Living (stained) deep-sea foraminifera off Hachinohe (NE Japan, Western Pacific): environmental interplay in oxygen-depleted ecosystems

Live (Rose-Bengal stained) deep-sea foraminiferal faunashave been studied at five stations between 500–2000-m depthalong the NE Japanese margin (western Pacific) tounderstand how complex environmental conditions (e.g.,oxygen depletion, organic matter) control their structure(i.e., diversity, standing stocks, and microhabitats). Allstations are characterized by silty sediments with no evidenceof recent physical disturbances. The three stations locatedbetween 760–1250 m are bathed by dysoxic bottom waters(,45 mmol/L). Although high organic-carbon contents arerecorded at all stations (.2.2% DW), only the oxygendepletedsites are characterized by higher concentrations ofsugars, lipids, and enzymatically hydrolysable amino acids(EHAA). Sedimentary contents in chlorophyllic pigmentsdecrease with water depth without any major change in theirfreshness (i.e., [Chl a/(Chl a + Pheo a)] ratios) . BothUvigerina akitaensis and Bolivina spissa are restricted to thestations bathed by dysoxic waters, proving their oxygendepletiontolerance. In such conditions, both phytophagoustaxa are obviously able to take advantage of labile organiccompounds (e.g., lipids and EHAA) contained in phytodetritus.Nonionella stella and Rutherfordoides cornuta survivein oxygen-depleted environments probably via alternativemetabolic pathways (e.g., denitrification ability) and a largeflexibility in trophic requirements. At stations where oxygenavailability is higher (i.e., .70 mmol/L in bottom water) andwhere bioavailable organic compounds are slightly lessabundant, diversity indices remain low, and more competitivespecies (e.g., Uvigerina curticosta, U. cf. U. graciliformis,Nonionella globosa, Nonionellina labradorica, and Elphidiumbatialis) are dominant.