Aim: Freshwater lake ecosystems in semi-arid regions are highly fragile and complex. However, the driving forces behind changes in temperate lake ecosystems at a regional scale in China remain largely unclear, primarily due to the lack of multidecadal to centennial regional studies. This study focuses on Hulun Lake, the largest freshwater lake in northern China's semi-arid region, with the aim of elucidating the characteristics and driving mechanisms of freshwater lake ecosystems in such areas. Location: Hulun Lake, the largest freshwater lake in northern China's semi-arid region (48.55 degrees similar to 49.33 degrees N, 116.97 degrees similar to 117.81 degrees E). Time Period: From the 1840s to 2010s AD. Major Taxa Studies: Zooplankton Cladocera. Methods: High-resolution multiproxy palaeoecological data (including biological and geochemical indices), human activities parameters and observational datasets from the lake basin are used to explore ecosystem shifts and Cladocera responses in Hulun Lake, located in a semi-arid region, over the past 170 years. The hypothesis that changes in the zooplankton Cladocera community composition within the ecosystem are driven by synergistic interactions among multiple factors was tested using effective approaches based on generalised additive models (GAM) and redundancy analysis (RDA). Results: The results indicate that both internal factors (such as lake sedimentary environment, fish catches, lake water level, organic matter and nutrients) within the lake and external factors (including population, urbanisation rate and livestock husbandry) in the surrounding basin have been key drivers influencing the structure of Cladocera in Hulun Lake over the past 170 years. The originally predominant littoral Cladocera species (Chydorus sphaericus. sl.) were replaced by planktonic (Bosminidae) since the 1980s accompanied by a decline in diversity, which corresponds with the process of eutrophication and pollution in the lake. Main Conclusions: The dynamics of the Cladocera assemblages indicate that the lake ecosystem is vulnerable to further imbalance since the 2000s due to the ongoing synergistic effects of declining fishery catches, increasing livestock numbers, rising populations, urbanisation rates and declining water levels in semi-arid regions.