A recipe for disaster
Lake Taihu, China's third-largest freshwater lake, covers a surface area of approximately 2,250 square kilometers. Despite its size, it is remarkably shallow, with an average depth of just over two meters. This shallowness makes it highly susceptible to pollution. For decades, the lake has been the endpoint for enormous quantities of agricultural runoff, industrial effluent, and untreated sewage from the rapidly growing cities in its basin. This has loaded the water with excessive levels of nitrogen and phosphorus.
These nutrients provide the perfect food for a specific type of cyanobacteria, Microcystis aeruginosa. When conditions are right—typically in the warm, calm months, this bacteria multiplies at an explosive rate, creating a vast, thick, paint-like green scum across the lake's surface. This is algae; Microcystis aeruginosa produces a class of potent liver toxins called microcystins. Ingesting these toxins can cause significant liver damage to both humans and animals.
The situation reached a crisis point in May 2007. A massive, early-season bloom overwhelmed the water treatment facilities for the city of Wuxi, cutting off the primary drinking water supply for more than two million people for at least a week. Residents turned on their taps to find a foul-smelling, greenish sludge. The event showed the environmental cost of unchecked economic growth.
The science of the bloom
The process behind the greening of Lake Taihu is eutrophication—the over-enrichment of a body of water with nutrients. The main sources are phosphorus and nitrogen. Scientific analysis has traced these pollutants to several sources. Diffuse sources, like synthetic fertilizers from farms, account for 90% of the river-exported nitrogen flowing into the lake. Point sources, such as sewage systems and industrial discharge, are responsible for 52% of the phosphorus. The sediment on the lakebed itself acts as an internal source, releasing stored phosphorus back into the water.
Genetic sequencing and metagenomic analysis of the blooms have confirmed the dominance of Microcystis aeruginosa and its direct link to high nutrient concentrations. The data shows more agricultural and urban runoff equals more potent and widespread blooms. Weather also is important. The bacteria thrive in warmer water, with optimal growth occurring between 27-37°C. Calm, sunny days allow the buoyant bacteria to float to the surface, forming the dense mats that starve the water below of sunlight and oxygen.
Following the 2007 crisis, the government initiated one of the largest lake restoration efforts ever attempted, costing over $41 billion. Measures included closing tens of thousands of factories, building hundreds of new wastewater treatment plants, and dredging nutrient-rich sediment. Water has also been diverted from the Yangtze River to help flush the lake and reduce its water replacement period from 300 to 250 days. While the water quality has improved from the lowest official grade to "fairly good," the threat of blooms remains.