Living deep-sea benthic foraminifera from the warm and oxygen-depleted environment of the Sulu Sea

The semi-enclosed, oligotrophic Sulu Sea is characterised by warm (ca. 10 degrees C) and oxygen-depleted (< 1.25 ml/l O-2) waters in the meso- and bathypelagic zones of the basin. The main objectives of this study were to examine the present-day distribution patterns of deep-sea benthic foraminiferal assemblages in relation to unique environmental conditions in the bottom waters of the Sulu Sea. Multiple-core sediments were obtained at seven sites in the Sulu Sea during the RN. Hakuho Maru KH-02-4 cruise at water depths between 534 and 4635 m. The investigation is based on the analysis of Rose Bengal-stained benthic foraminifera (> 32 mu m) from 56 samples (from surface down to 5 cm). The stained foraminiferal assemblages of the Sulu Sea comprised 285 species, including 137 agglutinated and 148 calcareous species. Standing stock values ranged from 595 individuals/10 cm(2) on the continental slope to 100 individuals/10 cm(2) in the abyssal zone. Generally, in the low-productivity and low-oxygen environment of the Sulu Sea, stained assemblages are characterized by low-standing stock values, moderate dominance and low diversity. The combination of poor rates of C org flux and low dissolved oxygen content in bottom waters are the main factors controlling the deep-sea benthic fauna in the Sulu Sea. The foraminiferal abundances in the upper bathyal zone respond to both food fluxes and oxygen concentrations. Most of the epifaunal, oxyphilic species have their lower limit of occurrence at a water depth of approximately 2000 m (dissolved oxygen < 1.25 ml/l) in the Sulu Sea. At constantly dysoxic (< 1 ml/l O-2) sites from the lower bathyal and abyssal zone, the assemblages comprised low-oxygen tolerant species and are more dependant on C org fluxes. The correlations between benthic foraminiferal faunas and C org flux and dissolved oxygen values are very good, with correlation coefficients of r(2) = 0.956 and 0.88, respectively. The living deep-sea foraminiferal assemblages consisted mainly of agglutinated tests, with dominant species such as Lagenammina difflugiformis, Ammoscalaria tenuimargo, various Reophax species, and small (< 70 mu m) trochamminaceans like Portatrochammina wiesneri. Stained individuals of the calcareous Parrelloides bradyi (type 2) occurred in low numbers at oxygen-poor sites, down to a depth of 5 cm in the sediment column. The only stained, calcareous, infaunal species found deeper than 4000 m was Valvulineria mexicana (average living depth of 3 cm). The general microhabitat preferences of common species were consistent with those from other studies, although due to the unique combination of poor food supply and dysoxic conditions, most of the infaunal species had shallower average living depths than those reported from other dysoxic locations. (c) 2006 Elsevier Ltd. All rights reserved.