Global elevation of algal bloom frequency in large lakes over the past two decades.

The recent increase in algal blooms in lakes, potentially exacerbated by climate warming, is of global concern. However, a spatially and temporally detailed characterization of algal bloom trends at a global scale has been lacking, posing challenges to definitively attribute the increase trend to warming as a primary driver. Here, we used daily MODIS satellite observations from 2003 to 2022 to analyze algal bloom trends in 1956 large freshwater lakes worldwide. Among these lakes, 620 have experienced algal bloom events in over half of the years during the past two decades, with an upward trend in bloom frequency observed in 504 lakes. This trend is particularly prominent in subtropical regions and has become most pronounced after 2015. The global median annual bloom frequency has significantly increased at a rate of +1.8%/yr over the past two decades, showing a significant correlation with air temperatures (r ² = 0.43, P < 0.05). Furthermore, in 44.8% of the bloom-affected lakes, we observed a strong correlation between air temperature and bloom frequency. Our study helps clarify the factors contributing to the global expansion of algal blooms and emphasizes the urgent need to recognize and address this growing environmental challenge within the context of climate warming.