178
and generally depleted in nutrients, and typically N 2 fixation has been associated
with these regions. However, in more recent years, N 2 fixation has also been discovered in coastal waters, with elevated fixed N compounds, and in polar seas.
With respect to the phototrophic cyanobacteria, Trichodesmium, Richelia and
Crocosphaera (UCYN-B) tend to be limited to warmer tropical and sub-tropical
waters above 20°C, whereas UCYN-A are found across a much broader range of
temperatures. Trichodesmium reaches some of its highest abundances in the western
boundary currents of the major ocean gyres such as the Gulf Stream and Kuroshio.
Its population densities tend to be much lower on the eastern boundaries and within
the gyre itself.
Interbasin and intrabasin difference in surface water populations are apparent as
well and are likely related to the relative availability of nutrients limiting to diazotrophs: available studies indicate that Trichodesmium is most abundant in the tropical North Atlantic where Fe is replete due to atmospheric inputs but phosphate is in
very short supply. In contract, Trichodesmium appears far less dominant in the
South Atlantic, and North and South Pacific gyres which are distant from atmospheric Fe inputs. Phosphate occurs in relatively high proportions, relative to inorganic N and Fe, through many of these areas, and UCYN-A are often the predominant
diazotrophs in surface waters.
The Diatom Diazotroph Associations (DDAs) have a unique requirement for silicon because of the frustules of their diatom host. While they are cosmopolitan in the
tropics and sub-tropics, their blooms are often associated with mesohaline river
plumes depleted of N such as the Amazon or Mekong rivers. Seasonal blooms are
also documented proximal to large island such as the Hawaii Islands.
With a current relative paucity of biogeographical data for diazotrophs in subeuphotic zone waters, it is difficult to make any generalizations. This remains an
area for future efforts.
10.8 N 2 Fixation in Ocean Basins
Trends in ocean nitrate concentrations, geochemical proxies of nitrate to phosphate
ratios (e.g. N* and P*) and the natural abundance N isotope ratios in key N pools
have all been used to determine the distribution of N 2 fixation and to estimate integrated basin scale rates of this process as well as its counterpart in the N cycle,
denitrification (including anammox). Paralleling the surface patterns of diazotroph
populations, the quantitative distributions of N 2 fixation indicates high levels in the
North Atlantic, and overall high basin-integrated rates in the North Pacific, with
intensive inputs also occurring in the equatorial western Pacific. Data for the Indian,
South Atlantic and South Pacific Oceans are relatively sparse. In contrast, denitrification is localized in the major ODZs and in shelf sediments.
It has been debated whether the N cycle is in balance and how closely N 2 fixation
and denitrification are coupled. Current evidence based on various modeling
approaches calibrated with limited field measurements suggests that total N 2 fixa10 Summary and Conclusions
and generally depleted in nutrients, and typically N 2 fixation has been associated
with these regions. However, in more recent years, N 2 fixation has also been discovered in coastal waters, with elevated fixed N compounds, and in polar seas.
With respect to the phototrophic cyanobacteria, Trichodesmium, Richelia and
Crocosphaera (UCYN-B) tend to be limited to warmer tropical and sub-tropical
waters above 20°C, whereas UCYN-A are found across a much broader range of
temperatures. Trichodesmium reaches some of its highest abundances in the western
boundary currents of the major ocean gyres such as the Gulf Stream and Kuroshio.
Its population densities tend to be much lower on the eastern boundaries and within
the gyre itself.
Interbasin and intrabasin difference in surface water populations are apparent as
well and are likely related to the relative availability of nutrients limiting to diazotrophs: available studies indicate that Trichodesmium is most abundant in the tropical North Atlantic where Fe is replete due to atmospheric inputs but phosphate is in
very short supply. In contract, Trichodesmium appears far less dominant in the
South Atlantic, and North and South Pacific gyres which are distant from atmospheric Fe inputs. Phosphate occurs in relatively high proportions, relative to inorganic N and Fe, through many of these areas, and UCYN-A are often the predominant
diazotrophs in surface waters.
The Diatom Diazotroph Associations (DDAs) have a unique requirement for silicon because of the frustules of their diatom host. While they are cosmopolitan in the
tropics and sub-tropics, their blooms are often associated with mesohaline river
plumes depleted of N such as the Amazon or Mekong rivers. Seasonal blooms are
also documented proximal to large island such as the Hawaii Islands.
With a current relative paucity of biogeographical data for diazotrophs in subeuphotic zone waters, it is difficult to make any generalizations. This remains an
area for future efforts.
10.8 N 2 Fixation in Ocean Basins
Trends in ocean nitrate concentrations, geochemical proxies of nitrate to phosphate
ratios (e.g. N* and P*) and the natural abundance N isotope ratios in key N pools
have all been used to determine the distribution of N 2 fixation and to estimate integrated basin scale rates of this process as well as its counterpart in the N cycle,
denitrification (including anammox). Paralleling the surface patterns of diazotroph
populations, the quantitative distributions of N 2 fixation indicates high levels in the
North Atlantic, and overall high basin-integrated rates in the North Pacific, with
intensive inputs also occurring in the equatorial western Pacific. Data for the Indian,
South Atlantic and South Pacific Oceans are relatively sparse. In contrast, denitrification is localized in the major ODZs and in shelf sediments.
It has been debated whether the N cycle is in balance and how closely N 2 fixation
and denitrification are coupled. Current evidence based on various modeling
approaches calibrated with limited field measurements suggests that total N 2 fixa10 Summary and Conclusions