Terrestrial contributions to the aquatic food web in the middle Yangtze River

Spatiotemporal response of dissolved organic matter diversity to natural and anthropogenic forces along the whole mainstream of the Yangtze River

The Yangtze River, the largest river in Asia, plays a crucial role in linking continental and oceanic ecosystems. However, the impact of natural and anthropogenic disturbances on composition and transformation of dissolved organic matter (DOM) during long-distance transport and seasonal cycle is not fully understood. By using a combination of elemental, isotopic and optical techniques, as well as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we investigated DOM abundance and composition along the whole mainstream at highly spatial resolution in the dry and early wet seasons. Our findings showed that the concentration and flux of dissolved organic carbon (DOC) in the Yangtze River was much lower compared with other worldwide larger rivers. The distribution of δ¹³C_DOC and higher abundance of humic-like fluorescent component and highly unsaturated and phenolics (HUPs) compound reflected a prominent contribution of allochthonous DOM. Further optical and molecular analysis revealed humic-like fluorescent components were coupled with CHO molecules and HUPs compound with higher aromatic, unsaturated, molecular weight and stable characteristics between upstream and midstream reaches. With increasing agricultural and urban land downstream, there were more heteroatomic formulae and labile aliphatic and protein-like compounds which were derived from human activities and in situ primary production. Meanwhile, DOM gradually accumulates with slow water flow and additional autochthonous organics. Weaker solar radiation and water dilution during the dry/cold season favours highly aromatic, unsaturated and oxygenated DOM compositions. Conversely, higher discharge during the wet/warm season diluted the terrestrial DOM, but warm temperatures could promote phytoplankton growth that releases labile aliphatic and protein-like DOM. Besides, chemical sulfurization, hydrogenation and oxygenation were found during molecular cycling processes. Our research emphasizes the active response of riverine DOM to natural and anthropogenic controls, and provides a valuable preliminary background to better understand the biogeochemical cycling of DOM in a larger river.

Variations of trophic structure and niche space in fish community along a highly regulated subtropical large river

The trophic interactions between consumers and resources play a vital role in the stability of communities. In river systems, fragmentation of natural habitats and environmental changes alters the energy basis and community composition, consequently leading to variations in the community's trophic structure and niche space. However, our understanding of how the trophic structure responds to environmental changes is still very limited. Here, based on stable isotope data, we explored and compared trophic positions (TPs), community-wide trophic metrics, and isotope niche space of fish communities in three reaches with different hydrogeomorphic conditions along a highly regulated subtropical river over three seasons. The community trophic structure and niche space showed notable spatiotemporal variations. Overall, the downstream reach had lower TPs, trophic diversity but higher trophic redundancy. The middle reach occupied a wider isotope niche space than other reaches, with the largest niche size during autumn. Furthermore, the niche overlap was relatively high in winter between reaches and in the downstream between seasons. The results implied a homogenization of feeding functional groups and energy flow pathways of species in the downstream community associated with the change of energy source and stability of hydrological conditions. The relationship between trophic structure and environmental factors suggested that the dam-induced alteration in hydrological-related aspects may drive the changes in the functional group composition, together with changes in energy basis, resulting in differences in the trophic structure of the community. The results of the present study deepen our understanding of how ecosystem functions respond to disturbance, thus contributing to improved ability to conserve river ecosystems.