EPIFAUNAL TAXA IN AN INFAUNAL WORLD

A series of sixteen multicores were taken on depth transects (360-3000 m) across the Southern California Bight oxygen minimum zone (OMZ) to investigate the ecology of living (rose Bengal stained) benthic foraminifera. Dissolved oxygen concentrations in bottom water at sampling sites varied from 0.47ml/L to 3.63ml/L. Bathygraphic highs were targeted in an effort to sample habitats with coarse sediments. Mean grain size varied from about 131μm (gravelly sand) to about 830μm (coarse sand with fine gravel). Dominant taxa in the >150μm size fraction assemblages from 0-2cm included Uvigerina peregrina, Cassidulina laevigata, Hoeglundina elegans, Cibicidoides wuellerstorfi, and Reophax dentaliniformis. Dominant taxa in the 63-150μm size fraction assemblages from 0-1cm included Uvigerina auberiana, Cassidulina carinata, Cibicidoides bradyi, and Bolivina spissa. Sediment-dwelling infaunal taxa, such as Uvigerina and Bolivina, traditionally associated with the supersaturated, unconsolidated mud, characteristic of OMZ habitats, are also able to live in coarse-grained sediment. Vertical distribution patterns (0-2cm) were consistent with those of conspecifics reported elsewhere, and reconfirm that Cibicidoides wuellerstorfi, Hanzawaia nipponica, and Hoeglundina elegans are epifaunal, living near or above the sediment-water interface. Despite oxygen-poor conditions (0.47 ml/L to 0.63 ml/L), epifaunal taxa comprised as much as 36 percent of the stained population at the five sites with the coarsest mean grain size. The presence of appreciable densities of epifaunal taxa in oxygen-poor habitats indicates that these species have much lower oxygen requirements than previously thought. We suggest that in environments with bottom water dissolved oxygen levels above their lowest requirements, these epifaunal taxa, including C. wuellerstorfi, prefer habitats with coarse grains that allow them to remain at or above the sediment-water interface. These results dispel the notion that abundant epifaunal foraminifera necessarily indicate well-oxygenated conditions, and strongly challenge methods that use epifaunal indicator taxa or groups of taxa to assess oxygen concentrations in deep water environments.