Macrobenthic assemblage structure and organismal stoichiometry control faunal processing of particulate organic carbon and nitrogen in oxygen minimum zone sediments

Abstract. The Arabian Sea oxygen minimum zone (OMZ) impinges on the western Indian continental margin between 150 and 1500 m, causing gradients in oxygen availability and sediment geochemistry at the sea floor. Oxygen availability and sediment geochemistry are important factors structuring macrofaunal assemblages in marine sediments. However, relationships between macrofaunal assemblage structure and sea-floor carbon and nitrogen cycling are poorly understood. We conducted in situ ¹³C:¹⁵N tracer experiments in the OMZ core (540 m [O₂] = 0.35 μmol l^–1) and lower OMZ boundary (800–1100 m, [O₂] = 2.2–15.0 μmol l^–1) to investigate how macrofaunal assemblage structure, affected by different oxygen levels, and C:N coupling influence the fate of particulate organic matter. No macrofauna were present in the OMZ core. Within the OMZ boundary, relatively high abundance and biomass resulted in the highest macrofaunal assimilation of particulate organic carbon (POC) and nitrogen (PON) at the lower oxygen 800 m stations ([O₂] = 2.2–2.36 μmol l^–1). At these stations the numerically dominant cirratulid polychaetes exhibited greatest POC and PON uptake. By contrast, at the higher oxygen 1100 m station ([O₂] = 15.0 μmol l^–1) macrofaunal C and N assimilation was lower, with POC assimilation dominated by one large solitary ascidian. Macrofaunal POC and PON assimilation were influenced by changes in oxygen availability, and significantly correlated to differences in macrofaunal assemblage structure between stations. However, macrofaunal feeding responses were ultimately characterised by preferential organic nitrogen assimilation, relative to their internal C:N budgets.