a ) and turbidity, but negatively associated with dissolved oxygen (DO) and pH, indicating that bloom biomass accumulation and associated physicochemical shifts promoted GHG accumulation in lake water. In the moisture-manipulation experiment with harvested CS, CH 4 release increased significantly with sludge moisture, reaching 2.3-fold higher levels at 93.17% moisture than at 78.21%, whereas N 2 O peaked at lower moisture levels. Sludge decomposition shifted archaeal communities toward methanogen dominance, with Methanosarcina and Methanobacterium becoming predominant. Structural equation modeling indicated that moisture-driven GHG responses were associated with shifts in sludge organic and nitrogen pools, particularly the coupled variation of dissolved total nitrogen (DTN), volatile solids and protein. These findings demonstrate that mechanical harvesting transfers part of the bloom-derived GHG production potential to post-harvest CS, where high moisture favors CH 4 accumulation. Post-harvest CS handling, including rapid dewatering, shorter wet storage, and low-carbon treatment, should therefore be integrated into lower-carbon bloom management.