High-resolution satellite observations reveal extensive algal blooms in both small and large lakes in China

Algal blooms in large lakes (such as Taihu, etc.) in China have attracted widespread concern, while the bloom patterns in small lakes (1–10 m², accounting for 74.3% of the lakes in China) remain unknown. Using high spatiotemporal resolution Sentinel-2 MultiSpectral Instrument (MSI) observations during 2019–2021, we investigated algal blooms in 17,549 lakes in China (with surface areas ranging from 0.1 to 2398.3 km²). All the examined lakes were divided into three classes (small: <10 km², medium: 10–100 km², and large: >100 km²) to understand the algal bloom patterns in lakes of different sizes. The results demonstrated that the bloom phenologies and intensities of all the studied lakes exhibited pronounced latitudinal gradients. The bloom months (BMs) increased with decreasing latitude, while the bloom start months (BSms) gradually delayed with increasing latitude. The difference in bloom intensity in lakes of different sizes was not pronounced, small lakes showed comparable bloom severities as large lakes, with a median of approximately 5 BMs per year. Many small lakes (i.e., <10 km²), especially some urban lakes, which have rarely received attention, are also experiencing severe bloom problems. Driving force analysis suggested that the algal blooms were primarily influenced by temperature either on a national scale or across different lake groups. These results could provide baseline information for the water quality management of lakes in China.