Assessing the impact of long-term changes in climate and atmospheric deposition on a shallow alpine lake from southeast Tibet

Regional warming and atmospheric nitrogen deposition have been widely recorded to impact remote catchments and alpine lakes; however, their independent roles and interactions have rarely been identified. Here, we combined down-core analyses of sedimentary mercury (Hg) and aluminum (Al) with multiple proxies (i.e. nitrogen stable isotope, chlorophyll a pigments, diatoms) for a radiometrically-dated sediment core of an alpine lake in southeast Tibet to track the atmospheric deposition of pollutants, and to examine possible effects of climate and catchment forcing over the past three centuries. The sediment data revealed that airborne deposition of Hg was recorded from the similar to 1860s, with an accelerating increase in anthropogenic Hg flux since the similar to 1960s. A synchronous decrease in reconstructed lake-water TOC indicated that acid deposition may have affected lake-water carbon concentrations and impaired catchment export of decomposed organic matter (OM). A moderate depletion of bulk sediment delta N-15 started from the similar to 1820s, but was followed by an enriching trend after the similar to 1970s. This positive shift of delta N-15 was associated with elevated sedimentOMand decreased catchment runoff of clastic materials (as inferred by Al). Sediment OM content displayed an accelerating increase from the similar to 1960s, with an increased input of autochthonous sources (i.e. lower bulk sediment C: N ratios), such as algae (as inferred by sedimentary chlorophyll a pigments). Meanwhile, climate warming and decreased lake-water TOC enhanced the production of algae, which was characterized by a more enriched delta N-15 signal than that of allochthonousOM. Furthermore, atmospheric acid deposition was significantly related to diatom assemblage changes, with an increase in acidophilous taxa. Our sediment evidence revealed the dominating impact of climate and catchment processes on lake-water chemistry and algal shifts in the context of atmospheric nitrogen deposition, and highlighted an increasing link of external forcing with in-lake processes in enriching sediment delta N-15 signal over the last few decades. (C) 2018 Elsevier B.V. All rights reserved.