| The Mangrove In Situ Underground Tube Ecofarming System（MSUTS）can allow fish（Bostrychus sinensis）culture in a underground tube while rapidly restoring mangrove areas.However,systematic and in-depth theoretical summary is needed to understand the key processes of MSUTS.Internal water environment,element budget,carrying capacity,and environmental effects are the four key scientific issues for characterizing MSUTS.The key processes of MSUTS in the Guangxi Beilun Estuary National Nature Reserve were studied from 2016 to 2018.Using field monitoring and experiments,we evaluated the below four aspects quantitatively.The main results were as follows.1.Rules of internal water quality in MSUTSThe water quality of B.sinensis was analyzed three times in the juvenile and adult stages,respectively,during the entire culture period（five months,from initial stocking to fish harvest）.Among the 15 water quality indexes,the level of dissolved oxygen indicated that the water quality declined within 15 days following each cleaning.The changing of the other indexes was irregular.Compared with the non-cultured control group,there was no significant difference in the measured water quality indexes.The total carbon,nitrogen,phosphorus,and sulfide of the sediments inside MSUTS were 5.07%,30.97%,73.90%and 204.31%higher than those of the control group,respectively.However,cleaning of every 15 days can effectively remove the stress of sediment pollution in MSUTS.These results indicated that the water quality in the internal MSUTS was close to the surrounding natural sea water.Regular cleaning can effectively eliminate nitrogen and phosphorus in sediments and this contributes to the success of MSUTS.2.Element budget of MSUTSThe percentages of carbon,nitrogen and phosphorus in the budget were obtained as follows.Carbon:Artificial feed（82.65%）+Other input（17.35%）=Harvest（19.70%）+System loss（6.06%）+Release out of MSUTS（74.24%）.Nitrogen:Artificial feed（82.33%）+Other input（17.67%）=Harvest（18.61%）+System loss（5.92%）+Release out of MSUTS（75.16%）.Phosphorus:Artificial feed（79.30%）+Other input（20.70%）=Harvest（16.97%）+System loss（5.84%）+Release out of MSUTS（77.19%）.In the above formulas,"Other input"includes stocked fish and natural feed,"System loss"includes dead fish and residual feed,and"Release out of MSUTS"included soluble loss,sediment,output in the water,and other outputs.Artificial feed and output in the water were the main channels for element input and output respectively.The results showed that the feed coefficient rate of B.sinensis in the MSUTS was 5.76,which was 50%higher than that of B.sinensis regularly cultured in the pond.The utilization ratio of carbon,nitrogen,and phosphorus was 14.76%,14.33%and 6.64%.The phosphorus in the feed of fresh fish mainly existed in the bones and scales（non-preferred parts for B.sinensis）,which was the main reason for the low phosphorus utilization rate.3.Carrying capacity of MSUTSThe results showed that,the concentration of dissolved oxygen（DO） in water was the first factor for limiting the aquaculture production in MSUTS.The threshold of DO concentration for normal feeding of B.sinensis was 2.59mg/L.The relationship between the culture biomass（B,kg）and flow velocity（V,m3/h）can be presented as below,while the DO concentration in water equals the threshold（2.59 mg/L）for normal feeding of B.sinensis:B=13.316V-11.395（B≤60kg）.The amount of MSUTS supported by an eco-polyculture pond can be calculated through the formula:n=S×（H1-H2）/4.24t.And the carrying capacity for aquaculture in one eco-polyculture pond can be determined through the formula:Ca=10.61×S×（H1-H2）/t.S:area of the eco-polyculture pond（m2）;H1:the elevation of the highest water surface during the latest flood tide in the eco-polyculture pond before the neap season without tidal prism（m）;H2:elevation of bottom of the eco-polyculture pond（m）;t:duration of neap season without tidal prism（h）.The above research has laid a theoretical foundation for the popularization and application of MSUTS and provided key parameters for engineering design of MSUTS.4.Impact of ecofarming using MSUTS on the environment,mangrove,and macrobenthosBased on the element budget equations,for every 1 ton of production of B.sinensis,338.02 kg of carbon,79.34 kg of nitrogen and 2.39 kg of phosphorus have been discharged to the outside from MSUTS through water,sediment,and soluble loss.After aquaculture using MSUTS（within one month after harvesting）,the total carbon,total nitrogen,total phosphorus,and sulfur content of the remaining water and sediment in the tidal flat were respectively72.86%,118.66%,89.50%,and 54.40%,and 100.01,100.15%,114.49%,and91.93%of the amounts before MSUTS aquaculture.There was no significant differences in water quality index,sediment index and mangrove morphology index between the pre-MSUTS aquaculture and the post-MSUTS aquaculture treatments.There was no significant difference in PSII maximum photochemical efficiency（Fv/Fm）of mangrove leaves and macrobenthic community structure（species amount,biomass,abundance,richness,evenness,and diversity index）between the aquaculture zones and the control zones.The simulation experiments showed that the isotopic signals(δ13C andδ15N)of mangrove plants and sediment were not significantly different between the pre-MSUTS aquaculture and the post-MSUTS aquaculture treatments.These results indicate that the application of MSUTS has little effect on the surrounding environment,macrobenthos community,and mangrove plants.This is mainly because the ecofarming effluent are discharged at a low concentration into the surrounding environment and then diluted by the everyday tide,instead of intensive effluent-discharge like those using regular pond aquaculture.In addition,the absorption of plants and the decomposition of microorganisms also contribute to the environment-friendly advantage of the MUSTS.5.Application of MSUTS and SuggestionsWe have developed a device that automatically conveys surface oxygen-enriched water.The device increased the average concentration of dissolved oxygen in water by 12.28%.It also contributes to the stability of running of MSUTS.The application of MSUTS is a sustainable way for the mangrove rehabilitation in bare flats and the control of Spartina alterniflora（cordgrass）along the coast of southern China. |
