Atmospheric dinitrogen fixation by benthic communities of Tikehau Lagoon (Tuamotu Archipelago, French Polynesia) and its contribution to benthic primary production

Acetylene reduction rates were measured in lagoonal sediments, cyanobacterial mats and limestone surfaces between 1991 and 1995 at many sites, depths and seasons; all the studied substrata contained cyanobacteria. The acetylene reduction/¹⁵N₂ fixation ratio was measured for the different communities and varied between 1.8 and 4.8, depending on substratum. Fixation rates were 1.7 to 7 times higher during daylight compared to night-time rates. N₂ fixation rates ranged from 0.4 to 3.9 mg N m^-2 day^-1 for the lagoonal sediment/mat communities, and the rate was about 2 mg N m^-2 day^-1 for the lagoonal limestone substrata. Total lagoonal benthic N₂ fixation contributed 24.4% of the total nitrogen requirement for the benthic primary production of benthic communities of the lagoon. The input of N₂ fixation by the microbial planktonic communities (including cyanobacteria) of the lagoon, which are highly productive, is unquantified but is likely to be large.