Annual sediment primary production and respiration in a large coral reef lagoon (SW New Caledonia)

Sediment photosynthetic production and community respiration were investigated for 1 yr in the south-west lagoon of New Caledonia (surface area: 2000 km'; mean depth: 21 m). Metabolic fluxes were measuredbat the water-sediment interface using benthic enclosures, at 60 sampling stations randomly distributed in space and time. Mean gross primary production (Pg) was 12.06 mol C m-' yr-l. Mean respiration (R) was 13.68 mol C m-'yr-'. Pg/R was thus 0.88, indicating that the whole lagoon was net heterotrophic. There were, however, large differences along a land-ocean gradient. Muddy bottoms (35% of the lagoon surface), located along the shore and in the deeper part of the lagoon, were significantly heterotrophic (Pg/R = 0.39), whereas the ratio for sandy bottoms (65 % of the area) towards the reef were close to 1 or slightly positive, indicating potentially autotrophic benthic communities. Both Pg and R followed a similar seasonal variation with higher values during the warm season (November to February), when maximum irradiance and the particulate organic carbon supply occurred. These results suggest that autotrophy, generally observed in coral reef flats, may also occur in lagoon sediments, indicating both a major importance of regeneration processes and a relative equilibrium of energy exchanges with adjacent ecosystems. Terrestrial influence results in a decrease of autochthonous production and a correlative increase of carbon sedimentation leading to benthic heterotrophy.