The quality of water in Anhui's Chao Lake, which suffered a massive blue-green algae attack in June, deteriorated again last month, but the local environment watchdog insisted it was only a seasonal problem. Pollution levels in the lake, one of the five biggest freshwater lakes in China, rose from intermediate in July to severe last month, and the quantity of nutrient substances, on which many algae live, grew from light to intermediate, according to a monthly water quality survey posted on the Anhui Environmental Protection Bureau's website. The water quality of several rivers that flow into the lake had also worsened, the report said. But Xiao Fu , director of the bureau's water department, said the deterioration in water quality was mainly due to heavy summer rain, which had flushed large amounts of phosphorus into the lake. 'The earth around Chao Lake is rich in phosphorus, and there are several phosphorus mines in the upper reach of a river that flows into the lake,' Mr Xiao said. 'The continuous rain in June and July flushed the contents into the lake, which made the water quality fall to category IV [heavily polluted].' Several big lakes, including Tai Lake in Wuxi, Jiangsu , and Chao Lake, saw severe algal blooms in June and July, sparking public demands for intensified efforts to battle worsening water pollution on the mainland. But Mr Xiao played down the impact of the pollution , saying it was not affecting the daily lives of the residents. 'Chao Lake is not the drinking water source for Hefei and the eastern part of the lake, which is the water source for [the city of] Chaohu , was not polluted by algae,' he said. 'The algae are affecting the diversity of flora and fauna, but it's not affecting the residents' lives.' Mr Xiao said it was impossible to treat water pollution and eliminate big algal outbreaks in the short term. Anhui is running a two-year test to divert water from the Yangtze River into the lake in an attempt to reduce the severity of algal blooms.