Assessment On Seaweed Beds Resources Based On BioSonics MX Echosounder

Seaweed bed is an important part of coastal marine ecosystem,and the ecological function plays an important role in the benign cycle of coastal marine ecosystem.In recent years,the ecological research on seaweed bed has been concerned by the international scholars engaged in marine research.The scale survey of seaweed bed is the basic data of relevant scientific research.At present,the survey methods mainly include diving survey,acoustic detection,remote sensing image,etc.The underwater acoustic survey method can directly present the scale of seaweed bed through quantitative way.In order to improve the accuracy of the scale evaluation of the seaweed bed in the underwater acoustic survey,according to the way of the acoustic survey and the underwater measurement in the condition of the indoor experimental pool,the parameters involved in the three evaluation indexes of seaweed bed,including plant height,growth depth and coverage of algae(Sargassum horneri),are verified and studied for the first time.So we can get the best threshold of measured acoustic parameters.Based on the above verification results,the Biosonics MX digital echosounder was used in this study from June to July 2018.Six islands of Shengshan,Bixia,LvHua,Sanheng,Huaniao and Gouqi in Ma'an archipelago are scanned around the island and at the same time,combined with the biological statistics of diving sampling,,to explore the species composition of seaweed resources,and to evaluate the coverage,canopy height and resource amount of the seaweed fields in the subtidal zone and some intertidal zones in each island reef area.In addition,taking Gouqi Island as an example,combined with the results of acoustic side scan survey in summer(May to July)when algae grow abundantly in different years,the scale of the seaweed bed is directly reflected by quantitative method,the change of coverage and canopy height are evaluated on a large time scale in the seaweed bed.With the continuous development of marine ranch construction,ecological protection and restoration of seaweed bed,the assessment of seaweed bed resources with the method of hydroacoustic survey aims to explore the distribution scale of seaweed field in Ma'an archipelago,which can provide theoretical guidance for analyzing the characteristics of seaweed bed,calculating the scale of seaweed field,and provide data support for its ecological restoration and reconstruction.The results are as follows:(1)Verification and analysis of hydroacoustics key parameters of seaweed field.In acoustic scanning,when the threshold range of the rising edge of the key parameters is-40?-50dB and the threshold range of the detection of the submerged vegetation is-70?-75db,the difference between the acoustic detection depth and the actual water depth is small,and it is very significant positive correlation(R=0.975,P<0.01);the difference between the measured plant height and the actual plant height is small,and it is very significant positive correlation(r=0.882,P<0.01).The seaweed coverage decreased with the increase of the velocity of the carrier,which was a power function relationship(y=23.633x-0.477,R2=0.8179).When the carrier speed of the echo detector is 1.5m/s(about three knots of ship speed),the rising edge threshold and the plant detection length threshold are-40dB and-75db respectively,more accurate acoustic detection results of algae can be obtained.(2)Study on the evaluation of underwater acoustics of seaweed field in Ma'an archipelago.43 species of macrobenthic algae,including 30 species of red alga,10 species of brown alga and 3 species of green alga,are collected from the diving survey in the islands and reefs of the Ma'an islands.Among them,the algae species are distributed in both the near-shore and mid-shore survey sections,mainly in the red alga phylum,and only a few brown alga phylum in the far shore survey section.The seaweed coverage was from high to low:Bixia(44.72%),Lvhua(43.26%),Huaniao(40.75%),Sanheng(24.79%),Gouqi(23.60%),Shengshan(21.31%).The canopy height was from high to low:Lvhua(58.63cm),Shengshan(44.85cm),Huaniao(37.22cm),Sanheng(26.74cm),Bixia(24.06cm),Gouqi(20.59cm).The distribution of seaweed resources in each reef area was estimated as follows:Lvhua(840016.10kg),Shengshan(387511.01kg),Bixia(274982.04kg),Huaniao(270381.93kg),Sanheng(111757.053kg),Gouqi(47576.48kg).(3)Study on the change of seaweed bed scale in Gouqi Island by hydroacoustic assessment.The results of spatial distribution show that the distribution area of seaweed bed in the north of Gouqi Island(mainly Huangshidong,Houtouwan and Yanggong reef)is larger than that in the South(mainly Ganxie and Lixi),and the difference over the years is small.The average coverage results show that the distribution scale of seaweed bed(54.88%)is the largest in 2011,the growth density of seaweed bed(30.26%)in 2013 and 2018(23.60%)is higher,the distribution uniformity of seaweed bed(32.46%)in 2017 is the highest,and the seaweed bed(20.19%)has the worst seaweed coverage in 2019.The average canopy height results show that 0.36m(2011)>0.21m(2018)>0.20m(2013)>0.15m(2019)>0.14m(2017).Among them,the growth of algae is the best in 2011,and the number of algae species in 2017 and 2019 is relatively small.The annual average coverage of Gouqi Island decreases year by year,but the annual average canopy height was not affected by the interannual time.