Spatial variability of the biogeochemical composition of surface sediments in an insular coral reef ecosystem: Moorea, French Polynesia

The spatial variability of the biogeochemical composition of surface sediments was investigated around Moorea, French Polynesia. This study complements other ones conducted in Moorea to assess coral reef ‘‘health’’. A total of 35 stations were sampled at four sites around the island, on the following geomorphological units: fringing reef of the bays, fringing reef of the lagoon, channel, barrier reef, pass, sandy slope, bay bottom. We found strong spatial heterogeneity in the biogeochemical composition of surface sediments. The multivariate analyses (PCA and clustering) distinguished four major sediment types around Moorea, and a gradient between the bayheads and the bay entrances. The first type corresponds to the inner half bay bottoms, and clearly represents the ‘‘terrigeneous’’ influence. This terrigeneous sediment type was mainly characterised by a clear deficit in carbonates and, conversely, by an enrichment in organic compounds (organic carbon (OC), nitrogen (N), as well as total amino acids (AA) and carbohydrates (CH) in mg g 1) that were highly degraded. Two other sediment types were influenced by the carbonated reef system. They were highly carbonated (inorganic carbon (IC) around 11%; i.e., carbonates around 90%), and showed low organic compound concentrations. They were also characterised by high hydrolysable organic carbon (HOC) concentrations. These two ‘‘reef-characteristic’’ sediment types can be distinguished by their N concentrations and C/N ratio. The fourth type represents a transition between ‘‘reef-characteristic’’ and ‘‘terrigeneous’’ sediment types. Our results showed that HOC, total AA and CH, examined for the first time in coral reef ecosystems, were reliable descriptors for spatial variability surveys, together with the more ‘classical’ descriptors: IC, OC and N.