Acoustic Observation And Evaluation Of Benthic Habitat Evolution Under The Influence Of Coastal Development

Offshore development has become the main way to expand spatial resources in the coastal areas.Large-scale exploitation activities such as reclamation and marine ranching construction have promoted economic and social development,but they also had a significant impact on the marine benthic habitat.Acoustic methods,which provide a new technique for benthic habitat survey have overcome the shortcomings of traditional means that are inefficient and provide incomplete information.Multibeam echo sounder system(MBES)is a high-tech product in modern marine survey.It is rare to apply multibeam sonar for offshore benthic habitat observation and evaluation in our country.Benthic habitat can be identified and classified by acoustic data combined with classification algorithm.However,there is a lack of research on the acoustic mapping and classification for benthic habitat in artificial reef(AR)water.In this paper,a method of automated mapping for benthic habitat was developed and applied to identify the habitat types in AR area.The micro-topography features of the AR area and reclamation area were analyzed by using MBES with high-resolution and high-precision.The main results of this paper are as follows:(1)An automated classification method is designed and tested to process bathymetric and backscatter data from MBES and transform the variables into simple,easily visualized maps.A suite of acoustic derived datasets was processed using ArcGISTM for further characterization of benthic habitats.These variables are valid indicators of substrate type and well represent variation in seafloor characteristics.To reduce the redundancy in acoustic variables,a principal component analysis(PCA)is used to condense the highly collinear dataset.An acoustic benthic map of bottom sediments is classified using an iterative self-organizing data analysis technique(ISODATA).The ISODATA clustering method can greatly reduce the likelihood of operator bias,reduce the time and cost of data interpretation,and ensure the consistency of the classification results.This method is implemented in a GIS environment,which allows further spatial analyses and algorithm development.The main benefit of ISODATA is that it combines multiple variables into a simplified comprehensive overview of the interested area.The approach is tested with MBES surveys in a 1.15 km2 fish farm with a high density of ARs off the Yantai coast in northern China.Using this method,3 basic benthic habitats(sandy bottom,muddy sediments,and ARs)are distinguished.The results of the classification are validated using sediment samples and underwater surveys.Our study shows that the use of MBES is an effective method for acoustic mapping and classification of ARs.(2)The benthic habitat in large-scale AR was monitored and evaluated by using MBES combined with geospatial analysis.The results showed that the multibeam sonar system can obtain detailed bathymetric data of ARs to identify their location,form and other information.The topographic variables such as slope,curvature,rugosity,terrain ruggedness index(TRI)and topographic relief(TR)were extracted by using terrain analysis tool to examine the ARs topographic features.The variables with high-value distributed around the ARs,and this can distinguish ARs from the flat seabed.ARs reduced the underlying flow rate and changed the current direction.Because of the AR's gravity and hydrodynamic effect,the seabed terrain changed significantly(depth from 5 to 10 m)and there were erosion(sinking depth about 0.45 m)and deposition appearance around ARs.Multibeam bathymetry data can be used to quantitatively analyze the changed characteristics of submarine terrain caused by ARs deployment and provide necessary data to evaluate the AR effect.Different kinds of ARs with distinctive structure formed different local flow and supplied diverse benthic habitat.Field surveys found the biomass around concrete reef was much larger than that of stones reef.It is possible to provide a new technique to analyze the micro topography changes caused by ARs and this could contribute to ARs examination and assessment work which has a strong practical significance.(3)Analysis of a series of historical bathymetric surveys has revealed large changes in morphology from 1960 s to 2010 s in Longkou Bay,China.Water depths were extracted from digitized admiralty charts to explore the accretion-erosion characteristics in a Geographical Information System(GIS)environment,providing quantitative estimates of morphological changes.Multibeam swath mapping were used to analyze the geomorphologic features caused by the construction of artificial islands.Results illustrated that the shoreline and bathymetry of Longkou Bay changed dramatically in recent decades.The subaqueous area decreased by 13.5 km2 in the study area during the last 50 years.From 1960 s to 1990 s,the evolution of Longkou Bay was mainly governed by natural processes with a patchy distribution of deposition and erosion,and there were few signs of being related to large-scale human activities.During the period of 1990 s to 2010 s,intensive coastal developments including large port engineering projects,channel dredging and artificial islands construction became the main processes affecting morphological changes in the Longkou Bay.The high-resolution bathymetric results near the artificial island showed that the seafloor was dredged at many sites and the mean water depth increased over 2 m,leaving large areas of borrow pits.The sudden change of the underwater topography will lead to the destruction of local benthic habitat and effective measures need to be taken to protect and remediate heavily disturbed subaqueous environment.In this study,the automated classification approach for benthic habitat in AR water we designed was an attempt in the field of seabed acoustic exploration and filled the gap that there was no application in AR area by using acoustic mapping method.The MBES combined with geospatial analysis was used to obtain the micro topography features in the AR area and reclamation area.This provided a new technique for monitoring the evolution of offshore benthic habitat which is largely influenced by large-scale human activities.