Lakes are sources of nitrogen to the atmosphere: A look at net nitrogen balance in Upper Midwest Lakes

Abstract

Lakes are critical to biogeochemical processing of nutrients because they provide long residence times with exposure to microbes which are the engines that drive these biogeochemical processes. Recently, research has been conducted to examine the net flux of CO2 between the lakes and the atmosphere, but less work has examined the N2 equilibrium and whether lakes are sources or sinks of N2. For N2, the relevance has little to do with the effects of freshwater release of N2 on atmospheric concentrations given the large size of this atmospheric pool, but rather, losses of N2 from freshwaters have important implications to N availability and cycling in freshwaters. A widely held paradigm is that freshwaters compensate for losses of N by increasing N-fixation relative to losses despite little empirical evidence of this on broad spatial scales. To determine whether lakes are more likely to be gaining or losing N to the atmosphere, we examined N2 percent saturation in 34 lakes across 5 degrees latitude in the Upper Midwest ranging in trophic status, mixing regime, origin, and bathymetry. By comparing water N2/Ar ratios measured versus the expected values given physical mixing properties and temperature, we observed that lakes were typically supersaturated with N2. More than 85% of the water samples taken from multiple depths within lakes and across all seasons demonstrated supersaturation (n=248). All the lakes except one experienced supersaturation of N2/Ar at some point in time. A mixed-model was used to determine what lake characteristics predicted N2 saturation. Both depth and temperature were positively correlated with N2 saturation, suggesting that there may be 'hot spots' where N2 saturating processes are optimized. These observations indicate that in most lakes, most of the time, denitrification and/or anammox exceed N-fixation with important implications for N losses and nutrient limitations in freshwater systems.