Study On The Effect Of Large-scale Aquaculture On Water Exchange Capacity In Qinhuangdao Sea Area

Due to the existence of aquaculture facilities and organisms,the scale of large-scale aquaculture directly affects the material transport and nutrient transfer in the aquaculture area,and also has a non-negligible impact on the physical process and ecological environment.In recent years,due to the blind pursuit of economic value,the aquaculture area has been expanding and the density has been increasing,which not only changes the hydrodynamic environment in the aquaculture area,but also poses a certain threat to the surrounding ecological environment.Therefore,studying the impact of large-scale aquaculture on water exchange and rationally planning aquaculture density are not only conducive to improving the resource utilization rate of aquaculture areas,but also conducive to rationally assessing the ecological environment of aquaculture areas and achieving the best economic,ecological and social benefits.Qinhuangdao sea area is an important aquaculture area and seaside resort in northern China.In recent years,red tides have occurred frequently in the area and have adversely affected water quality and fishery resources.In this paper,the sea area is taken as the research area.Firstly,the characteristics of tide,tidal current and residual current in the sea area during the high incidence of red tide are analyzed and the key physical processes are discussed by using the measured data of seabed-based platform.Secondly,a three-dimensional hydrodynamic numerical model is established based on the FVCOM to simulate the effects of large-scale aquaculture on hydrodynamics and water exchange in the sea area.Besides,the different effects of aquaculture on water exchange capacity in the sea area under different methods were comprehensively analyzed.Finally,the different characteristics of hydrodynamics and water exchange in the sea area under different aquaculture densities were simulated.The results are as follows:The results of measured data show that:(1)the tidal range of Qinhuangdao offshore area is small;the tidal type is all-day tide type,with regular all-day tide in the north and irregular all-day tide in the south.Due to the influence of M2 tide,the phenomenon of unequal tide days in this sea area is more significant,and the overall duration of rising tide is greater than that of ebb tide.(2)The evaluation of water exchange capacity is compared and analyzed by tracer and Lagrangian methods.The tracer simulation results showed that the existence of aquaculture activities reduced the water exchange capacity of the aquaculture area as a whole,resulting in an average half-exchange time of 2 % and a water exchange rate of 10 %.Affected by the accelerated current velocity on the northeast side,the tracer diffusion rate is higher than that without aquaculture.The calculation results of the Lagrangian method show that the particles in the aquaculture area mainly diffuse to the northeast.The presence of aquaculture significantly reduces the transport and diffusion capacity of particles in the middle of the aquaculture area,and the overall water exchange capacity of the region is reduced by 10 %.(3)There are many rivers in the sea area,and the runoff into the sea is large in summer.The pollutants into the sea from Tanghe River,Daihe River and Yanghe River estuary are easy to be transported directly to the bathing beach under the influence of the maximum velocity of the tidal current.Combined with the influence of the inflow of the offshore residual flow and the weak water exchange,it is easy to cause the aggregation of red tide organisms in the sea area near the harbor area on the north side of the bathing beach,which may cause the outbreak of red tide.The results show that:(1)the existence of aquaculture activities leads to the decrease of the overall flow velocity in the aquaculture area,and the decrease of the velocity in the middle part is the most obvious.The decrease of the velocity in outside area of the aquaculture area is smaller than that inside the aquaculture area.(2)The evaluation of water exchange capacity is compared and analyzed by tracer and Lagrangian methods.The tracer simulation results showed that the existence of aquaculture activities reduced the water exchange capacity as a whole,resulting in an average half-exchange time of 2% and a water exchange rate of 10%.Affected by the accelerated flow rate on the east side,the tracer diffusion rate increased compared with that without aquaculture.The calculation results of Lagrangian method show that the particles in the aquaculture area mainly diffuse to the northeast,the existence of aquaculture significantly reduces the transport and diffusion capacity of particles in the middle of the aquaculture area,and the overall water exchange capacity of the region is reduced by 10 %.(3)The comparison between the tracer method and the Lagrangian method shows that the overall water exchange rate calculated by the Lagrangian is lower than that of the tracer.Although the calculation results are different,the evaluation of water exchange capacity in the sea area under aquaculture activities is consistent,both of which show that aquaculture leads to a 10% decrease in water exchange capacity in the sea area.Under different aquaculture densities,the effects of aquaculture on hydrodynamics and water exchange are also different.When the aquaculture density is 0.5 times,1times and 2 times respectively,the maximum flow velocity in the aquaculture area is gradually decreasing compared and water exchange rate decreased with that without aquaculture.The water exchange rate of each layer in the vertical direction is obviously affected by the flow rate.When the aquaculture density is reduced to half of the current density,the water exchange rate in the middle and upper layers of the culture area is about 5% higher than the current density,and the bottom layer does not change significantly.When the aquaculture density is expanded to 2 times,the water exchange rate in the middle and upper layers is reduced by 5%,and the bottom layer is reduced by10%,indicating that the reduction of aquaculture density is conducive to improving the water exchange capacity of the upper and middle current in the aquaculture area,and the blind expansion of density will reduce the water exchange capacity of the sea area.Therefore,the aquaculture layout should be reasonably planned while comprehensively evaluating the ecological and economic benefits.