Hydrodynamic effect of a large offshore mussel suspended aquaculture farm

Hydrodynamic effect of a mussel suspended aquaculture farm on tidal currents off Gouqi Island, outside the Changjiang Estuary, China, was examined using observational data. The surface friction induced by suspended mussel aquaculture facilities in the top 3.5 m was much stronger than bottom friction. As a result, tidal current velocity was reduced by 75-90% in the top layer and by about 45% at the bottom. The mean surface hydraulic roughness z0s̅ and surface drag coefficient C500̅, were estimated to be around 1.392-1.890 m and 0.0875-0.1694, respectively. On the contrary, the mean bottom hydraulic roughness z0b̅ and bottom drag coefficient C100̅ within the farm, were estimated to be around 0.093-0.098 m and 0.0226-0.0252, respectively. The observed surface friction velocity u⁎s have the same order of magnitude with the bottom friction velocity u⁎b. However, the surface hydraulic roughness z0s is about 20 times greater than that at the bottom. The local surface drag coefficient C500̅ is 3.5-7.5 times of that at the local bottom. The aquaculture facilities not only inhibited the horizontal tidal currents, but also appeared to have induced downwelling, which was shown by the vertical profiles of salinity, temperature, and chlorophyll-a (chl-a) fluorescence. Surface chl-a fluorescence was depleted to below 1.0 μg L− 1 inside the farm area, which was 80% less than the outside. The reduction of horizontal velocity and the presence of downwelling can significantly decrease food supply for filter-feeding mussels.