Live benthic foraminifera in the Yellow Sea and the East China Sea: vertical distribution, nitrate storage, and potential denitrification

Abstract

Benthic foraminifera were investigated to determine their vertical distribution, nitrate storage, and potential denitrification in the Yellow Sea (YS) and the East China Sea (ECS). The phytodetritus content and freshness as well as sediment redox conditions were key factors determining the vertical distribution of foraminifera in sediments of the YS and the ECS. The intracellular nitrate (ICNO3) concentrations of Nonionella stella, Hanzawaia nipponica, Bolivina robusta, Cancris auriculus, and Globobulimina pacifica ranged from 3 to 114 mM, which in some cases was several hundred-fold more than the pore water nitrate (PWNO3) concentrations. The genus Hanzawaia was for the first time reported to store ICNO3 (316 +/- 73 pmol ind.(-1); 11 +/- 3 mM). The significant correlation between the ICNO3 concentration and foraminiferal abundance (Pearson correlation: r = 0.401, p < 0.01, n = 60) suggested that foraminifera may have an important role in nitrate storage in the sediments. The foraminiferal intracellular nitrate (FINO3) pool ranged from 9 to 74% of the ICNO3 pool in sediments, indicating that nitrate may be stored by other sediment organisms (e.g. diatoms). The chlorophyll a concentration, chloroplastic pigment equivalents (CPE), and chlorophyll a:phaeopigment ratio were all significantly positively correlated with the ICNO3 concentration (Pearson correlation: r = 0.563, 0.603, and 0.457, respectively; p < 0.01; n = 60), indicating that phytodetritus (e.g. diatoms) might also contribute to sedimentary ICNO3. Potential foraminiferal denitrification rates ranged from 9 to 92 mu mol m(-2) d(-1) in the YS and the ECS, indicating that benthic foraminifera might play a role in sedimentary denitrification.