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

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 these organisms make up a major percentage of the planktonic biomass. Planktonic nitrogen fixation tends to be low in oligotrophic and mesotrophic lakes ≪ 0.1 g N m−2 yr−1) but is often high in eutrophic lakes (0.2–9.2 g N m−2 yr−1). We found no data on planktonic nitrogen fixation in estuaries or coastal seas except for the Baltic Sea and for the Peel‐Harvey estuary in Western Australia. Fixation rates are quite high in the Peel‐Harvey estuary; rates are low offshore in Baltic waters but can be high near shore. As in lakes, fixation in these systems is associated with major blooms of planktonic, heterocystic cyanobacteria. However, nitrogen‐fixing cyanobacteria are much more abundant in the Baltic Sea and in the Peel‐Harvey estuary than in other estuaries or coastal waters. In most estuaries and coastal waters, species of nitrogen‐fixing cyanobacteria are absent or make up a very small percentage of the phytoplankton biomass (< 1%), suggesting insignificant amounts of nitrogen fixation. Annual average rates of nitrogen fixation reported for blooms of Trichodesmium and for Rhizosolenia mats in oceanic waters are also low. Unlike nitrogen fixation by planktonic organisms, there appear to be no major differences between freshwater and marine ecosystems with regard to fixation by benthic bacteria. Rates of nitrogen fixation in the sediments of most lakes and estuaries are low to moderate, generally <0.25 g N m−2 yr−1 except in extremely organic‐rich estuarine sediments. In estuarine sediments which are organic‐rich, nitrogen fixation rates range from 0.4 to 1.6 g N m−2 yr−1. Most benthic fixation in mesotrophic and eutrophic lakes and estuaries is mediated by heterotrophic (and perhaps chemoautotrophic) bacteria, but benthic fixation in oligotrophic ecosystems is often dominated by cyanobacteria. Rates of nitrogen fixation in freshwater and marine wetlands and seagrass beds appear similar to or somewhat greater than those in nonvegetated, organic‐rich sediments. Rates of fixation in cyanobacterial mats are high to very high (1.3–76 g N m−2 yr−1), but these mats usually cover only a small area of the ecosystems in which they reside, limiting the importance of fixation in the mats to the mats themselves. The importance of nitrogen fixation to the nitrogen economy of aquatic ecosystems is quite variable. For example, fixation by planktonic organisms appears unimportant as a nitrogen source to most oligotrophic and mesotrophic lakes (generally <1% of total nitrogen inputs) but accounts for 6–82% of the nitrogen inputs to eutrophic lakes. Planktonic fixation provides ≪ 1% of the nitrogen inputs to surface waters of the world’s oceans and is probably also of little importance in most estuaries, including eutrophic estuaries. However, planktonic fixation provides >20% of the nitrogen input to the Asko region of the Baltic Sea and 17% of the nitrogen input to the Peel‐Harvey 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.