Benthic nitrogen fixation : 

Nitrogen fixation in Tikehau atoll , results

Loïc Charpy, IRD

Dinitrogen fixation by benthic communities of Tikehau lagoon was measured between 1991 and 1995. The method used was the Acetylene reduction rates method, calibrated with ¹⁵N₂ for the different cyanobacterial communities. For more explanations about incubations, acetylene concentrations measurements, calibration, see the chapter methodology.

 Nitrogen fixation rate

Nitrogenase fixation was observed in all substrates studied and was detected from the start of incubations. Results appears in different tables and are commented in that page.

Acetylene reduction

Daylight rates varied greatly and were all higher than the night-time rates. Minor nitrogenase activity appeared on sand 0 communities, i.e. communities without apparent cyanobacteria, and the major activity occurred on sand 2 communities, i.e. substrate covered by a visible cyanobacterial community. Nitrogenase activity of the limestone of pinnacles was also very high and was sevenfold greater during the day than at night (Table 2). The results of diel experiments showed that ARR decreased at sunset and continued, though at a lower rate, throughout the night. 

Only Sand 3 showed a difference (49%) between the surface where most of the incubations are made and the mean depth of the lagoon (25m). At this depth, the communities of Sand 2 showed the highest acetylene reduction rate (Table 2).

Different communities nitrogen fixation

We have used our experimental values of C₂H₂ reduction/¹⁵N fixation ratios (1.8 to 4.8 mol N₂ fixed mol^-1 C₂H₂ reduced according to the community) to convert our C₂H₂ reduction rates to N₂ fixation. N₂ fixation rates of different substrates appear in Table 3 column (N₂Fix)S. Acetylene reduction activity was highest for limestone surface but, when using the experimentally determined conversion ratio, dinitrogen fixation by limestone surfaces was less than for the sand 2 community. Sand 2 was the community with the highest fixation per square meter among the lagoonal communities (3.9 mg N day^-1) followed by the limestone surface (2.12 mg N day^-1).

Total lagoonal benthic fixation

To estimate the total benthic N₂ fixation by the communities of the lagoon, we estimated the distribution percentage of each soft substratum community and limestone substratum (see table). 

For pinnacles, we assumed that each 1 m² of pinnacle was completely covered by limestone. The limestone surface area of pinnacles was estimated by multiplying the pinnacle projected area (2% of the lagoon surface) by 3  (Larkum et al., 1988).

For the whole lagoon, the nitrogen fixation rates of the communities, in terms of (N_2Fix)_T  of kg N day^-1 in Table 3, are equal to the nitrogen fixation rates of the communities per square meter of lagoon area, in terms of (N_2Fix)_M of kg N day^-1 in Table 3,  multiplied by the total area of the lagoon (400.2 km²).

The total contribution of the limestone surfaces to the lagoonal dinitrogen fixation was smaller than the soft bottom fixation because the pinnacles represented only 6% of the area of the lagoon (Table 3, column TA). 

 Contribution of benthic community dinitrogen fixers to primary production of Tikehau atoll  

To estimate the contribution of N₂ fixation to the total nitrogen requirement, to sustain the benthic primary production, we assumed that: 

(1) Nitrogen requirement for primary production is equal to the N new production

(2) N₂ fixation by soft bottom communities (sand 0 to sand 3) is an N supply to sandy microphytobenthic production (i.e. organisms growing at the sediment-water interface)

(3) N₂ fixation by limestone substrata was an N supply to the production of the pinnacles. 

We calculated the requirements of sand and pinnacle primary production using the Redfield ratio (6.6:1), together with a value of microphytobenthic primary production of 250 mg C m^-2 day^-1 (Charpy-Roubaud 1988) and additional production of the pinnacle of 100 mg N m^-2 day^-1 (Charpy and Charpy-Roubaud 1998). Based on these values, the nitrogen requirement for microphytobenthic primary production was 44.2 mg N m^-2 day^-1, compared to the observed dinitrogen fixation of 1.08 mg N m^-2 day^-1 (Table 3). Using a classic value of 0.1 for the F-ratio (Eppley and Peterson 1979), we can estimate new production to be on the order of 4.41 mg N m^-2 day^-1, of which microphytobenthic N₂ fixation contributes 24.4% of the new production. The new production associated with the limestone was 17.7 mg N m^-2 day^-1. Since the value of the additional limestone production is likely to be similar to new production (Hatcher 1990), it can be estimated that limestone N₂ fixation contributed 12.0% of the new production.    

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