Carbon processing at the deep-sea floor of the Arabian Sea oxygen minimum zone: A tracer approach

Abstract

We have elucidated the trophic interactions in the foodweb of sediments from and close to the oxygen minimum zone (OMZ) of the Arabian Sea. Sediment cores from inside (885m depth) and outside (1791m depth) the OMZ were manipulated onboard by adding 13C-enriched phytodetritus. The incorporation of phytodetritus by the benthic community was quantified after incubating for 7days. To assess the effect of bottom-water oxygenation on the processing of organic matter, the oxygen concentration in the overlying water of the incubated cores was also manipulated. Biomass values inside and outside the OMZ were comparable for bacteria (1068 and 1276mgCm−2) and macrofauna (2528 and 3263mgCm−2), but not for meiofauna (63 and 1338mgCm−2). Uptake values in percentage of total added tracer were 0.8 and 0.5% for bacteria suboxic and oxic treatments inside the OMZ, and 0.5 and 1.2% for suboxic and oxic treatments outside the OMZ. Macrofauna uptake accounted for 17.4 and 4.4% in the suboxic and oxic treatments inside the OMZ, and only for 0.1% and 1.3% respectively outside the OMZ. Respiration accounted for 13% of total tracer added inside the OMZ for both treatments, 4.6 and 6.8% for oxic and suboxic treatments outside the OMZ, respectively. Our results show that phytodetritus is most efficiently processed at in situ oxygen conditions, that foraminifera and bacteria remain active both under elevated and lowered bottom-water oxygen levels and that macrofauna was present in high abundance and showed high tracer uptake.