SEDIMENT NATURE AND MEIOBENTHOS OF AN ART IFICIAL REEF (ACADJA) USED FOR EXTENSIVE AQUACULTURE

In developing countries, recourse to extensive aquaculture systems bas ed on the exploitation of natural aquatic food webs is currently the s ubject of considerable interest. In this connection, the construction of artificial reefs (known as Acadja) for the development on substrate s of living communities, and the utilization of this new production to enhance fish growth have been evaluated in Cote d'Ivoire. This paper describes the granulometric, chemical and biological transformation of the sedimentary ecosystem as a consequence of this practice. In the a rtificial habitat, an increase in the sedimentation rate of the minera l fraction (definitive trapping of particle resuspension generated by the waves and induced by the wind) and of the organic fraction (direct and indirect sedimentation of the epiphytic biomass) constitutes the starting point of significant modifications in epibenthic communities. In the Acadja, the meiofauna, dominated by nematodes and copepods, is somewhat less diversified, some groups being absent or poorly represe nted (turbellarians, gastrotrichs, ostracods). These characteristics w ere confirmed by a specific study of the nematode assemblages. In fact , the relatively high diversity of the nematode community in the natur al lagoon sediment (dominated seasonally by Paraphanolaimus, Anonchus et Daptonema) contrasts with low diversity in the Acadja sediment, whe re the community is always dominated by Theristus sp. This lower speci fic diversity generates a reduction of the trophic types. In the Acadj a, only the non-selective detritus feeders colonize the fine and organ ic sediment. These sedimentological and ecological transformations, wh ich affect the entire ''brush park'' (Acadja), induce a strong eutroph ication of the benthic ecosystem. Sustainable fish production in such tropical lagoons, in an environment where anaerobic processes dominate (especially sulfate-reduction), calls for strategic spatio-temporal p lanning to take of account of this progressive eutrophication.