THE ECOLOGY OF LIVING (STAINED) DEEP-SEA BENTHIC FORAMINIFERA FROM THE SULU SEA

The distribution of living (rose bengal stained) deep-sea benthic fora minifera was determined in the upper 20 cm of sediments of eight Souta r box cores taken from two depth transects (510-4515 m) in the thermos pheric (> 10-degrees-C) Sulu Sea. Despite the uniformity of bottom wat er temperatures, salinities, and dissolved oxygen levels below 1000 m, significant faunal differences exist at different depths in the low-o xygen (approximately 1.25 mL/L below 1000 m) basin. The shallowest sit e (510 m) is dominated (> 10% of the calcareous fauna) by Cibicidoides , Uvigerina, (> 150 mum) and Bolivina (> 63 mum), while Siphonina is c odominant with Cibicidoides and Uvigerina in the 1005-m core. The 2000 -m cores are dominated by Cibicidoides, Gyroidinoides, and Oridorsalis , while, Cibicidoides bradyi and Oridorsalis umbonatus dominate the 30 00- and 4000-m cores. Infaunal assemblages of Valvulineria mexicana ar e found in the sediments of the 4515-m core. Relatively low bottom wat er oxygen values do not necessarily yield ''typical low-oxygen taxa'' such as Bolivina, Uvigerina, Chilostomella, Bulimina, and Globobulimin a. Changes in the abundances of these taxa in fossil assemblages have been used as indicators of changes in ancient bottom water oxygen leve ls but may instead reflect organic carbon contents of the sediments. A n examination of the vertical distributions of foraminiferal assemblag es from the > 63-mum and > 150-mum fractions reveals that taxa have mi crohabitat preferences similar to those observed in other regions. Tax a found in the upper 0- to 1-cm interval (epifaunal) include Cibicidoi des wuellerstorfi and Hoeglundina elegans, while taxa such as Chilosto mella and Globobulimina reach maximum abundances in subsurface sedimen ts and have infaunal microhabitat preferences. Cibicidoides bradyi and O. umbonatus live in sediment depths from 0- to 4-cm and have transit ional preferences with both epifaunal and infaunal occurrences. Intrag eneric differences in test morphologies, including pore distribution, rounded peripheries, and variable spire height, are observed in Cibici doides and Gyroidinoides and are suggested to be related to microhabit at preferences. Vertical distributions of a number of taxa found in bo th the 63- to 150-mum and > 150-mum fractions are similar, suggesting that juveniles and adults live under similar microhabitat conditions. Ontogenetic changes in microhabitat preferences of most species are no t observed in this study and therefore would not be expected to accoun t for isotopic vital effects reported for some taxa in previous studie s.