Algal blooms are becoming more frequent and intense in lakes around the world, posing increasing risks to water quality, ecosystem health, and carbon cycling. However, whether stronger blooms also occur earlier or last longer has remained unclear at the global scale.

A new study published in Communications Earth & Environment shows that bloom intensity and bloom timing often change independently across global lakes, revealing a more complex response of lake ecosystems to human activities and climate change than previously understood.

The study was conducted by researchers from the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, together with collaborators from Southwest University. Using two decades of satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS), the team analyzed algal bloom dynamics in 4,085 lakes larger than 20 km² worldwide. The researchers examined both bloom intensity, represented by fractional floating algal cover (FAC), and bloom timing, including the start and end dates of bloom seasons.

The results showed that about 71% of lakes exhibited increasing bloom intensity between 2003 and 2022. In contrast, changes in bloom timing were much more spatially heterogeneous. Some lakes showed earlier bloom onset, while others exhibited delayed onset or shifts in bloom ending. This pattern indicates a partial decoupling between bloom intensity and bloom phenology at the global scale.

The researchers further found that the drivers of these changes differ. Human-related factors, especially population density, agricultural pressure, and economic development, were more strongly associated with increasing bloom intensity. By comparison, climatic factors such as temperature, wind speed, and precipitation played a greater role in regulating bloom timing, particularly in lakes located in cold and temperate regions.

Future projections under different shared socioeconomic pathway scenarios suggest that this divergence may become even more pronounced under continued climate change. Tropical lakes are projected to experience rapid bloom intensification with relatively modest timing shifts, whereas cold-region lakes are expected to show contrasting phenological responses across regions. For example, European lakes may exhibit earlier bloom onset and later bloom ending, while North American lakes may show the opposite tendency.

According to the researchers, these findings highlight that global lake algal blooms cannot be understood solely in terms of increasing intensity. Changes in bloom timing may alter food-web structure, ecological stability, and carbon cycling, even when intensity changes are modest. The study therefore provides an important scientific basis for region-specific lake management and risk mitigation under climate change.