Scratching the surface: Biogeochemical consequences of faunal species loss in coastal sediments.

Coastal and shallow seas cover 7 % of the global seafloor but account for around 80 % of carbon dioxide fixation and burial in the oceans. Consequently, they are extremely important in the regulation of atmospheric carbon dioxide and marine nutrient cycles. Coastal ecosystems are sensitive to human-induced pressure from fisheries and pollution resulting in faunal species loss. In marine sediments, microbial activity is the dominant process that drives the carbon fixation, recycling and burial. Yet bacterial activity is mediated by the faunal community through processes such as bioturbation and deposit-feeding. As a consequence, changes in faunal activity may have indirect effects upon primary production via the microphytobenthos (primarily diatoms and cyanobacteria) at the sediment surface, and the recycling of organic matter by heterotrophic microorganisms. We conducted mesocosm-based experiments to test how changes in the presence/absence of three ecosystem engineers, the shore crab (Carcinus maenas), the ragworm (Alitta virens) and lugworm (Arenicola marina), affected micro-algal carbon fixation at the sediment surface, and the retention and transfer of newly fixed carbon within the sediment community. We used stable-isotope pulse chase techniques to directly trace the fixation of dissolved inorganic carbon (13C-labelled sodium bicarbonate) by the microphytobenthos and its preservation as sedimentary organic matter, alongside changes in sediment community respiration, primary production and dissolved nutrient fluxes. Here we will present the initial results from these studies and discuss the implications of faunal species-loss upon carbon cycling and the functioning of coastal sediment ecosystems.