『Abstract
　Human activities have greatly increased the transport of biologically available nitrogen (N) through watersheds to potentially sensitive coastal ecosystems. Lentic water bodies (lakes and reservoirs) have the potential to act as important sinks for this reactive N as it is transported across the landscape because they offer ideal conditions for N burial in sediments or permanent loss via denitrification. However, the patterns and controls on lentic N removal have not been explored in great detail at large regional to global scales. In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale, global model of lentic N removal called NiRReLa (Nitrogen Retention in Reservoirs and Lakes). The NiRReLa model incorporates small lakes and reservoirs than have been included in previous global analyses, and also allows for separate treatment and analysis of reservoirs and natural lakes. Model runs for the mid-1990s indicate that lentic systems are indeed important sinks for N and are conservatively estimated to remove 19.7 Tg N year^-1 from watersheds globally. Small lakes (＜50 km²) were critical in the analysis, retaining almost half (9.3 Tg N year^-1) of the global total. In model runs, capacity of lakes and reservoirs to remove watershed N varied substantially at the half-degree scale (0-100％) both as a function of climate and the density of lentic systems. Although reservoirs occupy just 6％ of the global lentic surface area, we estimate they retain 〜33％ of the total N removed by lentic systems, due to a combination of higher drainage ratios (catchment surface area:lake or reservoir surface area), higher apparent settling velocities for N, and greater average N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of NiRReLa suggests that, on-average, N removal within lentic systems will respond more strongly to changes in land use and N loading than to changes in climate at the global scale.

Keywords: Nitrogen; Lakes; Reservoirs; Denitrification; Global limnology; Nitrogen removal』

Introduction
Methods
　The NiRReLa model structure and calibration
　　Model structure
　　Model calibration
　Global application of NiRReLa
　　Spatial data
　　NiRReLa and small lakes and reservoirs
　　NiRReLa and large lakes and reservoirs
　Model sensitivity analysis
Results and discussion
　Apparent settling velocities
　NiRReLa model performance
　N removal by lakes and reservoirs at global scale
　Regional patterns of lake and reservoir N retention
　Sensitivity analysis
　Uncertainties and future directions
Acknowledgments
References