| Summary: | The bathyal continental margins (200 – 3000 m) account for 7 % of global sea floor area, but are responsible for recycling 30 % of sedimentary organic matter (OM) within the oceans. Climate-change driven expansion of oxygen minimum zones and increasing frequency of extreme weather events (storms) may have implications for OM processing at the continental margins, resulting in decreased oxygen availability and increases in terrigenous OM inputs. The present thesis tests how changes in oxygen availability influence sea floor community structure and OM processing across the OMZ-impacted Indian continental margin. Megafaunal and macrofaunal assemblages exhibit a distinct zonation, with faunal distributions controlled by changes in oxygen availability and sediment OM content. In situ stable-isotope pulse-chase experiments reveal that, following phytodetritus deposition, macrofaunal feeding responses were influenced by oxygen availability through changes in macrofaunal assemblage structure. Macrofaunal feeding was governed at the organismal level, with fauna maximising organic nitrogen assimilation against an internal carbon-nitrogen budget. Bacterial feeding responses were not directly influenced by changes in oxygen availability and instead were regulated by the macrofaunal assemblage. This thesis proposes that the macrofauna may control the availability of labile organic matter in OMZ-sediments, limiting bacterial activity. The thesis also investigates the differential fates of marine and terrigenous phytodetritus in the sediments of the Whittard canyon (NE Atlantic). In situ pulse-chase experiments reveal nitrogen demand to control macrofaunal feeding responses, inhibiting the utilisation of the nitrogen-poor terrigenous phytodetritus. Bacteria exhibit similar feeding responses to the fauna, whilst bacterial biomass was negatively correlated to increasing faunal activity. This demonstrates that increased inputs of terrigenous organic matter may alter ecosystem-scale carbon cycling pathways and trophic interactions within continental margin sediments.
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