Can nitrogen gas be deficient for nitrogen fixation in lakes?

Severe nitrogen gas (N-2) depletion in lakes may have implications for N-2 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 N-2 cycle in lakes. Second, we empirically analysed N-2 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 N-2 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 N-2 concentration. Simulations indicated that bottom waters and other isolated water bodies may show strong deviations from N-2 saturation during certain conditions differing from those during summers in Lake Erken. Simulated surface waters were always N-2 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 N-2 concentration in surface waters vastly exceeds the half-saturation constant for N uptake by phytoplankton, and concluded that it is unlikely that N-2 fixation is ever limited by the N-2 concentration. Modellers of lake eutrophication may regard N-2 as being available in excess. (c) 2006 Elsevier B.V. All rights reserved.