Classification of substrata

The lagoon bottom is formed of a fine to very fine calcareous sand. Extensive areas are covered with patches of relatively dense cyanobacterial communities or mats. 

Soft bottom sediments were classified into four groups (Table 1) according to the visual appearance and density of cyanobacterial communities: sand 0, white coloured, with no visible signs of cyanobacteria; sand 1, diffuse communities with few apparent signs of organisms, due to negative phototaxis of the species involved; sand 2, formed by a permanent and visible community, including cyanobacteria; sand 3, relatively dense filamentous cyanobacterial communities, which grew upwards as much as 20 cm from the bottom. A preliminary taxonomic study of cyanobacterial flora led to the checklist of the principal species (Table 1) in which there was a dominance of non-heterocystous species.
sand 1	sand 2	sand 3
Images provenant de http://www-cyanosite.bio.purdue.edu/images/

Biomass

58 stations were randomly selected in the lagoon to calculate the contribution of the four groups to the total cyanobacterial biomass of the lagoon. At each station five 0.25 m² quadrates were randomly positionned on the lagoon floor.

A map of Tikehau atoll showing the depth of the randomly selected stations used for community density measurements and the positions of stations (filled circles) used for nitrogenase activity measurements.

Community surface areas were calculated as square centimeters of communities per square meter of soft sediment. 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).

Table: Surface areas for the different benthic cyanobacterial communities of the lagoon of Tikehau

The table show that Sand 0 and Sand 1 cover the larger surfaces in the lagoon. Thera are many pinnacles in the lagoon but they represent only 6% of the surface area of the lagoon.