Trophic status of a shallow lake in Inner Mongolia: long-term, seasonal, and spatial variation

•Freezing drives trophic levels to rise during the glacial period.•Water supplements can reduce the trophic level of the lake.•Nutrients migrate from the ice body into the water during the winter.•The TLI in winter lakes is higher than in the other three seasons. The study of lake-trophic status drivers and their variable effects over space and time can assist in the management of lake eutrophication. Winter dynamics in lake trophic status are rarely evaluated, yet they could vary drastically from the more commonly sampled summer patterns. This represents a key blindspot in our knowledge because ice sheets affect nutrient distribution (e.g., nitrogen and phosphorus), light infiltration (translucency), and phytoplankton propagation (Chl-a concentration). We examined long-term (2011–2020) changes in the trophic status (trophic level index; TLI) of Lake Wuliangsuhai, a shallow lake situated in a cold and arid region, using Generalized Additive Models (GAMs). The results suggested a significant decrease in eutrophication of Wuliangsuhai Lake over the study period, with TLI decreasing from 61.7 (moderately eutrophic) in 2011 to 49.9 (mesotrophic) in 2020. TLI was significantly higher over winter (p summer (54.7) > autumn (54.2) > spring (54.0). TLI of the ice-covered period (58.7) significantly exceeded that of the ice-free period (54.3) (p