Nitrogen fixation: a poorly understood process along the freshwater-marine continuum.

Although N fixation is a major component of the global N cycle and has been extensively studied in open-ocean and terrestrial ecosystems, rates and ecological dynamics remain virtually unknown for the inland and coastal aquatic ecosystems (lakes, wetlands, rivers, streams, estuaries) that connect terrestrial and marine biomes. This is due to the diversity of these habitats, as well as the traditional paradigm that N fixation rates were low to nonexistent, and therefore not important, in these ecosystems. We identify three major research themes to advance understanding of aquatic N fixation: 1) the biological diversity of diazotrophs and variability of N fixation rates, 2) the ecological stoichiometry of N fixation, and 3) the upscaling of N fixation rates from genes to ecosystems. Coordinating research across these areas will advance limnology and oceanography by fully integrating N fixation into ecological dynamics of aquatic ecosystems from local to global scales.