Nitrogen fixation in lakes

Summary Determinations in the open waters of lakes using N¹⁵ as a tracer show that nitrogen fixation is generally associated with the presence of heterocystous blue-green algae and is light dependent. Although nitrogen-fixing blue-green algae tend to be abundant when the concentration of nitrate or ammonia in the water is low, fixation itself is not necessarily inhibited by the presence of these sources of combined nitrogen. The activity of nitrogen-fixing blue-green algae shows a direct relationship to concentration of dissolved organic nitrogen. As a result of the interaction of such factors, nitrogen fixation per unit area of lake surface per year tends to be greatest at an early stage of eutrophication. In relation to the total nitrogen budget of a lake the contribution of biological nitrogen fixation to the income is probably always small but at certain times and in particular water strata it may contribute a major part of the nitrogen assimilated by the phytoplankton.