Population Characteristics And Ecological Restoration Of The Seagrass Zostera Marina L. In The Yellow Sea And Bohai Sea

Seagrass beds have important ecological functions,providing habitats,nurseries,and food sources for many marine organisms.However,in recent decades,human activities in coastal areas have become increasingly frequent and intensified,as well as the multiple effects of global change and natural disasters,leading to the large-scale decline of seagrass beds.Eelgrass is the dominant species of seagrass in the Yellow Sea and Bohai Sea in China.However,due to the lack of information about the eelgrass distribution in the Yellow Sea and Bohai Sea,the population characteristics of eelgrass in the Yellow Sea and Bohai Sea are not clear.Meanwhile,there are still some problems and weakness that exist in the process of the ecological restoration of seagrass bed.These issues greatly hinder the further development of the protection and restoration of seagrass in China.In this study,the population resources of eelgrass in the Yellow Sea and Bohai Sea was systematically investigated.Based on the information of the eelgrass distribution in the Yellow Sea and Bohai Sea,the typical eelgrass beds were selected as the research objects to investigate and study the characteristics of population reproduction.Based on eelgrass population characteristics,studies on ecological restoration of eelgrass beds based on seeds and adult shoots were carried out,and the studies include development and promotion of key technologies for seagrass restoration based on seeds,exploration of the effects of sediment type,burial depth,and water depth on seed germination and seedling establishment,and the effects of water depth and high water temperature on transplanted eelgrass shoots.The main research contents and results are as follows:1.The eelgrass distribution in the Yellow Sea and Bohai SeaFrom 2015 to 2020,the field survey was carried out according to the literature, satellite map,distribution of eelgrass debris on the beach,and other environmental information,and the eelgrass distribution area was estimated based on 3S technology.In recent years,many Chinese researchers have carried out a large number of seagrass resources investigation and seagrass protection effort in coastal waters of Weihai and Rongcheng,while the investigation and research in other coastal waters in the Yellow Sea and Bohai Sea are relatively backward.Therefore,this study focuses on the coastal waters of Dandong-Yantai and Rushan-Ganyu.The results showed that there were some large-area eelgrass beds or eelgrass zones in the Yellow Sea and Bohai Sea,including the eelgrass bed with an area of 2916.81 ha in Tangshan,Hebei Province,the eelgrass zone with an area of 1253.47 ha in Dalian,Liaoning Province,and the eelgrass bed with an area of 694.36 ha in Huludao,Liaoning Province.However,the populations are seriously threatened by human activities,such as destructive digging and aquaculture,and many populations are seriously degraded or even disappeared.In addition,taking Tangshan seagrass bed,the largest seagrass bed in the Yellow Sea and Bohai Sea,as an example,the spatio-temporal variations of eelgrass beds in Tangshan in the past half century were studied.It is found that reclamation and storm surge were the main reasons for the degradation of seagrass beds in Tangshan.2.Population reproduction of eelgrass in Tangshan and HuludaoFrom May 2018 to December 2019,we investigated the population reproduction of two populations in Tangshan,Hebei Province and Huludao,Liaoning Province.The results showed that asexual reproduction was the main way for population recruitment,and the asexual reproduction mainly occurred in autumn.In June 2018,the proportion of reproductive investment in Tangshan population was about 50%,which decreased to24%in June 2019;the maximum potential seed yield in 2018 was about 10074 seeds m^-2,which decreased to 5216 seeds m^-2 in 2019;the density of seed bank showed obvious interannual variations,and the maximum seed bank density in 2019 decreased by about 80%compared with 2018.During the survey period,these interannual variations may be due to the fact that the water temperature in spring 2018 was 3-4°C higher than that in spring 2019.In June 2019,the colonization of eelgrass seedlings could not be completed after its establishment in the vegetated area,but could be completed in the blank area between seagrass patches.Asexual reproduction was the main way for eelgrass population recruitment in Huludao,and the asexual reproduction mainly occurred in autumn.The reproductive investment of eelgrass population in 2018 and 2019 is about 20%-30%.The maximum potential seed yield in 2019 is about 830-1020 seeds m^-2,which is much lower than that of other perennial eelgrass populations(6200 seeds m^-2)in the world.This may be due to the low proportion of reproductive shoots（about 5%）in 2019.The density of seed bank showed obvious interannual variations.In 2018,the maximum seed bank density(about 280 seeds m^-2)was more than 10 times that of 2019(about 24 seeds m^-2),which may be related to the difference of reproductive shoot density in this two years.During the survey period,these interannual variations may be related to the extreme high temperature event in Northeast Asia in 2018.The colonization of eelgrass seedlings could not be completed after its establishment in the vegetated area,but could be completed in the blank area between seagrass patches.3.Elemental content characteristics of eelgrass in the Yellow Sea and Bohai Sea In this study,9 typical seagrass beds in the Yellow Sea and Bohai Sea were selected as the research objects to explore the spatial-temporal variations of carbon,nitrogen,and phosphorus contents and their ratios,and the spatial variations of stable carbon and nitrogen isotopes in different tissues（leaves,leaf sheaths,rhizomes,and roots）of eelgrass.In addition,the relationship between the height of leaf sheath and the height of shoot was explored.The results showed that the contents of nitrogen and phosphorus in eelgrass have shown obvious spatial-temporal variations,while the stable carbon and nitrogen isotope contents have shown obvious spatial variations.The contents of nitrogen and phosphorus in eelgrass aboveground tissues from Swan Lake showed a significant seasonal trend.The average nitrogen contents（%）of leaves and leaf sheaths in Swan Lake were 2.56±0.89 and 2.53±0.96,respectively,and the average phosphorus contents of leaves and leaf sheaths were 0.31±0.13 and 0.41±0.17,respectively.The contents of nitrogen and phosphorus in aboveground tissues were higher than those in underground tissues.Theδ¹⁵N isotopic contents in leaves and sheaths of different populations were 5.57-9.64 and 5.89-9.79,respectively.There is a significant positive correlation between the heights of leaf sheath and shoot,which indicates that the height of leaf sheath can be used to estimate the height of shoot;there is a significant positive correlation between leaf sheath and leaf blade in terms of the content of nitrogen,phosphorus,and stable carbon and nitrogen isotope,which indicates that leaf sheath can be used as a new ecological indicator of nitrogen,phosphorus and stable isotope content of leaf blade.In addition,one seagrass bed in Tangshan and three seagrass beds（qingyutan,nanwodao,wawushi）in Ailian bay were selected as the research objects to explore the heavy metal content（Cr,Cu,Zn,CD,Pb）and its enrichment characteristics of different eelgrass tissues,and to select the plant with more efficient in heavy metal enrichment by comparing the heavy metal enrichment ability of eelgrass and Saccharina japonica in Ailian Bay.The results showed that the contents of Cu,Zn,and Cd in the aboveground tissues were higher than those in the underground tissues.The average values of BCF and TF of seagrass underground tissue were in the order of[Cd]_mean>[Zn]_mean>[Cu]_mean>[Pb]_mean>[Cr]_mean.The BF of Cd ranged from 1.24 to 15.23,while these of the other heavy metal were less than 1.The TF of Cd ranged from 1.57 to 3.84.In Ailian Bay,the content of Cr in Saccharina japonica was generally higher than that in eelgrass,while the content of Zn,Cd,and Pb in kelp was generally lower than that in eelgrass.The BCFs of Cu,Zn,and Cd in the aboveground tissues were higher than that in Saccharina japonica.4.Ecological restoration of eelgrass bedIn this study,we tried to select high-quality eelgrass seeds by salt water,and compared the effects of different antimicrobial agents on the long-term seed storage.The results showed that the high-quality seeds of eelgrass could be selected by using salt water with a specific gravity of 1.20.The use of nano silver and copper sulfate antibacterial agents in the long-term seed storage could effectively inhibit microbial reproduction and seed decay,reduce seed loss rate,and improve seed retention rate.In order to explore the effects of sediment type,burial depth,and water depth on seed germination and seedling establishment,two experiments were carried out with PVC boxes suspended on the raft in Ailian Bay.The results showed that the seed burial depth was the main factor to determine the seed germination and seedling establishment.The proportions of seed germination and seedling establishment were higher in shallow burial depth.When the seed burial depth was 2 cm,the sediment type had no significant effect on the seed germination and seedling establishment;when the seed burial depth was 5 cm,the type of sediment had a great influence on the seed germination and seedling establishment.In addition,water depth had no significant effect on seed germination and seedling establishment.In this study,four transplanting experiments of eelgrass were carried out by hanging PVC/PE boxes on the raft in Ailian bay to study the effect of water depth on the transplanted adult shoots.The results showed that the transplanted eelgrass could complete a one-year life cycle in the submerged environment of less than 3 m,and the light decreased to 6.48%-10.08%of the surface radiance at the depth of 4 m in Ailian Bay,which had a negative impact on shoot density,sexual and asexual reproduction.In addition,from 2015 to 2021,eelgrass adult shoots and seeds were transplanted from Qingdao to Rizhao,in order to explore the effect of high water temperature on ecological eelgrass restoration in Rizhao,and Rizhao was the southern limits of geographical distribution of eelgrass in west coast of Yellow Sea.The results showed that under the high temperature stress in summer,the leaves of transplanted eelgrass became black,the density and height of shoots decreased significantly,the underground tissues became small（the diameter of underground rhizomes decreased significantly）,and eelgrass finally died;the seedlings and adult shoots of eelgrass could not survive through the first summer or the second summer.The present study fills in the gap of the distribution information of eelgrass in theYellow Sea and Bohai Sea,systematically reveals the characteristics of eelgrass populations in the Yellow Sea and Bohai Sea,and explores the effects of different environmental factors on the ecological restoration of eelgrass.The results of this study will provide scientific and effective theoretical guidance for the protection and restoration of eelgrass.