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

Abstract

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 hydrodynamic conditions, eddy covariance has the potential to obtain true in situ benthic oxygen fluxes, giving greater insight into aquaculture-environmental interactions.