Impacts Of The Crassostrea Hongkongensis Raft Culture On The Hydrodynamics And Water Quality In Semi-closed Bays

The impact of high-density raft aquaculture on the water environment of the bay has become a hot issue in the fields of marine environmental science,marine dynamics,ecology,and environmental management.The raft aquaculture of Crassostrea hongkongensis is generally carried out with a standard floating raft or submerged floating-raft hanging about 1.2 meters of cement strings attached to different sizes of the oysters,which are densely covered in semi-enclosed bays and produce frictional resistance to the wave,affecting in turn water exchange and water quality.To quantify the impact of the oyster raft aquaculture on the hydrodynamic and water quality environment of semi-closed bays,this study takes the Qinzhou Bay,the largest oyster aquaculture area in China,as the research object.Firstly,typical samples of submerged floating-raft and floating rafts were collected,the on-site tidal flow was measured by Acoustic Doppler Current Profiler(ADCP),the impact of raft aquaculture on the tidal structural characteristics was quantitatively analyzed,and the surface roughness height coefficient representing frictional resistance was calculated,and the correlation between oyster string size,breeding density and raft surface roughness height was analyzed;based on tidal current measurement,the breeding resistance was parameterized and brought into a two-dimensional hydrodynamic-water quality coupled model.By comparing the two scenarios with and without raft aquaculture,the tidal volume of the raft aquaculture to the semi-closed bay in Qinzhou was estimated,as well as the impact on the half-life time and the concentration distribution of inorganic nitrogen and reactive phosphate in the bay.The main conclusions are as follows:(1)The results of ADCP field observation show that the surface flow velocity of seawater passing through aquaculture facilities will be attenuated.The average reduction rate of the surface flow velocity by a single aquaculture facility in the submerged floating-raft aquaculture area of the Maowei Sea is between 30.8 and 65.2%,and that in the floating raft aquaculture area in the bay neck is between 15.0 and 56.6%.After the wave passes through the aquaculture facility,the vertical profile velocity of the upper boundary layer is logarithmic.By fitting the logarithmic profile of the surface vertical velocity,the upper boundary layer parameters of each observation station are estimated,and the average surface friction flow velocity at the floating raft observation stations is 0.127 m/s,the average surface roughness length 0.377 m,and the average surface roughness height is 11.339 m;those at the submerged floating-raft observation station are respectively 0.121 m/s,0.545 m,and16.341 m.There was a significant positive linear correlation between breeding density and surface roughness height.(2)The simulation results of the two-dimensional hydrodynamic-water quality coupled model show that The average reduction rate of the flow velocities in the submerged floating-raft aquaculture area of the Maowei Sea,the floating raft aquaculture area in the bay neck,and the floating raft aquaculture area in the bay mouth with aquaculture compared to without aquaculture was 60.3%,40.2%,and 35.7%,respectively in the neap tide period;Under the condition of culture or not,the average tidal volume in Qinzhou Bay was 10.20×10~8 m~3 and 11.28×10~8 m~3,which decreased by 9.6%.Neap tide was used as the beginning time of simulation,With and without aquaculture,the half-life time are 46.1 d and 36.8 d respectively,indicating that the resistance generated by the high-intensity aquaculture of Crassostrea hongkong ensis increased the average half-exchange period of the semi-closed bay by 9.3 d.It can be found that the presence of aquaculture activities will hinder the normal movement of seawater,thus affecting the water exchange capacity of the Bay Area.(3)The simulation results of the two-dimensional hydrodynamic-water quality coupled model show that:The variation range of inorganic nitrogen concentration increased by 45.6%and 32.4%,and the variation range of active phosphate concentration increased by 45.5%and 30.8%at two typical stations in the high-density culture area of the bay.The variation range of pollutant concentration at two typical stations in the low-density culture area of the bay did not change significantly with or without culture.At the same time,after the absence of aquaculture activities,the inorganic nitrogen concentration of two typical stations in the high-density culture area in the bay would be reduced by3.0%and 3.4%,and the active phosphate concentration would be reduced by3.2%and 3.8%.The pollutant concentration of two typical stations in the low-density culture area near the bay mouth would not change much with or without aquaculture.Thus,the existence of high density raft culture activities,will lead to weaker bay water exchange capacity,pollutant concentration change with tidal amplitude decreases,and thus influence culture zones within the convective diffusion of pollutants,pollutants are more likely to accumulate in this area,result in higher concentration of pollutants,and for breeding area of low density and water exchange capacity is strong,The existence of aquaculture activities had little effect on the variation of pollutant concentration in this region.