Abstract

Living (rose-Bengal-stained) foraminifera were investigated from the continental margin off the Pacific Coast of Costa Rica and Nicaragua. Assemblages from seven stations, including mounds and mud volcanoes, were compared in terms of foraminiferal density, distribution patterns, diversity and species composition and related to environmental factors (satellite chlorophyll a imagery, organic carbon content in the sediment, available oxygen data, particle grain sizes). Sediments from mud volcanoes and mounds did not show direct influence of seepage. Highest foraminiferal densities were found on the slope off Nicaragua. The lowest foraminiferal densities were found in the north off Costa Rica where the smooth Cocos Plate segment subducts beneath the Caribbean Plate. Intermediate values were counted in the southern investigated area, where the down-going plate has a rough topography. Observed foraminiferal densities did not always correspond to chlorophyll a imagery or measured organic carbon content in the sediment, suggesting a possible influence of time-limited food supply, lateral transport of sinking organic material and local variability in the investigated area. In total, 187 species were recognized, distributed across the seven stations, which all showed high diversity. Foraminiferal associations contained many Buliminida, pointing to low-oxygen concentrations at the sediment-water interface. Some taxa indicated a possible bathymetric or regional limitation. The assemblage composition varied among the stations to such a degree that no commonalities were identified, based on results from multivariate analysis. This was also the case for samples originating from mounds and mud volcanoes, which did not show any outstandingly different assemblages, meaning that no characteristic foraminiferal assemblage could be used to identify these areas. This mosaic-like distribution pointed to a large degree of habitat heterogeneity in the relatively small area; the habitat heterogeneity could explain the overall high diversity of benthic foraminifera in the region.

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