Mollusc-shell debris can mitigate the deleterious effects of organic pollution on marine sediments

1. Organic pollution is widespread in coastal areas and can have profound impacts on the seabed. Coastal sediments play an important role at a global scale in the recycling of organic matter, and this process is influenced by the habitat complexity of the sediments, among other factors. Mollusc shells are produced as a waste product from a range of anthropogenic activities, but we demonstrate that they can be used to increase the habitat complexity of sediments. 2. We studied the effect of mussel-shell debris (shell-hash) on the biogeochemical processes of marine sediments affected by organic pollution, using a mesocosm experiment simulating the bioturbation effects of macrofauna. 3. We found that shell-hash improved the ecological status of organically polluted sediments by reducing the accumulation of sulphide from anaerobic metabolic pathways. 4. Additionally, when shell-hash was present in an organically polluted sediment, there was a decrease in ammonium release to the water column, thus preventing the negative ecological consequences of eutrophication. 5. Synthesis and applications. Our study indicates that shell-hash debris can be used as a potential tool to mitigate the effects of organic enrichment on marine sediments. A density of shell-hash debris of 1900 g m-2 in the sediment can diminish toxic by-products (sulphides and ammonium) derived from the stimulation of anaerobic metabolic pathways by organic pollution, at levels that are biologically relevant. The mitigation effect of shell-hash is more pronounced in sediments where macrofauna is not present.This work has been funded by the projects: GRE14-19 from the University of Alicante, GV/2015/001 from the ‘Conselleria de Educación, Cultura y Deporte’ of the government of the Valencia region and CGL2015-70136-R from the Ministerio de Economía y Competitividad (MINECO/FEDER) of Spain. CS was funded by the contract ‘Juan de la Cierva’ (ref. JCI-2012-12413) from MINECO