Abstract
Rates of epilithic N 2 fixation in the rocky littoral zones of Lake Malawi, determined by in situ incubations and the acetylene reduction method, declined with depth, were highly correlated to light intensity and heterocyst biovolume, and are among the highest observed values for freshwater or marine systems. Daytime N 2 fixation rates were similar between sites at similar depths, except at sediment and nutrient impacted sites, where rates were lower and more variable. Daytime N 2 fixation rates were measurable in all transparent chambers and were negligible in opaque chambers, which indicates that phototrophic diazotrophs (organisms capable of utilizing atmospheric nitrogen) were responsible for all day‐time N 2 fixation. Nocturnal N 2 fixation was ~60% of daytime fixation. A model that integrated N 2 fixation over depth and time predicted that epilithic N 2 fixation may contribute up to 35% of N inputs to the epilimnion of Lake Malawi.