Modeling the impact of culture facilities on hydrodynamic and solute transport in marine aquaculture waters of the North Yellow Sea

: An increasing number of marine aquaculture facilities are being placed in shallow bays and 8 open sea. In this study, we present a coupled hydrodynamic and solute transport model with high-resolution 9 schemes in marine aquaculture waters based on depth-averaged shallow water equations. A new expression 10 of drag force is incorporated into momentum equations to express the resistance of suspended culture cages. 11 The coupled model is used to simulate the effect of suspended structures on the tidal current and the motion 12 of a contaminant cloud in the marine aquaculture of the North Yellow Sea, China. Simulation results show 13 a low-velocity area inside the aquaculture cage area, with a maximum reduction rate of velocity close to 14 45% under high-density culture. The results also show that the tidal currents are sensitive to the suspended 15 cage densities, cage length and drag coefficients of cages. The transport processes of pollutants inside 16 aquaculture facilities are inhibited away from the vicinity of the culture cage area because of the reduction 17 in tidal currents; therefore, the suspended cages significantly affect the transport processes of pollutants in 18 coastal aquaculture waters. Furthermore, the reduction in the horizontal velocity can significantly decrease 19 the food supply for aquaculture areas from the outside sea. The results of this study provide new insight 20 into the planning of high-density suspended facilities for coastal aquaculture activities.


Introduction 24
The global demand for seafood continues to rise sharply because of population growth and increased 25 per capita consumption; marine aquaculture has thus become one of the world's fastest-growing food 26 sectors [1]. Although estuaries, bays and coastal waters occupy a small fraction of the Earth's surface, the 27 large-scale expansion of marine aquaculture has been concentrated in these areas. The rapid growth in 28 marine aquaculture over the last two decades has also been accompanied by the development of diversified 29 culture systems ranging from ponds and floating rafts [2] to suspended cages in coastal waters [3][4]. These 30 types of rafts or cages in estuaries offer many advantages, including high utilization rate of breezing space, 31 low unit cost and reduced disease probability [5]. However, floating aquaculture structures significantly 32 alter the water exchange ability and the food supply of plankton in the aquaculture structure; furthermore, 33 high-density culture can directly affect the transport process of pollutants released during the production 34 process [6] and thus may have consequences for the environment as well as for the productivity of 35 aquaculture farms [7]. 36  Large-scale interactions between a whole farm and its surrounding circulation have been investigated 56 using numerical approaches in bays and larger-scale coastal waters [21][22][23][24][25]. The numerical results have 57 indicated that current speeds passing through an aquaculture area might be significantly affected by farms; 58 more generally, these studies have established an approach that can be used to provide insight into 59 production, environmental effects and disease interactions. Recent developments in computational behavior 60 of the fate and transport of contaminants have permitted the transient loading over the complicated 61 geometry of the real estuarine systems in 2D models to be accurately described [26][27]. These studies have 62 shown that the effect of suspended aquaculture facilities on coastal circulation is important for the 63 efficiency of aquaculture management practices. Although much research has examined the hydrodynamic 64 characteristics of coastal waters near aquaculture facilities, quantitative studies examining how marine 65 aquaculture facilities can affect the hydrodynamic flow field and solute transport in large-scale coastal 66 waters have been rarely conducted. Moreover, few studies have described the dispersal of aquaculture 67 where t is time; h indicates the flow depth h= d+; d is the still water depth;  is the water surface elevation 86 above the still water; u and v indicate the depth-averaged velocities in the x and y directions, respectively; v t 87 is the eddy viscosity coefficient; g is gravitational acceleration; f c is the Coriolis parameter; f x and f y indicate 88 the drag force of suspended cages in the x and y directions, respectively; bx  and by  indicate the bed 89 friction in the x and y directions, respectively; τ wx and τ wy are the surface wind stresses in the x and y 90 directions, respectively; and z b is the bed elevation. 91 When planning and assessing the aquaculture cages at larger-scale waters, the drag force imparted by 92 the distributed structure on the fluid needs to be represented [29]. The loss of energy of the mean flow 93 through cages is proportional to the drag force exerted by a single cage and the cage density within the 94 marine aquaculture area [16,26,30,31]. Therefore, the drag force caused by suspended cages (Fig. 1 (a)) in 95 the ocean can be parameterized as: 96 where C d is the drag force coefficient of the suspended cages, N is the cage density defined as the number 98 of cage elements per unit area,  is the diameter of a cage element, and L c is the length of the cage 99 elements (see Fig. 1

Solute Transport Model 114
The scalar's vertical distribution can be assumed to be uniform for shallow estuaries. The 115 depth-averaged equation for solute transport subjected to advection and diffusion process can be derived as 116 follows: 117

Mooring lines
Long lines where C is the depth-averaged substance concentration; S C represents the source contribution per unit area, 119 which is obtained from external loading; and K is the decay rate of the substance. 120 where c is Chèzy's coefficient, and l ε and t ε are the dimensionless coefficients of longitudinal

Study Area 132
The study area is located in the northern Yellow Sea, China (Fig. 2 a); it is one of the major estuaries 133 for marine aquaculture in China with approximately 60 years of maricultural activities. This water 134 represents a typical coastal ecosystem of China that is influenced by the freshwater influx from the 135 Yalujiang River, Biliu River and Zhuang River as well as saline water influx from the Yellow Sea. The 136 mudflats along the coastline make the region highly productive and support several economically important 137 shellfish species, especially Patinopecten yessoensis (Fig. 2 b). In addition to bottom-seeded culture, one of 138 the most important aquaculture modes for Patinopecten yessoensis consists of suspended facilities (rafts or 139 cages) in coastal waters ( Fig. 2 (c)). This structure is used to suspend spat-laden culture cages from the sea 140 surface to 3 m deep, sometimes more, depending on the water depth. Generally, the suspended aquaculture

Model Setting and Verification 156
In the simulation experiments, the computational area is discretized into 38257 unstructured triangular 157 cells, which provides more flexibility in defining the fine spatial resolution across channel, island and P6) the cage domain in the study area (Fig. 7), where the depth-averaged current speeds within the culture 209 cages significantly decrease as the cage density increases. Compared with cases in which the culture cages 210 are not considered, the velocity reduction is larger in the culture cage area than outside the culture cage area. 211 and hydrodynamics (Fig. 11); increases in the drag coefficient result in greater reductions in the average 229 tidal speed. Figure 12 shows the corresponding velocity fields in the presence of culture cages (cage 230 density=0.08) during the periods of flood and ebb tide; the results are also compared with the absence of 231 culture cages (Fig. 6). Generally speaking, flow patterns are similar whether or not the marine cages are 232 included in the simulations; however, reductions in water speed can be observed in areas inside culture 233 cages. These areas outside of marine aquaculture facilities show a small change in depth-average velocity. 234 The simulated results indicate that the culture cages in these coastal waters have a significant effect on the 235 currents traveling through suspended cages. Higher densities of the suspended cage culture result in a 236 weaker current area in the culture area.

Effect of Marine Cages on the Waste Transport 255
Shell aquaculture (Patinopecten yessoensis) tends to release additional fecal material into the water 256 column. To understand the effect of suspended cages on mass transport and dispersal in aquaculture waters, 257 a hypothetical release scenario is considered using a tracer to represent the amount of fecal material, which 258 is discharged into the water body at the P2 station from aquaculture areas. It can be thought of as a waste 259 effluent. The emission intensity and duration are 100 g/s and 4.1 h, respectively. The pollutant 260 concentration at the boundary of the sea area and the boundary of land was set to 0. The effects of cage 261 densities on waste transport are illustrated in Fig. 13; the maximum concentration occurs inside cage areas 262 in the presence of advection and diffusion. The pollutant concentrations are higher in the inner parts of 263 cages at higher cage densities than in the absence of cage densities (see P1, P2 and P3 in Fig. 13). However, 264 in the P4 gauge outside the culture area, the pollutant concentration is significantly higher in the absence of 265 suspended cages than at high cage densities. In most of the culture area, cage densities reduced the 266 dispersal of waste from advection because of the reduction in tidal current speeds. That is to say, the 267 aquaculture facilities limit the water exchange rate inside the aquaculture ground with the outside cage 268 environment. A decreased water exchange rate means a lower supply of nutrients and food and vice versa. 269 As a result, the food availability may be insufficient for high cage densities. Figure 14  Sea. Their runoff is controlled by the season, and runoff is highest in the flood season. There is a reservoir 282 upstream with flood control and the urban water supply as its main mode of operation; it also features the 283 comprehensive utilization of irrigation, fish farming and power generation. These rivers carry large 284 amounts of suspended detritus into the North Yellow Sea, which supplies food to shell aquaculture. Here, a 285 continuous food supply scenario is considered to test the impact of suspended aquaculture on the food 286 supply following the expansion of high-density aquaculture facilities. The point source is located at the 287 mouth of the rivers near Shicheng Island; the release rate is 100 g/s. The initial concentration of the 288 computational domain is assumed to be 0 mg/L. Figure 15 shows the change in detritus concentration at 289 several time points in the absence of aquaculture facilities and in the presence of aquaculture facilities. 290 After entering the bay at the mouth of the Zhuanghe River, the suspended debris mainly spreads to the 291 southwest. When the suspended debris does not enter the culture sea area, there is no obvious effect of the 292 culture facility on the debris concentration transport; however, the debris food enters the culture sea area 293 associated with current circulation, and the attenuation of the flow field produced by the culture facilities 294 has a great effect on the transportation of the debris. In our simulation, aquaculture facilities limit the water 295 exchange rate inside the aquaculture ground; a decreased water exchange rate means a lower supply of food 296 in suspended detritus. A relative food shortage for Patinopecten yessoensis in this study domain is more 297 serious during neap tides, especially in areas with high densities of aquaculture. Generally, the food supply 298 occurring in shallow coastal waters is closely tied to the tidal current; thus, the aquaculture density in 299 marine aquaculture waters of the North Yellow Sea needs to be carefully designed.

Conclusions 318
This study presents results from highly resolved 2D depth-averaged numerical simulations of the 319 hydrodynamic characteristics and current changes caused by suspended cages in marine aquaculture waters. 320 The effect of suspended cages on the transport processes of pollutants is also discussed. The finite volume 321 method is used to discretize the shallow water equations and solute transport equation. This model uses an 322 unstructured grid to compute flow movement in the coastal ocean. The high-resolution scheme is evaluated 323 to solve the advection and diffusion terms for pollutant transport in near-coastal environments. The  (2) The related parameters of cage densities, cage lengths and the drag coefficients are significantly related 332 to the reductions in tidal current speed in the aquaculture domain; thus, models must incorporate the 333 resistance effect caused by culture facilities to calculate the suspended cage-induced changes on 334 hydrodynamics in the design of the suitable densities, layouts and depth of the suspended cages. 335 (3) The mixing and dispersion of the pollutant field with additional resistance force from the suspended 336 cages is obviously different from cases in which the effect of suspended cages is absent; the 337 importance of the reduction in water speeds caused by marine culture cages is the fact that they affect 338 the transport and dispersal of particulate matter and dissolved nutrients, which affect environmental 339 quality and are sub-optimal for shellfish health in the aquaculture environment. Moreover, the 340 cage-induced changes on hydrodynamic conditions in marine aquaculture waters directly affect the 341 food supply of suspended detritus from the outside sea during the production process. 342 Shellfish aquaculture tends to release additional material into the water column, including fecal 343 material and food wastes; there is thus a need to accurately characterize the hydrodynamic conditions using 344 numerical models at particular shellfish farm sites because these conditions dramatically affect how far a 345 waste plume from a shellfish farm site would spread. Such modeling requires the use of accurate 346 parameters for reliable calculations of the water renewal rates and nutrient supplies. Future numerical 347 modeling studies should be conducted to aid the selection of suitable locations for shellfish cages; a 348 coupled ecological-hydrodynamic model should also be developed to improve predictions of nutrition 349 transport and cage productivity.  Sketch of the study domain and topography in the North Yellow Sea Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Mesh, eld gauges and marine aquaculture domain in the North Yellow Sea. The white domain is the island, and the orange domain is the suspended aquaculture cage area. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 4
Comparison of the measured and simulated water levels at Dachangshan tidal station. Comparison of the measured data and simulated results in tidal currents and ow directions Flow elds within a tidal period in which the effect of aquaculture facilities is not considered Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Comparison of the simulated current directions with different culture cage densities Comparison of the simulated velocity change with different cage densities in a tidal cycle Figure 10 Changes in depth-averaged mean current speeds as a function of the ratio of the cage length.

Figure 11
Changes in depth-averaged mean current speeds as a function of the drag force coe cients.

Figure 12
Flow elds within a tidal period in which the effect of aquaculture facilities is considered Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 13
Time series of concentration pro les of fecal material at several different locations near the culture cage embayment Figure 14 Development of the pollutant concentration eld at several time points Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 15
The transport process of suspended detritus in the culture waters in absence of cages (left) and in the presence of cages (right) Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.