The microphytes biomass, measured as spectrophotometric concentrations of chlorophyll a in acetonic extracts, was studied at 120 randomly sampled stations in the south-western lagoon of New Caledonia (South West Pacific). The mean concentrations of pigments in the top centimetre of sediment (0-1 cm) were found to be 47.2 mg m-2 (s.e. 1.2) for chlorophyll a and 78.4 mg m-2 (s.e. 1.4) for phaeopigments. Small-scale, spatial scale and temporal scale distributions were heterogeneous. The spatial distribution of benthic chlorophyll a is affected by a combination of several parameters such as depth (and therefore light), sediment type and macrophyte interactions. Chlorophyll a maxima were found close to the reef on coarse white and well oxygenated sediment, whereas phaeopigment maxima existed close to the coast and were associated with fine sediment and detrital areas. Temporal variations were observed. An attempt to express the benthic chlorophyll a biomass as organic carbon was made using the C / Chl a ratios. This resulted in the benthic microflora trophic compartment being assessed at 2.4 gC m-2. From the trophic level point of view, the primary producers, including micro and macrophytobenthos, dominate the system in terms of carbon biomass with 7.6 gC m-2.