ALBERT

Description: To assess the contribution of soft-bottoms to the carbon cycle in coral reefs, the net community production (p) was measured in winter at 3 stations on La Saline inner reef flat (Reunion Island). Changes in pH and total alkalinity at different irradiances (I) were assessed using benthic chambers (0.2 m2) during a 1-h incubation. Mean grain size, the silt and clay load and chlorophyll a content of the sediments were analysed in each chamber. Daily community production (P), gross community production (Pg) and community respiration (R) were estimated from p-I curves and daily irradiance variations (PAR, 400-700 nm). Sediment characteristics and chlorophyll a contents did not differ between the three sites, except for the silt and clay fraction at one station. R being higher than Pg (84.88 ± 7.36 and -62.29 ± 3.34 mmolC m-2 d-1 respectively), P value reached 22.59 ± 5.66 mmolC m-2 d-1. The sediments were therefore heterotrophic with a mean Pg/R lower than 1 (0.74 ± 0.05) and appear to be a carbon source. Our data suggested the importance of the degradation process in the functioning of near-reef sediments.

Description: Coral reefs are very productive ecosystems. Soft-sediment plays generally a role in storing place, in transforming and in producing of matter, and therefore it could play a key role in coral reefs ecosystems. The goal of this study was to evaluate the role of soft-bottom compartment during the transfer of matter and energy in the reef of La Saline (Reunion Island). Three main lines of research were developed: the physicochemical characterisation of studied site, the metabolism of soft bottom in view to estimate the trophic production and its status and finally, the characterisation of biological component by the study of macrobentos and megabentos (i.e. Holothurians). A limited disturbance was detected by physicochemical data although locally enrichment of freshwater by nutrients (nitrates+nitrites+silicates) were measured (reef back hollow of Planch'Alize and Grand Trou d'Eau). At the reef scale (9 stations and 2 seasons), the soft-sediment was found heterotrophic (net production = 31.6, respiration R = 109.6, gross production Pg = 77.9 mmolC m-2 d-1 et PgR = 0.7). Opposite to the two holothurians studied population Holothuria atra et H. leucospilota, macrobenthos (〉 1 mm) did not constitute a major group in the food web (134 taxa, with a mean of 32.86 individuals per 0.2 m-2 et 0.059 gPS 0.2m-2. compared to Echinoderm biomass that could reach 7.92 gPS m-2. At the station scale, spatial heterogeneity of metabolism was in agreement with the physicochemical characteristics of environment reflecting the double influence of both ocean and continent (freshwater enrichment). The most productive and heterotrophic stations were located in the reef back hollow where the oceanic influence was the lowest. Finally, macrofauna was under hydrodynamic influence although holothurians are growing in the most productive area. The soft-bottom sediment played a key role in filtrating organic matter provided from the back reef. This mechanism was deeply influenced by the high hydrodynamism (narrow reef and low water level), which modulated the loss of mater and energy of the reef. These losses were however limited by the action of holothurians (high density ind m-2), which store organic matter such as biomass and enhanced probably local production via excretion and bioturbation. To conclude, at the ecosystem scale, soft-bottom was responsible of recycling, which was reinforced by holothurians that limited the loss of matter and energy due to hydrodynamism.