Fifty-Year Trends Reveal Reversal from Recovery to Re-eutrophication and Reinforced Anoxia in a Managed Mountain Lake.

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Anoxia in lakes has intensified in recent decades, threatening ecosystem functioning. Yet, the mechanisms driving long-term trends in anoxia intensity and duration are complex, especially in managed ecosystems, where field data are limited. Using a 50-year dataset from a lake affected by both eutrophication and restoration measures, we examined annual oxygen dynamics, assessing the effect of external drivers, such as climate warming and hypolimnetic withdrawal effectiveness, and of in-lake processes influencing anoxia. Breakpoint analysis revealed a major ecosystem regime shift around 1996, reversing the earlier recovery trend. Between 1972 and 1996, both the anoxic factor and hypolimnetic total phosphorus concentrations declined, but both rose significantly afterward, with phosphorus concentrations eventually exceeding pre-restoration levels, despite declining watershed inputs. This reversal coincided with a marked increase in thermal stratification duration, which likely intensified deoxygenation by limiting oxygen renewal in the hypolimnion. Our results also show that higher anoxia levels in 1 year significantly reinforced anoxia in the following year, suggesting a self-sustaining feedback mechanism. In addition, our results provide evidence that anaerobic mineralization is important to this feedback, accumulating reduced compounds that further enhance deoxygenation. Despite management efforts, the intensification of internal phosphorus loading and the accumulation of reduced substances have progressively diminished the effectiveness of the cost-effective hypolimnetic withdrawal system implemented since 1970. Our findings demonstrate how the emergence of reinforcing feedbacks, linking oxygen depletion, internal phosphorus release, and climate-driven stratification, can undermine traditional restoration strategies. This highlights the urgent need for adaptive management that explicitly addresses these interacting mechanisms among oxygen dynamics, nutrient cycling, and climate warming.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10021-025-01003-5.