Dynamics of nutrients and organic matter and their control mechanisms at the river mouths of major Chinese rivers

The high-intensity human activities have had a strong impact on the flux and composition of nutrients and organic matter exported from major rivers to the sea worldwide. The goal of this study was to identify the mechanisms that control the distributions and variations of terrestrial nutrients and organic matter transported from the eight major Chinese rivers to the oceans. The continuous construction of reservoirs and dams in river basins causes them to resemble reservoirs, which results in intensified fragmentation of water channels, more transparent water bodies, longer water retention times, greater retention of nutrients, and enhanced biomass and production of phytoplankton. Herein we provide a preliminary definition of “reservoir degree” (i.e., the extent to which a river channel resembles a reservoir or a lake) and offer an empirical equation to roughly quantify the reservoir degree of rivers. We collected surface water samples at the river mouths of China's eight major rivers from different seasons and measured concentrations and compositions of nutrients and organic matter in these samples. Rotated principal component analysis of our data showed that reservoir degree and pollution degree are the two most important factors determining the distributions and variations of nutrients and organic matter in these rivers. We found that the reservoir degrees were often higher in small rivers than in large rivers and in plain rivers versus mountainous rivers. As reservoirs and dams are still being constructed in all the eight river basins, the reservoir degrees of the rivers will continue to increase. Under the implementation of environmental protection projects, however, their pollution degrees would decrease. Therefore, the different biogeochemical parameters will show divergent variation trends in the future. The results of this study help clarify the control mechanisms and the future evolution trends of nutrients and organic matter transported by rivers worldwide.