Benthic biotopes remote sensing using acoustics

Abstract

The present work applies novel methodologies to the study of sublittoral benthic biotopes, by combining the information given by underwater acoustic and biological survey techniques. The acoustic seabed classification system QTC VIEW™ was used to map the acoustic diversity between 5 and 40 m water depth on the shelf off “Ria de Aveiro”, Western coast of Portugal. Ground-truth was undertaken using an analysis of superficial sediments grain-size, and compared to the species composition and distribution of macrofaunal communities. Sedimentary and biological data were submitted to ordination analysis, and the acoustic data to both ordination and cluster analysis. The acoustic classes identified were mapped using a geographical information system.

The acoustic results showed a very clear geographic pattern, with the acoustic classification being coincident where survey lines crossed, confirming the stability of the classification procedure. At the optimal splitting level, three acoustic classes were obtained. These classes were correlated to differences in coarse, fine and very fine sands. Additional real bottom differences in the grain-size of the coarser sand classes were not detected by the acoustic method, possibly due to the fact that they showed a similar degree of compactness. The benthic ecological data suggests only two main communities, which correspond to the outcomes of a two-class acoustic split. Therefore, a mismatch was noticed between the optimal acoustic split and the number of major biological communities present. However, by dropping the acoustic solution down to two classes, an optimal relationship to benthic communities is achieved. Overall, results suggest that the acoustic system provides very valuable and important data for mapping soft sediment biotopes, even in areas of relative bottom monotony such as the one analysed, but careful ground-truth is required to ensure that the acoustic class splits are biologically relevant.