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

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 < 0.05), with the rank of seasons in terms of TLI of: winter (58.7) > 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 < 0.05). There were also significant spatial differences in TLI (p < 0.05). Factors driving the spatial and temporal heterogeneity in lake trophic levels included the water supplement, water depth, water temperature, external inputs of nutrients, hydrodynamic conditions (flow velocity), distribution of phragmites, and the location of the main diversion channel.