Human and natural controls over ecosystem nitrogen retention in lakes

Background/Question/Methods

Human driven increases in concentrations of reactive nitrogen (N) strongly impact biodiversity, biogeochemical processes, and water quality in earth’s ecosystems. The sources leading to excess N are well known, but the removal processes that reduce negative effects of excess N are poorly understood. For example, lakes have been shown to be strong sinks for watershed N via benthic denitrification, and may substantially reduce N transport to downstream rivers and coastal zones, yet some lakes show rising levels of nitrate. I used a synthesis of annual whole ecosystem mass balance measurements to explore the factors that regulate N retention in lakes. To interpret variation in N retention, I analyzed data for physical and biological factors known to affect N cycling and transport in coupled lake-watershed systems, focusing on factors that often co-occur with increased N loading to lakes.

Results/Conclusions

Total N removal rates in lakes increased with N loading, exhibiting little decrease in removal under the highest N loading rates. N removal efficiency, the proportion of external N load removed annually, was primarily influenced by lake hydrology and not N loading rate. N removal and N removal efficiency were enhanced by the availability of phosphorus (P). Phosphorus availability increased annual rates of N removal, via positive effects on lake productivity, which stimulate N uptake in the water column as well as sedimentary denitrification and burial.  Human activities and natural processes that increase P availability stimulate N removal thereby reducing N transfer to downstream ecosystems.  Conversely, reductions in lake productivity due to P control measures appear to promote accumulation of N, especially in large lakes.  Because lake productivity levels are strongly affected by human activities, management of increasingly N-saturated landscapes must consider multi-element interactions to predict future changes in N concentration and flux through watersheds.