Harvesting cyanobacterial blooms can remove algae from a lake, but the harvested sludge may continue to release methane. A Phys.org report on a study of Lake Dianchi in southwestern China says the amount released depended in part on how wet the sludge remained. In the experiments, reducing its moisture content from 93% to 78% cut methane emissions by half.
The work examined an often overlooked stage of bloom management: what happens after algae are skimmed from the water. The report describes a research collaboration involving two Chinese research institutions and the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), including IGB researcher Hans-Peter Grossart. The study’s publication details list Shenghe Cai and colleagues as authors and identify the Journal of Cleaner Production as the journal.
The researchers conducted two experiments to investigate methane production in Lake Dianchi and in harvested cyanobacterial sludge. One tested different sludge moisture levels. The reported 15-percentage-point reduction—from 93% moisture to 78%—was associated with a 50% reduction in methane emissions under those experimental conditions. That is a result from the reported experiments, not a guarantee of the same reduction at every lake or in routine sludge treatment.
Moisture also mattered as the sludge decomposed. The report says wetter sludge produced significantly higher methane emissions, while bacterial communities shifted toward species associated with methane production, including Methanosarcina and Methanobacterium. Together, those observations point to the wet material left after harvesting as a potential source of emissions in its own right.
The study also reported that greenhouse-gas concentrations were positively correlated with chlorophyll a and water turbidity, and negatively correlated with dissolved oxygen and pH. The authors interpreted these patterns as suggesting that greater algal biomass and related changes in water conditions promote greenhouse-gas emissions. Correlations, however, do not by themselves establish that any one factor caused the emissions.
Cyanobacterial blooms can create water-quality problems as large numbers of cells die and decompose, consuming oxygen; the organisms can also produce toxins. Mechanical skimming is used in China as a rapid-response measure during blooms and is increasingly used in some lakes elsewhere to improve water quality and protect drinking-water supplies. The report says that removing the bloom does not, by itself, prevent greenhouse-gas emissions: both affected lakes and harvested sludge can release them.
The findings make post-harvest handling part of the emissions question. Grossart said the sludge should be dewatered more quickly or stored wet for less time. The experiments support that as a direction for managing sludge, while the report does not specify a universally effective drying schedule or quantify how much emissions would change across different lakes and operating conditions. The study’s central implication is that bloom removal and sludge treatment need to be considered together when assessing greenhouse-gas releases.
<a href="https://phys.org/news/2026-11-drying-harvested-blue-green-algae.html">Source: Phys.org report on methane emissions from harvested cyanobacterial sludge</a>