Tidal modulation of benthic oxygen fluxes in a suspended mussel aquaculture

Benthic oxygen uptake in coastal mussel farms is enhanced by organic bio-deposition and can trigger hypoxic or anoxic conditions. Despite this, previous studies have determined benthic oxygen uptake under simulated conditions using traditional flux methods, overlooking the dynamic nature of coastal regions that can affect oxygen exchanges with processes occurring on different time scales. In this study, tidal variability of benthic oxygen fluxes in a mussel farm was examined using the aquatic eddy covariance (AEC) technique. Four deployments were conducted during neap and spring tides in the Ría de Vigo (NW Spain). Two AEC systems were concurrently deployed, one on each side of a mussel raft to measure benthic oxygen fluxes during tidal ebbs and floods. During neap tides, significantly higher oxygen flux was observed during floods (mean ± SD = -64 ± 34 mmol m^-2 d^-1) than ebbs (-42 ± 25 mmol m^-2 d^-1). This was due to higher velocities and associated shear stress. Conversely, during spring tides, oxygen fluxes were significantly higher during ebbs (-67 ± 34 mmol m^-2 d^-1) than floods (-51 ± 29 mmol m^-2 d^-1) despite higher velocities for floods. We suggest that this might be due to spatial differences in the shear stress, and possibly in resuspension, caused by bottom roughness variations. These differences in bottom roughness with tidal current direction is associated to the inhomogeneous distribution of mussel shells below the rafts. Measurements showed that the magnitude and direction of tidal currents modulated benthic oxygen fluxes by resuspending organic-rich sediments in shallow coastal ecosystems. Our results indicated that tidal driven resuspension enhanced oxygen consumption, and suggest that future mussel farms should be placed in deeper marine environments where this effect is reduced.