Can nitrogen gas be deficient for nitrogen fixation in lakes?

Severe nitrogen gas (N2) depletion in lakes may have implications for N2 fixation and for model structures describing the nitrogen (N) cycle. To determine whether this is likely to occur, we first reviewed the general processes influencing the N2 cycle in lakes. Second, we empirically analysed N2 concentrations in various stations and depths of Lake Erken, Sweden during the beginning and the end of the summer stratification period of 2004. Overall, no significant deviations from N2 saturation from any stations or depths were observed during the study period. Third, we developed a simple, one-box gas model to investigate possible effects on lake water from processes described in the literature that affect the N2 concentration. Simulations indicated that bottom waters and other isolated water bodies may show strong deviations from N2 saturation during certain conditions differing from those during summers in Lake Erken. Simulated surface waters were always N2 saturated in all types of lakes where the water column was in direct contact with the air. We quantified all probable fluxes, showed that the N2 concentration in surface waters vastly exceeds the half-saturation constant for N uptake by phytoplankton, and concluded that it is unlikely that N2 fixation is ever limited by the N2 concentration. Modellers of lake eutrophication may regard N2 as being available in excess.