Sandy beach macrofauna along the shore of Fiji: Low species richness due to beach morphodynamics or low productivity in coastal waters?

Six sandy beaches located on the south coast of Viti-Levu, Fiji, were sampled to provide as a first aim, an environmental description based upon their physical and biological attributes, to serve as a reference tool for further monitoring programs. Beach face slopes were measured at 4 replicated transects stretching from the front dunes or the seaward reach of the tree vegetation (upper shore level) to the low tide level. Samples for analyses of sand particle size were collected at 4 tidal levels: the upper shore, the drift and effluent lines, and the low tide level. Samples for macroinvertebrates were sampled at the dry, retention, and resurgence/swash zones. Beach sites were categorized in terms of their morphodynamic types by means of photographs taken on site and expert analysis, taking into consideration beach width, beach face slopes and mean grain sizes of sands. Three of the studied beaches were labeled as reflective, two were intermediate and one was in between these two morphodynamic types. The mean number of intertidal species and abundance of macroinvertebrates (dominated by crustaceans and polychaetes) across all sites were 4.1 taxa (±1.7, ranges = 2–6) and 917 individuals per linear meter of beach (m⁻¹) (±490, ranges = 133–1154). The second aim was to test the hypothesis that, along this coast, those biotic attributes are related to beach morphodynamic types. Our data allowed us to reject this hypothesis, since i) species richness and abundances of macroinvertebrates, did not differ significantly among beaches, ii) beach slope, beach width, and sediment grain size did not account for significant variations in the number of species and abundance of macroinvertebrates, and iii) multivariate analysis indicated that sediment grain size and beach slope just accounted for 31 % of the biotic richness variation. The third aim, was to evaluate the hypotheses that biotic richness in open ocean sandy beaches is not only affected by local environmental factors, but also by regional scale processes, such as ocean productivity. Thus, biotic information and coastal productivity data of ocean exposed sandy beaches along the tropical and subtropical belt at global scale were gathered from scientific literature. Based on upwelling and downwelling measurements, we conclude that oligotrophic ocean conditions, account for the low biodiversity and abundance of macroinvertebrate assemblages inhabiting the Fijian beaches studied. Our study establishes, for the first time, a reliable baseline for future studies and projects aimed at the protection of natural sandy beaches along the shores of the Fiji Islands and other isolated archipelagos of the tropical Pacific Ocean, where oligotrophic conditions prevail in their aquatic environment.