Nutrient pumping triggers algal overgrowth in oligotrophic water bodies

Algal overgrowth is a global environmental concern and is often considered to occur primarily in eutrophic water bodies. However, this phenomenon has been observed in oligotrophic water bodies during water diversion operations, and the underlying mechanisms remain unclear. Here, in the world's largest water-transfer canal (a typical oligotrophic water body), we discovered that pulsed formation of high-nutrient environments triggered rapid algal proliferation, with the maximum algal cell density (ACD) reaching nearly 2 × 10⁷ cells/L. The algae function as a nutrient pump, persistently drawing nutrients (C/N/P) from both the water column and atmosphere, and these nutrients are subsequently deposited in localized slow-flow areas as the algae senesce. Following hydrological fluctuations (daily flow variations > 10%), these locally accumulated nutrients are rapidly released into the overlying water column, triggering algal overgrowth again. If left uncontrolled, the imbalance in nutrient cycling is expected to gradually increase, further promoting extensive algal overgrowth. This study reveals the mechanisms behind algal overgrowth in oligotrophic water bodies and provides a fresh theoretical foundation for ensuring water quality in large-scale water diversion projects.