| The unique physical structure and shellfish raft culture with the existence of shellfish strings occupy most of the space in the upper ocean,which has a nonnegligible impact on the physical and ecological processes of the culture area,and these impacts react on the growth and development of shellfish individuals in the raft area,forming a unique ecological dynamic process of the shellfish raft culture area.The scale of shellfish raft culture affects the material transport and nutrient transfer in the sea area of the culture area,and also affects the yield and quality of shellfish in the culture area.In recent years,due to the blind pursuit of economic value,the floating raft culture area of shellfish has been expanding and the culture density has been increasing.This not only causes unnecessary investment,but also increases the threat to the environment.Therefore,the study on the physico-ecology and the multi-level ecological dynamics of individual growth in the typical mussel raft culture area is helpful to understand the special material transport and nutrient transfer processes in the mussel raft culture area,to provide more reasonable culture strategies,to improve the resource utilization rate of the culture area,and to evaluate the production and carbon sink capacity of the culture area.In this paper,a shellfish individual growth model was established and coupled with the three-dimensional ecological dynamic model established by the research group,and a physical-ecological-shellfish growth coupled numerical model for a typical mussel raft culture area was established to simulate the multi-level ecological dynamic process of the shellfish culture area.The effects of key physical and ecological processes on the individual growth of shellfish and the carrying capacity of shellfish population in the culture area were discussed,and the relationship between mussel culture and marine environment was also comprehensively analyzed.By setting different horizontal and vertical culture densities and different culture water layers,the time differences of individual growth,culture capacity and carbon sink capacity of cultured shellfish were simulated.The main conclusions are as follows:The measured data show that the existence of aquaculture activities leads to the weakening of the surface velocity in the aquaculture area,and the velocity is basically not more than 0.2 m/s.The blocking effect of aquaculture activities on the water flow is particularly obvious at the time of rapid fluctuation,and the maximum vertical velocity appears below the aquaculture water layer,but at the time of rapid fluctuation,the blocking effect of aquaculture activities on the water flow does not exist or is very small due to the very small velocity of the sea water itself.The sailing velocity data show that there is a certain transition distance between the velocity difference inside and outside the aquaculture area at a certain time,that is,the section of the sailing velocity shows a wake structure.Generally speaking,the hydrodynamic structure of the aquaculture area presents the vertical characteristics of low velocity in the aquaculture water layer and high velocity in the middle layer.The weakening of surface current velocity will inevitably affect the exchange capacity of seawater and indirectly affect the material transport process in the aquaculture area.The water exchange capacity in the culture area is weak,and the renewal of nutrients and plankton is slow.This phenomenon is particularly evident in summer,when phytoplankton grow in large numbers on the surface of the ocean.In the horizontal direction,the phytoplankton in the culture area was consumed due to the filter feeding of mussels,and the phytoplankton could not be replenished in time due to the weak water exchange capacity in the culture area,resulting in the low content of phytoplankton and chlorophyll in the cultivation area.In summer,the chlorophyll concentration was lower than 2 mg/m3 in the culture area,while it was higher than 3 mg/m3 outside the culture area,and even reached 5.6 mg/m3 at some stations,which was many times higher than that inside the culture area.Vertically,the chlorophyll concentration in the culture water layer was significantly lower than that in the non-culture water layer in summer,and the chlorophyll concentration was lower in the surface layer and higher in the middle layer.In addition,the sampling results of the May 2021 cruise showed that the chlorophyll concentration in the surface layer of the off-site S6 station was lower than that in the middle layer.Combined with the numerical simulation results,it was indicated that the S6 station was located in the wake area of the culture area at the observation time,that is,the surface layer of S6 was the seawater flowing out of the culture area after the mussel filter feeding.The results showed that the distribution of phytoplankton in the culture area was affected by the high feeding pressure of mussels,the hydrodynamic hindrance of culture activities and the phytoplankton biomass in the external seawater.On the spatial plane,the phytoplankton biomass in the culture area was lower than that outside the culture area during the neap tide due to the filtration of mussels and the weak hydrodynamic force.During the spring tide,the seawater in the culture area was replenished by the exogenous seawater with high biomass,and the phytoplankton in the culture area could still be replenished despite the high feeding pressure of mussels.In summer,the biomass of phytoplankton in the euphotic layer above 10 m was large,while the mussel culture layer occupied the water layer above 3.8m,so the mussel filter feeding did not have a greater consumption of phytoplankton in the whole water layer.In the vertical direction,phytoplankton in the euphotic layer of the non-aquaculture water layer was not directly affected by the filter feeding of mussels,so it had a high biomass,showing the characteristics of low surface,high middle and low bottom.The results of shellfish growth simulation experiment showed that the individual growth of mussels in the upstream area of the culture area was significantly better than that in other areas in terms of spatial distribution,and the northeast side of the large area culture area in the northwest of Gouqi Island was located in the upstream area during the flood tide,where the sea water flow velocity was large,the material transport capacity was strong,and the mussels could filter enough food,so the individual biomass was higher than that in other areas.The two small culture areas in the southwest of Gouqi Island are surrounded by terrain on three sides,and only the south side is the inflow surface.The individual biomass of mussels in the south side of the culture area is higher than that in the interior.In summer,when the water temperature is suitable,the organic pellet food is relatively abundant,and the growth rate of Mytilus coruscus is high,the individual growth difference of Mytilus coruscus in the culture area is obvious.The biomass of individual mussels at the edge of the culture area was always higher than that inside the culture area.This is because,at any time,the seawater filtered by the mussels in the culture area is always filtered by the mussels at the edge of the culture area,which has a relatively low feed content;the vertical distribution of the individual biomass of mussels is also affected by the blocking effect of culture activities and the vertical distribution of phytoplankton.In the mussel culture area in the northwest of Gouqi Island,the culture area decreases slightly from west to east,and the blocking effect of culture activities is large.In the horizontal direction,it is difficult for high biomass seawater to directly enter the culture area at the depth of the culture water layer.The seawater of the non-culture water layer can easily enter the culture area and preferentially supply bait for the mussels at the bottom of the culture water layer.Therefore,in the deeper water area,the biomass of mussels at the bottom is higher than that at the surface.After one year of culture,the individual biomass of Mytilus coruscus ranged from 6.8 to 14.4 mol N/ind,the carbon content of tissue dry weight was about0.5 to 1.1 g C,and the carbon content of shell was 3.9 to 8.6 g C in different water layers and culture positions.The highest primary productivity was about 1766 mg C/m2/day,and the cumulative carbon sequestration was about 12124 t C/year.The average carbon sink of shellfish was about 478 kg C/hm2/year,and the cumulative carbon sequestration was 40617 t C/year,accounting for about 33% of the primary productivity in the sea area.The difference of mussel individual biomass shows that the current culture scale is unreasonable in the utilization of space resources.The most efficient use of space can be achieved by adjusting or dislocating the culture water layer in the culture area,dividing a number of small culture areas,and setting the waterway along the direction of the main flow axis of the ebb and flow tide to slow down the blocking effect of large culture areas and increase the transport of off-site seawater into the farm.Improve the feed supply capacity,thereby improving the overall yield and carbon sink capacity of the thick shell mussel in the culture area. |
PDF Full Text Request