Impact of suspended mussel aquaculture on benthic oxygen fluxes

Global aquaculture has experienced rapid growth in recent decades to meet the food demands of a growing population. Among non-fed aquaculture, suspension culture of mussels exerts a great control on the coupling of pelagic and benthic processes. These organisms feed on the seston of the water column and produce a large amount of faeces which are rapidly deposited on the sediment surface. This large biodeposition increases the benthic remineralization, causing an impact on the benthic-pelagic coupling and, to large extent, on the net ecosystem metabolism. The main objective of this study was to quantify the impact of one of the most intensive mussel aquaculture, located in the NW Iberian coastal upwelling system, on benthic oxygen fluxes by applying the aquatic eddy covariance (AEC) technique. Two AEC systems were deployed concurrently; one below a mussel raft and one in a reference location during summer upwelling season. Benthic oxygen fluxes were 2.4 times larger below the mussel raft compared with the reference position (mean ± SD): -48 ± 36 mmol m^-2 d^-1 vs. -20 ± 17 mmol m^-2 d^-1). This increased benthic oxygen consumption was consistent with the large vertical carbon fluxes previously observed below mussel rafts due to biodeposition. The large benthic oxygen consumption enhanced heterotrophic conditions in the mussel farm area accounting for 40% of the total ecosystem respiration (water column respiration + mussel respiration + benthic oxygen uptake). This study emphasizes the impact that mussel aquaculture has on the trophic state of coastal regions and the need of monitoring strategies for benthic oxygen uptake in aquaculture areas.