Bioturbation may not always enhance the metabolic capacity of organic polluted sediments

Marine sediments are a major sink of organic matter, playing a crucial role in the global cycling of major elements. Macrofauna, through the reworking of particles and movement of solutes (bioturbation), enhances oxic conditions and the sediment metabolic capacity. Increases in the inputs of organic matter can lead to profound changes in the seabed and impact benthic ecological functions. Through a microcosm experiment, the effect of bioturbation of the polychaete Lumbrineris latreilli on biogeochemical fluxes under scenarios of increasing loads of organic matter was quantified. We found that bioturbation can buffer the negative consequences of anoxic conditions produced by organic enrichment, preventing the build-up of toxic by-products derived from anaerobic metabolic pathways by maintaining oxic conditions. However, the maintenance of oxic conditions by bioturbation is at the expense of limiting the sediment metabolic capacity. The maintenance of oxic conditions may limit anaerobic metabolic pathways, and consequently, the metabolic capacity of sediment. Thus, under organic matter pollution conditions, bioturbation may lessen the metabolic capacity of the sediment.