Diverse effects of climate changes and anthropogenic activities on sedimentary nitrogen isotope (δ15Nsed.) in lakes across different regions of China since the late Holocene.

Nitrogen plays a vital role in the Earth's systems. Nitrogen isotopes have been widely used in environmental and climatic research. Various biogeochemical processes and nitrogen sources contribute to the sedimentary organic matter. Therefore, environmental implications of sedimentary nitrogen isotope (δ15Nsed.) require further investigation. In this study, we report and compile the δ15Nsed. records from eight lakes in China influenced by either climate changes or anthropogenic activities since the late Holocene to determine the major forcing factors of δ15Nsed.. Our results showed that: (1) During the late Holocene, lacustrine δ15Nsed. was mainly regulated by natural climate changes, such as temperature, precipitation, or both as anthropogenic activities were weak. (2) The impact of gradually increasing anthropogenic disturbances on δ15Nsed. in the different lakes varied. Globally, the declining lacustrine δ15Nsed. values could be attributed to atmospheric reactive nitrogen deposition. However, on the regional catchment scale, the increased δ15Nsed. values in lakes may be related to increased nutrient input and higher primary production caused by increased anthropogenic activities, whereas the rapid decline of δ15Nsed. values in lakes is possibly related to more nutrient input and 15N-depleted organic matter input resulting excessive anthropogenic activities. Our study suggests that the impact of anthropogenic activities on lake ecosystems gradually increased in recent decades, and the amplitudes in variations of δ15Nsed. values in some lakes exceeded those during periods of weak anthropogenic disturbances. If not properly managed, anthropogenic disturbances may outweigh nature (i.e., temperature, precipitation) as the leading cause of lake ecosystem changes in the future.

Sedimentary organic molecular compositions reveal the influence of glacier retreat on ecology on the Tibetan Plateau.

Global warming and glacier retreat have significant impacts on the structure and function of natural ecosystems. However, little is known about how glacier retreat affects the long-term evolution of ecosystems at high-altitude regions. In this study, we explored the possible effects of glacier retreat on catchment vegetation and lake productivity in Lake Puma Yumco, southeastern Tibetan Plateau, based on detailed organic molecular compositions determined by an ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and combined with various sedimentary geochemical indicators. The glaciers in the catchment keep retreating since 1870 CE, as inferred from the multiple indices of total organic carbon content (TOC), total nitrogen content (TN), C/N ratios, and carbonate contents. Accompanying modern global warming and glacier shrinkage, the relative abundance of soil- and vegetation-derived large molecular compounds (e.g., vascular plant-derived polyphenols, highly unsaturated and phenolic compounds, and condensed aromatics) increased gradually in lake sediments, suggesting that ice-covered land was exposed under warming condition, and gradually revegetation occurred. Both increases in relative abundance of nitrogen-containing compounds (e.g., CHNO) and chlorophyll derivative contents in the lake sediments were observed since 1870 CE, suggesting that stronger catchment weathering and increasing terrestrial nutrient loads enhanced the downstream lake productivity after glacier retreat. Our results imply that continued global warming and alpine glacier retreat in the future may further promote vegetation expansion and increases in lake productivity on the Tibetan Plateau.

Sedimentary organic matter molecular composition reveals the eutrophication of the past 500 years in Lake Daihai, Inner Mongolia.

Lake eutrophication seriously threatens water quality and human health. Under continuous global warming and intensified human activity, increasing attention is being paid to how lake trophic status responds to climate change and anthropogenic impacts. Based on the sedimentary organic matter (SOM) molecular composition determined by the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) technology, and combined with the SOM stable nitrogen isotopes (δ15Norg), we studied how lake trophic status and ecology respond to both climatic changes and anthropogenic impacts of the past 500 yrs at Lake Daihai, Inner Mongolia. The results show that the relative abundance of lipids, proteins, and carbohydrates in lake sediments kept relatively low before AD ∼1850, and increased gradually thereafter, especially after AD ∼1950, suggesting that the lake trophic status was low before AD ∼1850, but obviously increased during the past one more century. On the other hand, the relative abundance of allochthonous condensed aromatics and vascular plant-derived polyphenols compounds gradually decreased after AD ∼1850, which is most likely due to the intensified land-use changes in the catchment. Our results show that the SOM molecular composition is more sensitive to trace the land-use changes than the δ15Norg ratios, suggesting a potential use of this technique to trace even earlier human land uses (e.g., during the prehistorical times) in a catchment. The results of this study suggest that intensified land-use change, increased discharges of human sewage and industrial wastewater, cropland runoff, and concentrated effects caused by lake level drops may have combinedly increased nutrient concentration and accelerated lake eutrophication at Lake Daihai. Therefore, proper policy is necessary to slow down anthropogenic impacts and limit further eutrophication for lakes like Lake Daihai.