Nitrogen fixation is the process whereby molecular N 2 gas is converted to reactive, biologically available forms of nitrogen. The vast majority of nitrogen on Earth is present as molecular N 2, and before the advent of the agricultural and industrial revolutions, biological nitrogen fixation was the only significant process creating reactive nitrogen. Since nitrogen is essential for the maintenance of life as well as an element frequently limiting primary productivity, nitrogen fixation is an essential process at the global scale. Most inland aquatic ecosystems receive a majority of their reactive nitrogen from upstream ecosystems, but fixation can also be an important internal nitrogen input. Prokaryotic bacteria are the sole organisms able to convert highly stable N 2 into a biologically useful form. A variety of types of bacteria are capable of nitrogen fixation, including heterotrophs, cyanobacteria and other photoautotrophs, and chemo-autotrophs. In inland waters heterotrophic bacteria and cyanobacteria are responsible for most of the nitrogen fixation that occurs. The rates of nitrogen fixation and the resulting ‘new’ nitrogen input to an ecosystem from this process vary greatly, and are controlled by a number of physical, chemical and ecological factors related to both the internal characteristics of a water body and external inputs.