20th century climate warming and human disturbance triggered high aquatic production and strong water-column mixing in maar Lake Xiaolongwan, northeastern China

Lake ecosystems in northeastern (NE) China are sensitive to global environmental change, and are currently under threats of eutrophication and hypolimnetic anoxia. However, the lack of long-term records of lake production and anoxia in this region makes it difficult to discriminate the impacts of recent anthropogenic activity on lake ecosystems from natural ecosystem variability. This study investigates varved sediments from remote maar Lake Xiaolongwan, NE China and reconstructs high-resolution changes in lake primary production, anoxia, nutrient cycling and catchment processes over the past 1500 years using hyperspectral imaging inferred sedimentary total chlorophyll-a and bacteriopheophytin-a (Bphe-a), combined with sedimentary iron and phosphorus fractions data. Results show that, prior to discernible human impacts in this area from ∼600 CE to 1900 CE, lake primary production was higher during warm periods and reduced during cold periods. Bphe-a records show that hypolimnetic anoxia persisted regardless of temperature variability. Cluster analysis suggests that lake algal communities and biogeochemical conditions in the twentieth-century warm period are unprecedented and significantly different from any other time over the past 1500 years. This phenomenon mostly results from global warming and stronger local catchment disturbance in the early 1900s, combined with atmospheric pollution after the 1950s. Human-driven climate warming has caused stronger seasonal mixing (due to shortened ice-cover duration) and overall better oxygenation in the lake. This study demonstrates clear anthropogenic impacts to lake ecosytems in a relatively pristine region in NE China. We anticipate that our findings will have implications for understanding the status of aquatic ecosystems in NE China under future interacting stressors of anthropogenic climate warming and pollution.