Investigating benthic impacts at salmon farms using eddy covariance measurements of benthic oxygen fluxes

Benthic oxygen fluxes were measured in situ using the eddy covariance method at 2 Chinook salmon farms located in the New Zealand Marlborough Sounds in 29–35 m depth and compared to sediment nutrient (C, N and P) and sulphide concentrations. Observations from 3 sites at a high-flow location (near-bed RMS velocities of 0.11 to 0.17 m s−1) showed oxygen fluxes increasing with sediment enrichment. Mean ± SE oxygen fluxes of −102 ± 4 mmol O2 m−2 d−1 were observed immediately adjacent the farm. Higher fluxes were observed at 120 m from the farm (−53 mmol O2 m−2 d−1) than at 50 m (−48 mmol O2 m−2 d−1), consistent with higher sediment nutrient concentrations at the more distant site, and reflecting the effect of currents on deposition patterns. Ratios of C, N and P in sediments indicated a reduction of N mineralisation rates as sediment enrichment increased. The low-flow site (near-bed RMS velocities of 0.033 m s−1) had similar oxygen fluxes (−108 ± 9 mmol O2 m−2 d−1) to the high-flow site closest to the farm, but sediments were more highly enriched with high sulphide concentrations. Oxygen fluxes at the low-flow site were close to the estimated maximum potential flux that could be achieved under the ambient hydrodynamic conditions. Less than 0.2% of surface PAR reached the sediments, and no evidence of benthic primary production was observed at any of the sites. By incorporating ambient hydro-dynamic conditions, eddy covariance has the potential to obtain true in situ benthic oxygen fluxes, giving greater insight into aquaculture–environmental interactions.