Nitrogen fixation in freshwater, estuarine, and marine ecosystems. 1. Rates and importance1

TLDR: Nitrogen fixation appears important in making up deficits in nitrogen availability relative to phosphorus availability in many lakes, contributing to the phosphorus-limited status of these systems.

Abstract: Nitrogen fixation is mediated by a variety of autotrophic and heterotrophic bacteria. Cyanobacteria appear responsible for most planktonic fixation in aquatic ecosystems, and rates of fixation are high only when thcsc organisms make up a major percentage of the planktonic biomass, Planktonic nitrogen fixation tends to be low in oligotrophic and mesotrophic lakes (generally 20% of the nitrogen input to the Asko region of the Baltic Sea and 17% of the nitrogen input to the PeelHarvey estuary in Australia. Fixation in sediments of estuaries and eutrophic and mesotrophic lakes usually constitutes a small percentage of the nitrogen inputs to these systems. However, benthic fixation appears to be a major source of nitrogen for many oligotrophic tropical lagoons and for some oligotrophic lakes, even though fixation rates are moderate because other nitrogen inputs tend to be low. Nitrogen fixation probably is a fairly minor input of nitrogen to marine wetlands, which are generally open to other inputs, but contributes roughly half the total nitrogen input to some freshwater wetlands (bogs, cypress domes), where other inputs are more limited. Nitrogen fixation appears important in making up deficits in nitrogen availability relative to phosphorus availability in many lakes, contributing to the phosphorus-limited status of these systems. That many estuaries and coastal seas are nitrogen limited is due in part to the generally low rates of nitrogen fixation found in these systems.