Thirty years ago, the waters of Lake Tai, China’s third-largest lake, were clear of algae. But the lake is surrounded by several high-density cities, including Shanghai, Suzhou and Changzhou, metropolitan areas that have grown rapidly in the past few decades. Rampant sewer dumping and livestock drainage, combined with shifting agricultural practices, allowed the algae blooms to flourish, and now human mismanagement and global warming have entrenched them. ‘‘They love warm, stagnant, nutrient-rich conditions,’’ said Hans Paerl, a professor of marine and environmental sciences at the University of North Carolina, Chapel Hill. Over the past decade, the blooms have significantly expanded, and their season has grown longer. In 2007, the ‘‘pea soup’’ conditions of the lake were so bad, Paerl said, that the cities surrounding the basin ‘‘had green slime coming out of their faucets, and the central government had to bring in drinking water.’’ At least two million people were without fresh water. Image by George Steinmetz. China, 2017.