The Hydrological and Nutrient Conditions Mediate Algal Biomass Response to Compound Heatwave and Drought Events in Large Shallow Lakes

ABSTRACT Compound heatwave and drought (CHWD) events are becoming increasingly frequent, posing significant challenges to aquatic ecosystems. This study assesses the impact of the 2022 CHWD on chlorophyll‐ a (Chl a ) concentrations in 40 large shallow lakes (> 50 km 2 ) in the Chinese Eastern Plains ecoregion. The findings reveal that the impact of CHWD on algal biomass varies depending on hydrological and nutrient conditions in a lake. In rapidly flushing lakes, CHWD did not change Chl a concentration from 32.9 ± 3.8 μg/L in the reference year to 34.1 ± 4.3 μg/L in the CHWD year ( Z = −1.098, p = 0.272), but in long retention lakes Chl a concentration decreased from 35.9 ± 4.1 μg/L to 33.7 ± 4.8 μg/L ( Z = −3.552, p < 0.001). Field‐monitoring data suggest that the Chl a decline in long retention lakes is primarily due to reduced external nutrient inputs from decreased rainfall. In rapid flushing lakes, Chl a shows minimal sensitivity to variations in TN or TP. Consequently, the direct effect of climate warming on algal biomass in these lakes may be limited. The effect of CHWD on long retention lakes is nutrient‐dependent. For example, Lake Gehu, a long retention lake, experienced an overall increase in Chl a , likely because its surplus nitrogen and phosphorus made it less susceptible to drought‐induced reductions in external nutrient loads. This study highlights the complex responses of lake ecosystems to climate extremes and provides insights for managing eutrophication in a changing climate.