Diatom-inferred water pH variability in response to climate change and acid deposition in subtropical peatlands

Lake acidification emerged as one of the most pressing environmental issues in the 20th century, while peatland acidification is less investigated. Diatom assemblages in one hundred fourteen surface samples of eleven peatlands in South China and in peat cores of two peatlands covering a 200-year period were investigated. Ordination analyses revealed that water pH was the most important variable influencing diatom distribution in surface samples. Accordingly, a diatom-based pH transfer function was developed using a weighted averaging model with tolerance downweighting and inverse deshrinking, and applied to diatom records of two ²¹⁰Pb-dated sediment cores from Dajiuhu Peatland (Hubei Province) and Niangniangshan Peatland (Guizhou Province). Before the 1950s, declines in acid-tolerant diatom taxa coincided with rising diatom-inferred pH in both peatlands, indicating a regional shift toward less acidic conditions. This regional shift probably resulted from warming-accelerated bedrock weathering and alkalinity generation. Thereafter, the expansion of acid-tolerant diatoms mainly responded to enhanced atmospheric acid deposition, suggesting that its impacts outweigh those of climate warming. Temporal changes in diatom assemblages showed similar trends in the two distant peatlands, suggesting that acidification might be a widespread phenomenon in subtropical peatlands of South China after the 1950s. In spite of recent reductions in atmospheric deposition from 2010, diatom-inferred water pH continued to decline, indicative of delayed recovery from anthropogenic acidification in these headwater peatlands. The results provide novel insight into the timing and extent of peatland acidification in South China, and can inform sustainable management of semi-aquatic ecosystems under a changing environment.