Background/Question/Methods
Human activities have greatly increased the transport of biologically available N through watersheds to potentially sensitive freshwater and coastal ecosystems.  Lentic water bodies (lakes and reservoirs) may 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 smaller lakes and reservoirs than have been included in previous global analyses, and also allows for separate treatment and analysis of reservoirs and natural lakes. 

Results/Conclusions
Model runs for the mid-1990s indicate:

• Lentic systems are indeed important sinks for N and are conservatively estimated to remove 23 Tg N yr^-1 from watersheds globally.  Small lakes (< 50 km²) were critical in the analysis, retaining almost half (10.9 Tg N yr^-1) of the global total.

• Capacity of lakes and reservoirs to remove watershed N varied substantially (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 they retained approximately 35% 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 N uptake velocities, and greater N loading rates in reservoirs than in lakes.

• 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.