Regional variation of total microbial biomass in sediments of the deep Arabian Sea

Eight different sites from 2300 to 4420 m water depth in the Arabian Sea we re sampled for a biochemical quantification of phospholipid concentrations in the sediments. This method serves as a measure of microbial biomass in m arine sediments comprising all small-sized organisms, including bacteria, f ungi, protozoa and metazoa. Phospholipid concentrations can be converted to carbon units as an estimate of total microbial biomass in the sediments. T he average phospholipid concentrations in the surface sediments (0-1 cm) of the 4 abyssal sites ranged from 7 nmol cm(-3) at the southern site (SAST, 10 degrees N 65 degrees E, 4425 m) to 29 nmol cm(-3) at the western site (W AST, 16 degrees N 60 degrees E, 4045 m). The high values detected at the ab yssal station WAST exceeded those in the literature for other abyssal sites and were comparable to values from the upper continental slope of the NE-A tlantic and the Arctic. At the four continental slope sites in the Arabian Sea, average phospholipid concentrations ranged from 9 to 53 nmol cm(-3) Wi th the maximum values at stations A (2314 m) and D (3142 m) close to the Om ani coast. Records of particulate organic carbon flux to the deep sea are a vailable for four of the investigated locations, allowing a test of the hyp othesis that the standing stock of benthic microorganisms in the deep sea i s controlled by substrate availability, i.e. particle sedimentation. Total microbial biomass in the surface sediments of the Arabian Sea was positivel y correlated with sedimentation rates, consistent with previous studies of other oceans. The use of the measurement of phospholipid concentrations as a proxy for input of particulate organic matter is discussed. (C) 1999 Else vier Science Ltd. All rights reserved.