Influence of land cover types and phytoplankton community on the distribution and fate of dissolved organic matter in a typical river located in the semi-arid regions of China

Land use can impact dissolved organic matter (DOM) dynamics directly by changing terrestrial loading in anthropogenic areas, and indirectly by promoting in-situ phytoplankton production due to the high load of nutrients. However, understanding how land use and phytoplankton community interact to affect DOM quantity and quality is deficient. To tease apart the synergistic effects of land use and phytoplankton on DOM dynamics, fluorescence spectroscopy with parallel factor analysis (PARAFAC) and mediation analysis was performed in this study. PARAFAC produced three components that had two humic-like (C1 and C2) and a protein-like (C3) fluorescent components, significantly related to anthropogenic land-use (cropland + urban area) and phytoplankton concentration (p < 0.05). Through the mediation analysis, the altered C1 and C3 at sites with a higher proportion of anthropogenic land-use appeared to originate partly as a result of the phytoplankton production fueled by a high load of nutrients from anthropogenic run-off (26.6% and 12.5%). These results suggested phytoplankton significantly mediated the effects of land use on microbial humic-like and protein-like DOM. Additionally, Redundancy analysis (RDA) results further showed that Phormidium (Cyanophyta) had a notable influence on the production of the protein-like component via phytoplankton. Conversely, the decreases in terrestrial humic-like C2 were mainly attributed to the reduced terrestrial loading in anthropogenic areas. These findings provide insights into the roles of land use and phytoplankton communities in DOM dynamics and advance our understanding of the mechanisms that drive DOM composition patterns.