Abstract
Oxygen minimum zones (OMZs) currently impinge upon >1 million km(2) of sea floor and are predicted to expand with climate change. We investigated how changes in oxygen availability, macrofaunal biomass and retention of labile organic matter (OM) regulate heterotrophic bacterial C and N incorporation in the sediments of the OMZ-impacted Indian continental margin (540-1100 m; [O(2)]=0.35-15 μmol l(-1)). In situ pulse-chase experiments traced (13)C:(15)N-labelled phytodetritus into bulk sediment OM and hydrolysable amino acids, including the bacterial biomarker D-alanine. Where oxygen availability was lowest ([O(2)]=0.35 μmol l(-1)), metazoan macrofauna were absent and bacteria assimilated 30-90% of the labelled phytodetritus within the sediment. At higher oxygen levels ([O(2)]=2-15 μmol l(-1)) the macrofaunal presence and lower phytodetritus retention with the sediment occur concomitantly, and bacterial phytodetrital incorporation was reduced and retarded. Bacterial C and N incorporation exhibited a significant negative relationship with macrofaunal biomass across the OMZ. We hypothesise that fauna-bacterial interactions significantly influence OM recycling in low-oxygen sediments and need to be considered when assessing the consequences of global change on biogeochemical cycles.
Highlights
The bathyal continental margins (200–2000 m) are regions of high biogeochemical activity in the deep sea accounting for B7% of global sea floor area, but recycle up to 30% of oceanic sedimentary organic matter (OM) (Middelburg et al, 1997)
hydrolysable amino acids (HAA) made up 1.3–3.0% of the sediment organic C content and 15–30% of the sediment N content, across all stations
Low oxygen mediates the preservation of biomolecules resulting in a relatively large amino-acid pool within OMZimpacted sediments (Cowie et al, 1999; Vandewiele et al, 2009)
Summary
The bathyal continental margins (200–2000 m) are regions of high biogeochemical activity in the deep sea accounting for B7% of global sea floor area, but recycle up to 30% of oceanic sedimentary organic matter (OM) (Middelburg et al, 1997). At oxygen concentrations of 15 mmol l À 1, C: N phyto phyto ratios were lower than the C:N ratio of the added phytodetritus, indicating enhanced N phyto retention within sediment HAAs. Between 4 and 7 days changes in C: N phyto phyto ratios at T2 1100 m, suggest a progressive loss of N phyto (Figure 6b).
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