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 thcsc organisms make up a major percentage of the planktonic biomass, Planktonic nitrogen fixation tends to be low in oligotrophic and mesotrophic lakes (generally <O. 1 g N m-2 yr-I) but is often high in eutrophic lakes (0.2-9.2 g N m-2 yr-I).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 co.25 g N m-2 yr-' 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 mm2 yr-I. 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 nonvcgetated, organic-rich sediments. Rates of fixation in cyanobacterial mats are high to very high (1.3-76 g N m-2 yr-I), 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 K 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 regi...

Benthic decomposition rates and pathways in plantations of the mangrove Rhizophora apiculata in the Mekong delta, Vietnam

“…data). The rates of nitrogen fixation were rapid compared with those recorded in previous studies of mangrove forests (Alongi et al 1992), and are at the upper end of the range of N, fixation rates measured in other marine and estuarine environments (Howarth et al 1988).…”

Heterotrophic nitrogen fixation in oligotrophic tropical marshes: changes after phosphorus addition

“…Cyanobacterial N fixation has been described for a variety of freshwater and salt marshes, as well as mangroves (Paerl et al, 1994;Rejmánková & Komárková, 2000;Tyler et al, 2003;Inglett et al, 2004;Steppe & Paerl, 2005;Scott et al, 2007;Vargas & Novelo, 2007). The contribution of cyanobacterial N fixation to the ecosystem N economy is relatively high, contributing on average 1-10 (and up to 76) g N m -2 y -1 (Howarth et al, 1988a).…”

“…Herbaceous wetland ecosystems generally lack symbiosis-involving legumes and thus the most important N fixers are N-fixing cyanobacteria and sediment heterotrophic bacteria (Howarth et al, 1988a). Cyanobacterial N fixation has been described for a variety of freshwater and salt marshes, as well as mangroves (Paerl et al, 1994;Rejmánková & Komárková, 2000;Tyler et al, 2003;Inglett et al, 2004;Steppe & Paerl, 2005;Scott et al, 2007;Vargas & Novelo, 2007).…”

Nutrient enrichment in tropical wetlands: shifts from autotrophic to heterotrophic nitrogen fixation

“…The continual input of easily available C into the plant rhizosphere sustains high activity of root associated microflora (Boyle and Patriquin 1981;Bergholz et al 2001;Nielsen et al 2001;Moseman 2007), and thus diazotrophs are closely associated with plant roots (Dakora and Drake 2000;Bürgmann et al 2005). In addition to C availability, the activity of diazotrophs in wetland sediments is repressed by high concentrations of ammonium (Howarth et al 1988;Č erná et al 2009) and low concentrations of available phosphorus (P) (Reed et al 2007). …”

“…The theoretical reduction ratio is 3:1, reflecting the number of reducing equivalents required by nitrogenase to reduce C 2 H 2 to C 2 H 4 relative to the number required to reduce N 2 to NH 4 (Zehr and Montoya 2007). However, this ratio can vary from 1.5:1 to 100:1 (summarized in Howarth et al 1988) and a direct calibration through 15 N fixation assay is needed to get reliable estimates of N 2 input to the ecosystem.…”