ABSTRACT: The Orinoco is the worldâ€™s fourth largest river, outflow from it forming an extensive hypopycnal plume around Trinidad and Tobago and over much of the eastern Caribbean Sea. The plumeâ€™s areal extent fluctuates through the year, being at its greatest during the northern South American, July-December rainy season. The area perennially-impacted by the plume is thus surrounded by a rim that is only seasonally-impacted. The plume is transported NW across the Caribbean Sea by the Caribbean Current. Two upper bathyal piston cores were used in this study. Core BGT-031 (water depth 260 m) was taken from the perennially-impacted area east of Trinidad, while JPC-ASC-001 (water depth 428 m) was collected from the seasonally-impacted area northwest of Tobago. Both cores are ~80 cm long. They were sampled at 10 cm intervals, four replicates being collected from each of nine horizons. Qualitative comparisons are made with a previously-reported core (BGT-086) collected from the seasonally-impacted area east of Trinidad. Distinct differences in foraminiferal assemblages were found between JPC-ASC-001 and BGT-036. These differences are ascribed to the coresâ€™ differing relationships with the Orinoco Plume. The assemblage in the perennially-impacted Core BGT-031 was dominated by Uvigerina parvula with subdominant Cibicidoides umbonatus and Cibicides pachyderma. That in the seasonally-impacted JPC-ASC-001 was dominated by Sphaeroidina bulloides, with (in rank order) subdominant Globocassidulina subglobosa subcalifornica, C. umbonatus and C. pachyderma. ANOVA indicated that U. parvula and S. bulloides are both specialist species with relatively narrow niches, the mean proportional abundances differing between horizons in their respective cores. Cibicidoides umbonatus is in contrast shown to be a generalist species. The mean Shannonâ€™s index H for replicates from JPC-ASC-001 was significantly greater than that for replicates from BGT-031, but qualitatively comparable to that for BGT-086. Meanwhile, mean dominance, measured using max(pi), was significantly lower in JPC-ASC-001 than in BGT-031, that in the former being comparable to that in BGT-086. Epistominella exigua, which is indicative of a seasonal phytodetrital flux, was recovered from BGT-086, but not from JPC-ASC-001, even though both cores were recovered from the seasonally-impacted rim. This difference is ascribed to the development of a wake and an eddy in the Caribbean Current to the lee of Tobago that locally change the nature of surface productivity.
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