China is a country with many lakes,about one-third of which are freshwater mainly distributed in the middle and lower reaches of the Yangtze River.Currently most of the lakes are mesotrophic or eutrophic.Lake eutrophication has become one of the major ecological and environmental problems faced by lakes in China and can lead to a series of abnormal ecosystem responses,including extinction of submerged plants,frequent occurrence of cyanobacterial blooms,increased microbial biomass and productivity,decreased biodiversity,accelerated cycles,and a change in the efficient use of nutrients.With development of eutrophication,the whole lake ecosystem suffers decreased biodiversity,simplification of biotic community structure,instability of the ecosystem,and ultimately the clear-water,macrophyte-dominated ecosystem gradually shifts to a turbid-water,algae-dominated ecosystem.This ecosystem succession mechanism is speculated to be caused by different nutrient utilization efficiencies of macrophytes and phytoplankton.The ultimate ecosystem succession trend of seriously eutrophic lakes is that a phytoplankton-dominated autotrophic lake shifts to a heterotrophic lake dominated by micro-organisms,protozoans.
The global application of nitrogen is far greater than phosphorus, and it is widely involved in the eutrophication of lakes and reservoirs. We used a bibliometric method to quantitatively and qualitatively evaluate nitrogen research in eutrophic lakes and reservoirs to reveal research developments, current research hotspots, and emerging trends in this area. A total of 2695 articles in the past 25 years from the online database of the Scientific Citation Index Expended（SCI-Expanded） were analyzed. Articles in this area increased exponentially from 1991 to 2015.Although the USA was the most productive country over the past 25 years, China achieved the top position in terms of yearly publications after 2010. The most active keywords related to nitrogen in the past 25 years included phosphorus, nutrients, sediment, chlorophyll-a, carbon,phytoplankton, cyanobacteria, water quality, modeling, and stable isotopes, based on analysis within 5-year intervals from 1991 to 2015 as well as the entire past 25 years. In addition, researchers have drawn increasing attention to denitrification, climate change, and internal loading. Future trends in this area should focus on：（1） nutrient amounts, ratios, and major nitrogen sources leading to eutrophication;（2） nitrogen transformation and the bioavailability of different nitrogen forms;（3）nitrogen budget, mass balance model, control, and management;（4） ecosystem responses to nitrogen enrichment and reduction, as well as the relationships between these responses; and（5）interactions between nitrogen and other stressors（e.g., light intensity, carbon, phosphorus, toxic contaminants, climate change, and hydrological variations） in terms of eutrophication.
