A Study On Sonar Detection Of Typical Zostera Marina Seagrass Beds In The Yellow-Bohai Seas And Eelgrass Seed Storage

The eelgrass Zostera marina L.is a dominant seagrass species circumglobally distributed in temperate coastal environments in the North Hemisphere,which mainly occurs in coastal areas of Hebei,Shandong,Liaoning and other provinces in China.Human activities are increasingly threatening the health of seagrass beds,and it is urgent to carry out research on important coastal seagrass resources and habitats.In this paper,we used an in-situ sampling survey,sonar detection technology and underwater observation system to investigate the distribution characteristics and habitats of typical eelgrass meadows in temperate China;and we studied the effects of temperature and salinity on the long-term storage of eelgrass seeds in laboratory conditions.The results may facilitate the monitoring,conservation and restoration of seagrass resources.The main results are summarized as followed:1.The advantages and analysis process of sonar detection technology in monitoring the distribution characteristics of seagrass beds(1)In contrast to direct sampling methods and optical remote sensing,active hydroacoustic techniques are relatively inexpensive and efficient for the detection of seagrass meadows.In this paper,we introduced the processes of sonar data acquisition,data processing and data analysis in detail,and obtained the seagrass distribution maps,the seagrass coverage distribution maps,and water depth distribution maps of the target seagrass beds.(2)We proposed a method for determining the accuracy of interpolation analyses.Though fully use the tools in ArcGIS and the principle of interpolation analysis,we may design the sonar detection route and verification line in advance,and randomly extract the verification points from the verification line.According to the real values and the prediction values of the verification points,the accuracy of the interpolation analysis can be calculated.(3)We used three interpolation analysis methods to analyze the same sonar data in August 2018.These methods include Topo to Raster,Kriging,and IDW.The raster surface obtained from Topo to Raster is considerably smoother compared to those from the other two methods.Topo to Raster has the highest interpolation accuracy and is the most conservative among three interpolation methods.Thus,we concluded that Topo to Raster was the most suitable interpolation method for sonar detection of seagrass beds.2.Sonar detection of seagrass beds(1)In the present study,we used a single beam echosounder to investigate the spatial and temporal distribution characteristics of Swan Lake seagrass beds.The distribution of seagrass in Swan Lake varies with seasons,with maximum distribution area in summer.The maximum distribution area of seagrass beds in Swan Lake was199.09 ha-231.67 ha,accounting for 41.48%-48.26%of the area of Swan Lake.The area of seagrass beds in Swan Lake was significantly reduced in winter(88.46ha).The distribution of seagrass in Swan Lake was concentrated in the central area of the lake and got patchy around the lagoon.The average canopy height of seagrass in August2018 was 0.482 m.The average water depth in Swan Lake was approximately 1.6 m,and deep gullies existed only in the southeast.We could distinguish the echogram of the bloom-forming macroalgae Chaetomorpha linum K.and the eelgrass Z.marina.We observed that C.linum was increasingly threatening seagrass beds in Swan Lake,which might lead to a decrease in seagrass bed area and biomass in Swan Lake.(2)Based on the sonar detection technology,the largest Z.marina seagrass beds in China was discovered in the coastal waters of Tangshan in Bohai sea with a seagrass bed area of 2916.81ha.The average canopy height in this eelgrass beds was 0.457m;the area of seagrass meadow whose canopy height in the range from 0.2 m~0.6 m was2644.69 ha,accounting for 90.67%of the total seagrass beds;the area of seagrass meadow with seagrass coverage higher than 60%was 1959.38 ha,accounting for 67.18%of the total seagrass beds.A certain proportion of reproductive branches appeared in May-June.After the seeds matured,the numbers of reproductive branches began to decline,and the peak growth period of the vegetative branches was the late July.3.In-situ sampling survey and underwater observation system research resultsIn the coastal areas of Xingcheng city and Changhai city,Liaoning Province,we found a large area of seagrass distribution.The seagrass bed area in Xingcheng was ca.800ha,which had important research value.The underwater observation system had great potential for in-situ observing the biodiversity,animal habits and behavioral characteristics of the seagrass bed ecosystems.4.Results of long-term eelgrass seed storage experiment(1)During the long-term eelgrass seed storage experiment,the seeds stored in the control group germinated rapidly at a salinity of 30 psu and under variable room temperature(5–20 ~oC)conditions,where the percentage seed loss was 68.65%after three months of storage.There was a high seed loss percentage at 4~oC with a salinity of less than 60 psu,and the highest seed loss was observed in the 30 psu salinity and 4~oC treatment,with 12%after 1 month,37%after 2 months,and 48.36%after 3 months.Conversely,after storage for 12 months,all of the treatments at 0~oC kept lower seed loss percentages,which were not more than 20%,and some treatments(0~oC/40psu,0~oC/50psu and 0~oC/70psu)were even not exceed 10%.In addition,compared with the seeds stored at 4~oC,the seeds stored at 0~oC were less likely to be infected by bacteria.(2)After storage for 12 months,both salinity and temperature had significant effects on seed value.There was high seed loss and lower seed vigor in the 4 ~oC treatment groups,therefore,4°C is not suitable for the long-term storage of eelgrass seeds.Although the seed loss rate during storage under high salinity conditions(60psu and 70psu)was very low,the seed vigor after storage was also low,which was also not suitable for the long-term storage of eelgrass seeds.Seeds stored in conditions of0°C/30psu,0°C/40psu and 0°/C50psu maintained high seed vigor(>60%)after 12months of storage,but a higher seed loss rate(17.67%)at 0°C/30psu.Considering the seed loss rate and the seed vigor after storage,a salinity of 40-50 psu and a temperature of 0°C were the optimal conditions for long-term wet storage of Z.marina seeds.