Abstract:
Regions of the Tibetan Plateau are experiencing increasing temperatures and decreasing glacial mass at a faster rate than the global average. Our study area, the Parlung Zangbo River basin, is within the southeastern region of the Tibetan Plateau which is experiencing the greatest degree of glacial mass loss. Glacial loss could mobilize organic matter from the terrestrial landscape to lakes and rivers and thereby influence watershed carbon cycles, water quality, and aquatic ecological functioning. However, the present-day dissolved organic matter (DOM) pool of the river, which could serve as a baseline for future predictions, has not been characterized. This project will characterize spatial distribution of the amount, source, and biodegradability of DOM exported from the Parlung Zangbo River basin. Samples were collected along the main stem of the river over 170 km, nearby tributaries, and a headwater lake and analyzed for DOC concentration and DOM quality based on absorbance and fluorescence properties. The excitation emission matrix coupled with parallel factor analysis (EEM-PARAFAC) yielded three humic-like fluorophores and two protein-like fluorophores. A subset of samples analyzed with Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) analysis showed that the DOM pool was dominated by lignins. Incubation experiments showed microbial utilization of DOM increased with increasing temperature up to an optimum temperature (between 17°C and 25°C) before declining. Lake samples showed characteristics indicative of glacial contribution, including greater proportions of Nitrogen-rich DOM and microbially-derived DOM, as well as higher β:α values that suggest lower degrees of diagenesis and higher bioreactivity. Our results suggest that increasing glacial loss in the future may shift the composition and increase the bioreactivity of DOM pool in Tibetan rivers. Consequently, these shifts in the DOM pool will contribute to global warming through a positive feedback loop.
