NGOUDA DIONE ON the shore of Senegal's Pink Lake, salt farmer Pape Sira Ba has raked in what he fears may be his last harvest. Extreme floods contaminated the 3-square-km lake five months ago, turning its famous waters green and threatening thousands of people who depend on it for their livelihoods. Officially known as Lake Retba, the lake's high salinity and rare microbiome long-fostered algae that turned it a pinkish colour, making it one of the West African country's most visited attractions and under consideration as a UNESCO World Heritage site. Everything changed last September when torrential rains hit the…