Abstract
In this work, multibeam echosounder (MBES) and dual frequency sidescan sonar (SSS) data are combined to map the shallow (5–100 m) benthic habitats of the National Marine Park of Zakynthos (NMPZ), Greece, a Marine Protected Area (MPA). NMPZ hosts extensive prairies of the protected Mediterranean phanerogams Posidonia oceanica and Cymodocea nodosa, as well as reefs and sandbanks. Seafloor characterization is achieved using the multi-frequency acoustic backscatter of: (a) the two simultaneous frequencies of the SSS (100 and 400 kHz) and (b) the MBES (180 kHz), as well as the MBES bathymetry. Overall, these high-resolution datasets cover an area of 84 km2 with ground coverage varying from 50% to 100%. Image texture, terrain and backscatter angular response analyses are applied to the above, to extract a range of statistical features. Those have different spatial densities and so they are combined through an object-based approach based on the full-coverage 100-kHz SSS mosaic. Supervised classification is applied to data models composed of operationally meaningful combinations between the above features, reflecting single-sonar or multi-sonar mapping scenarios. Classification results are validated against a detailed expert interpretation habitat map making use of extensive ground-truth data. The relative gain of one system or one feature extraction method or another are thoroughly examined. The frequency-dependent separation of benthic habitats showcases the potentials of multi-frequency backscatter and bathymetry from different sonars, improving evidence-based interpretations of shallow benthic habitats.
Highlights
The cumulative human-induced threats on marine biodiversity and ecosystem goods and services are increasing worldwide while habitat loss and degradation, pollution, overfishing and climate change effects are classified among the most important threats that affect the marine realm [1,2]
The two different sub-categories of Posidonia oceanica beds, low-relief meadows and high-relief matte, have almost identical angular backscatter response curves (ARCs). This is interesting as, their textural appearance and relief are totally different in the backscatter and bathymetry data, it is their physical properties that governs the shape of the ARCs, i.e., both are variations of the same species
We exploit the efficacy of the combined use of multibeam echosounders (MBES) and sidescan sonars (SSS) for gaining more acoustic backscatter spectral bands in a single survey, as well as dense bathymetric and angular backscatter responses information
Summary
The cumulative human-induced threats on marine biodiversity and ecosystem goods and services are increasing worldwide while habitat loss and degradation, pollution, overfishing and climate change effects are classified among the most important threats that affect the marine realm [1,2]. This common scale was achieved through a segmentation process on the 100 kHz mosaic (being the only full seabed coverage dataset), which provided the means for an Object Based Image Analysis (OBIA) This way, regardless of the different data densities and non-stationarity of some data sources, all their features can be joined into common minute regions covering the full seafloor scene. This work is an effort to exploit the synergistic use of MBES and SSS as complementary sources of information for improved benthic habitat mapping with emphasis on Mediterranean seagrass habitats By weighing their relative importance for habitat discrimination, we identify optimal mapping strategies, informing future surveys and extending interpretation of the huge amount of existing datasets.
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