Distributions of ostracod (Crustacea) biofacies on the continental shelf off south-east Trinidad, western central Atlantic Ocean, suggest the location of an offshore river-induced front within the Orinoco Plume

Abstract

Thirty two seafloor samples were taken over an area of 3600 km2 off south-east Trinidad (water depths 38–102 m), north-east of the Orinoco Delta. A total of 7097 ostracod carapaces and valves from 37 species were picked from them. Total recovery was co-dominated byBairdoppilata dorsoangulata andBradleya ex gr.dictyon, with lesserRadimella ovata, Loxoconcha sp. 2sensuBreman, 1982 andCosta variabilicostata recticostata. Cluster analysis revealed four biofacies approximately perpendicular to the bathymetric contours. This arrangement is contrary to the usual pattern, in which biofacies parallel contours, and is suggested to reflect the occurrence of a shelfal, river-induced front entrained within the hypopycnal Orinoco Plume. The front marks where outflow from the Essequibo and Orinoco rivers abuts against surface water rich in outflow from the Amazon. The biofacies immediately under the front yielded particularly high proportions ofB. ex. gr.dictyon andB. dorsoangulata, but relatively fewLoxoconcha sp. 2. Application of SHE Analysis for Biozone Identification (SHEBI) indicated that within the biofacies there occur abundance biozone boundaries at ∼55m, 75 m and 100 m that separate areas with differing population structures. SHE Analysis for Community Structure Investigation (SHESCI) indicated that, although the species proportions within most abundance biozones have a logarithmic series distribution, the rate of increase in species richness as specimens were accumulated was lowest in an abundance biozone below the front.