Influence Of Tropical Cyclones On Important Habitat Of Marine Ranching In Shengsi Zhejiang Province

The construction of marine ranching has the functions of conserving marine fisheries resources,improving the marine ecological environment,promoting the upgrading and transformation of the aquatic industry.Shengsi Ma'an Archipelago Marine Ranching was built in 2004,and at the end of 2015,Ma'an Archipelago Marine Ranching was selected into the first 20 national-level marine ranching demonstration areas approved by the Ministry of Agriculture and Rural.As of 2018,a total of 56.18million yuan has been invested in the construction of Shengsi Ma'an Archipelago Marine Ranching,with a total of 164.75 million square meters of artificial fish reefs,forming a seaweed beds of 30 hm~2.Shengsi Ma'an Archipelago Marine Ranching has rich bait in the sea.The temperature is moderate,which is conducive to the growth of mussels in Shengsi.It is a type of shellfish production area designated by the Ministry of Agriculture and Rural Affairs.Mussels in Shengsi are cultured by floating rafts.The cultivation area in 2018 is 1487 hm~2,Shengsi's large-scale mussel breeding habitat also creates feeding and nursery grounds for marine resources.Shengsi's large-scale mussel breeding habitat has important functions of proliferation and ecological conservation in Shengsi marine ranching.Tropical cyclones are low-pressure vortices generated on the surface of tropical or subtropical oceans.The tropical cyclones usually produce strong storms and have strong destructive power.They exert an important influence on the temporal and spatial distribution of biogenic materials in the upper layer of the sea area and even marine resources.The impact of tropical cyclones in Zhejiang's coastal areas is frequent,and the characteristics and extent of tropical cyclones encountered in Shengsi Ma'an Archipelago Marine Ranching which in the northeast coast of Zhejiang Province are rarely reported.This paper will try to explore the spatial and temporal distribution characteristics of tropical cyclones in the coastal areas of Zhejiang and Shengsi Ma'an Archipelago Marine Ranching,evaluat the response characteristics of the marine environment and biological resources to different tropical cyclone events,analyze the damage of marine ranching breeding facilities caused by tropical cyclones,and explore the suitable areas for mussel culture in the Shengsi Ma'an Archipelago Marine Ranching under the influence of tropical cyclones.Finally,with a view to providing a reference for the disaster prevention and mitigation and construction of the artificial habitat of Shengsi Ma'an Archipelago Marine Ranching.First,this study uses the best track data of tropical cyclones from 2000 to 2018provided by the China Meteorological Administration to analyze the characteristics of the occurrence time,moving path and intensity distribution of tropical cyclones affecting Zhejiang in recent years,and to evaluate the characteristics and changes of tropical cyclones along the coast of Zhejiang.Secondly,combined with the measured and remote sensing data of environmental factors of wind,waves and currents,the changes of wind,significant wave height and current of tropical cyclones affecting Shengsi Marine Ranching in Zhejiang in recent years were analyzed,and a joint probability model of wind and significant wave height was established to explore which type path and intensity of tropical cyclones has the most significant impact on the environmental factors in the sea area of Shengsi Marine Ranching,and the typical tropical cyclone events are selected for further research and analysis.Again,taking the sever typhoon Haikui in 2012 as an example,the bottom sediment survey and wave exposure index experimental data of Gouqi Island,the biological survey data before and after Haikui of the macroalgae and macrobenthos in Gouqi Island,the changes of wind,wave and current environmental factors of Gouqi Island under the action of Haikui were analyzed to explore the spatial distribution characteristics of macrobenthos under the action of wind and waves,explore the possibility of damage to mussels which have a sessile life on floating raft aquaculture impacted by tropical cyclones.Then,the tropical cyclone event data,mussel culture area and yield data,marine measured data and remote sensing data of environmental elements,and remote sensing image data of mussel culture areas from 2010 to 2018 were screened using the combined probability model of wind and wave distribution were used to analyze the impact of tropical cyclone events on the marine environment field,location and area of mussel culture in Shengsi,and identify the types and characteristics of tropical cyclones that most seriously affect the mussel aquaculture facilities.Finally,combining the historical tropical cyclone movement path and the statistical characteristics of environmental factors in the study area,the results of multi-year changes of mussel culture area,the distribution of sediments,water depth and waterway functional zoning in Ma'an Archipelago,comprehensively put forward suggestions on Shengsi mussel culture location selection.The main results of this study are as follows:(1)The source of tropical cyclones affecting Zhejiang is the Northwest Pacific Ocean,and there are four types of tropical cyclones moving paths,of which type D tropical cyclones have the largest number,followed by type A tropical cyclones.The average annual tropical cyclone frequency number during the study period is 4,the annual maximum tropical cyclone number is 7 and the annual minimum tropical cyclone number is 2.July to September is the prevalence of tropical cyclones in Zhejiang,and the number of tropical cyclones is the largest in August.During the 17years study period,the ACE value is 213.09×10~4 m~2/s~2,and the annual average ACE value is(12.53±0.11)×10~4 m~2/s~2.The monthly ACE value of tropical cyclones has Seasonal changes,the ACE value increases month by month from May,and reaches a maximum in August.The month with the most active tropical cyclones in the affected area have largest energy is September.The tropical cyclones that affect the sea area of Shengsi Marine Ranching should belong to Type A and D moving path,and the activity time mainly occurs from August to September.(2)Type D tropical cyclones have the widest influence on the wind speed of Shengsi Marine Ranching,with the greatest intensity,followed by type A tropical cyclones,and type B and C tropical cyclones have less influence.As the center of tropical cyclones is closer to Shengsi Marine Ranching,The significant wave height is gradually increasing,and the wave direction changes from south to northeast.The current distribution is different when it is affected by tropical cyclones and when it is not affected by tropical cyclones,but the difference under each type of tropical cyclone is not significant,and varies from station to station.In the analysis of the change in average velocity between the two sites,there is no significant difference in the average current and direction at the same site under various types of tropical cyclones.The average distance between the tropical cyclone center and Shengsi Marine Ranching has a significant linear relationship with the average wind speed and effective wave height.The extreme wind speed and significant wave height distribution caused by the tropical cyclone in Shengsi Marine Ranching conforms to the Gumbel joint probability distribution model.Since 2000,Shengsi Marine Ranching experienced the strongest tropical cyclone is Muifa.According to the wind speed and significant wave height joint cumulative probability model calculation,the wind wave recurrence period of Muifa is11.20 years with sever intensity.(3)Affected by Haikui,the maximum wind speed in the waters near Gouqi Island is 20 m/s,at Beaufort scale 9 level.The wind direction is southeast to east-southeast.Under the action of typhoon waves generated by Haikui,the intensity of storm wave at the south sites of Gouqi Island is greater than that at the north sites.On the vertical water depth distribution,with the increase of the water depth,the wave exposure index shows a weakening trend.The wave exposure index is highest at a water depth of 0?1m,and lowest at a water depth of 4?5 m.The Haikui activity did not cause changes in the species composition of the macroalgae on Gouqi Island,but the biomass distribution of the dominant species and the main species of macroalgae decreased with the increase of the wave exposure index.Four broad temperature species of macroalgae(Dictyota dichotoma,Symphyocladia latiuscula,Jania decussato-dichotoma,Corallina officinalis)were selected for analysis the biomass affected by Haikui,their biomass distribution is related to their underlying sedimentary environment and morphological structure.The proportion of non-fixed organisms such as Mitrella burchardi,Paguridae sp.and Gammaridea sp.were greatly affected by Haikui.Due to the influence of typhoons,the biological abundance decreases significantly in the short term.Thewave exposure index in the 0?3 m shallow water area caused by Haikui is highest,and the sever intensity of the wind wave caused the abundance of macrobenthos and the species number decreasing.After Haikui,the abundance of polychaetes and crustaceans at the HSD site increased since macroalgae rotted by wind wave and transformed to humus sediment.The wind and wave generated by tropical cyclone may cause damage to floating raft aquaculture mussels in shallow waters of 0?3 m depth.(4)According to the wind-wave combined cumulative probability distribution model and the regression relationship between the cyclone distance and the wind speed and significant wave height of Shengsi Marine Ranching,11 tropical cyclones were filtered out.The coincidence rate between those 11 tropical cyclones and tropical cyclone disaster events recorded in the historical records of Shengsi mussel culture from 2007 to 2018 reached 71.43%,and type D tropical cyclones in those 11 tropical cyclones were all caused losses to Shengsi mussel aquaculture.Tropical cyclone activities have significantly damaged and reduced production on Shengsi mussel culture area and unit area production.The damage caused by tropical cyclones in different movement path and intensity is different.The great northeast wind and storm wave generated by Muifa in the east side and has closer distance with Shengsi marine ranching caused extensive damage in Shengsi mussel culture area.But the southeast wind and wave generated by Talim in the south side and has further distance with Shengsi marine ranching just caused little damage to the culture area in the southeast Gouqi Island.The distance of type A path tropical cyclones is 200?350 km away from Ma'an Archipelago,and these type tropical cyclones caused little damage to Shengsi mussel culture area.While type D tropical cyclones with sever intensity were close to the study area,those tropical cyclones mainly were typhoon or sever typhoon intensity,when this type of tropical cyclones encountered during the growth period of mussel,they would cause significant loss of Shengsi mussel.(5)Period from August to September is the key period of revenue for Shengsi mussel,combined with the historical analysis of the activities of A and D type tropical cyclones and the change in environmental characteristics and the change in the culture area distribution,it can be judged that type D tropical cyclones moving through the east or southeast of Ma'an Archipelago make destructive damage to Shengsi mussel culture facilities.According to the wind,significant wave height and current generated by the type D tropical cyclones,the distribution of sediments,water depth and waterway functional zoning in Ma'an Archipelago,suggestions were put forward for Shengsi mussel culture location selection.Those locations include the arc island chains of Lyuhua Island,Huaniao Island,Hudie Island,Zhangqishan Island and Sanhengshan Island can cover over 228 hm~2 district area,which can annual increase about 24,500tons mussel production and about 68.562 million yuan yield value.