The annual green tide
Chaohu Lake is one of China's five largest freshwater lakes, with a surface area of about 780 square kilometers. For decades, this important body of water has been plagued by severe eutrophication, a process driven by excessive nutrient enrichment. Each year from May to November, the lake experiences vast harmful algal blooms (HABs), primarily composed of cyanobacteria. The dominant species are Microcystis aeruginosa and Dolichospermum flos-aquae, which thrive in the nutrient conditions. These blooms turn large portions of the lake into a thick, green soup, depleting the water of oxygen and creating hypoxic zones where fish cannot survive.
The cause is the massive influx of nitrogen and phosphorus from the surrounding watershed. These nutrients originate from industrial effluent, untreated domestic sewage, and agricultural runoff containing fertilizers. The water quality is often rated Class IV or V, the worst grades in China's national standards. Total phosphorus content in the water has been measured at 0.131 mg/L and total nitrogen at 2.04 mg/L, creating an ideal nitrogen-to-phosphorus ratio for cyanobacterial growth. The blooms are most frequent and severe in the western part of the lake, which receives the highest nutrient loads.
Finding the sources
finding the specific pathways for this pollution is a complex task. The Chaohu Lake basin is fed by 33 inflowing rivers, each contributing to the nutrient load. Decades of monitoring and data analysis have been necessary to trace the primary culprits. Studies have shown that rivers flowing through urban areas carry significantly higher concentrations of phosphorus and nitrogen than those in rural areas.
The Nanfei and Paihe rivers, which flow through the major city of Hefei, have been identified as bearing substantial nitrogen and phosphorus loads into the lake. Research comparing urban and rural rivers found that total phosphorus levels in urban-adjacent rivers were, on average, 0.684 mg/L, compared to just 0.120 mg/L in rural rivers. Total nitrogen was 11.0 mg/L in urban rivers versus 2.09 mg/L in rural ones. This data allows for targeted management strategies, focusing on upgrading wastewater treatment facilities and controlling runoff in the most heavily impacted river systems. Between 2014 and 2018, studies calculated that the lake received external inputs of 1.3 grams of phosphorus per square meter per year, retaining about 48% of it annually.