This research developed an innovative approach to reveal nitrogen sources, transformation, and transport in large and complex river networks in the Taihu Lake basin using measurement of dual stable isotopes of nitrate. The spatial patterns of δN corresponded to the urbanization level, and the nitrogen cycle was associated with the hydrological regime at the basin level. During the high flow season of summer, nonpoint sources from fertilizer/soils and atmospheric deposition constituted the highest proportion of the total nitrogen load. The point sources from sewage/manure, with high ammonium concentrations and high δN and δO contents in the form of nitrate, accounted for the largest inputs among all sources during the low flow season of winter. Hot spot areas with heavy point source pollution were identified, and the pollutant transport routes were revealed. Nitrification occurred widely during the warm seasons, with decreased δO values; whereas great potential for denitrification existed during the low flow seasons of autumn and spring. The study showed that point source reduction could have effects over the short-term; however, long-term efforts to substantially control agriculture nonpoint sources are essential to eutrophication alleviation for the receiving lake, which clarifies the relationship between point and nonpoint source control.

Hydropower development is the key strategy in many developing countries for energy supply, climate-change mitigation and economic development. However, it is commonly assumed that river dams retain nutrients and therefore reduce downstream primary productivity and fishery catches, compromising food security and causing trans-boundary disputes. Contrary to expectation, here we found that a cascade of reservoirs along the upper Mekong River increased downstream bioavailability of nitrogen and phosphorus. The dams caused phytoplankton density to increase with hydraulic residence time and stratification of the stagnant reservoirs caused hypoxia at depth. This allowed the release of bioavailable phosphorus from the sediment and an increase in dissolved inorganic nitrogen as well as a shift in nitrogen species from nitrate to ammonium, which were transported downstream by the discharge of water from the base of the dam. Our findings provide a new perspective on the environmental impacts of river dams on nutrient cycling and ecosystem functioning, with potential implications for sustainable development of hydropower worldwide.

The dynamics of riparian vegetation in a reach of the Lijiang River, China, are investigated. A new process-based model is developed based on cellular automata, which simulates the key processes in the life cycle of the ten most occurring plant species: germination, normal growth, response to floods and droughts, destruction by high velocities, consumption of resources, colonization and competition. The parameterization of these processes is based on controlled experiments on plants sampled in the study area. A traditional statistical model is also developed, which relates the vegetation state to four flow-related variables. Both models are assessed based on data from 12 field surveys in the period from 2009 to 2011, during the dry season, the wet season and at the end of the growing season. Both models predict satisfactorily the spatial distribution of the vegetation cover at the end of the growing season. Although the statistical model is by definition limited to the steady state conditions at the end of the growing season, the process-based model also satisfactorily simulates the temporal dynamics of the vegetation during the dry season and the wet season. Contrary to the statistical model, the process-based model also satisfactorily simulates the vegetation cover outside the area used for the model parameterization. Thus, process-based models are more robust under flow regimes with spatial heterogeneity and important temporal variations. Field observations and process-based model predictions indicate that the regime of dry season and wet season floods is the main regulator of the vegetation cover in the study area.