Effect of sediment grain size and bioturbation on decomposition of organic matter from aquaculture

Sediment grain size plays a major role in sediment biogeochemistry and sediments with different grain size are expected to react differently to organic enrichment. Through a mesocosm approach we tested the behavior of sediments with two types of predominant grain size (sandy and muddy sediments) under two levels of organic enrichment, related to mussel and fish farming. The polychaete Hediste diversicolor was used to simulate macrofauna bioturbation and bioirrigation. H. diversicolor stimulated organic matter (OM) mineralization and nutrient recycling. Muddy sediments had more OM from nature, resulting in higher sulfate reduction rates than sandy sediments. Under low levels of organic enrichment grain size did not have any effect on benthic fluxes (sediment oxygen uptake, total CO2, ammonium, nitrate, nitrite and phosphate). However, at high levels of organic enrichment, sandy sediments accumulated less OM, less sulfide and less ammonium than muddy sediments, while sediment oxygen uptake and total CO2 showed similar levels between sandy and muddy sediments. Thus, grain size should be considered a key parameter for site selection of fish farming facilities when aiming for a sustainable aquaculture industry.