High-resolution multibeam mapping of habitats in the extremely shallow waters of the Venice Lagoon.

Recent advances in multibeam technology have opened new possibilities in the field of mapping bottom morphologies, substrates and habitats. In particular, substrate mapping by mean of backscatter and water column multibeam data is relatively new in extremely shallow environments, where multibeam systems have rarely been used. In this study we present the first results of an extensive survey carried out during 2013in the very shallow channels of the Venice Lagoon using a Kongsberg EM-2040 DC multibeam system. Although numerous biological studies have been carried out to study habitats in the tidal flats and salt marshes of the Venice lagoon, the channel habitats are still almost unexplored. The new applications in habitat mapping given by the analysis of high-resolution bathymetry, backscatter and water column data will be discussed. In particular, we will show the first results of a survey carried out in a natural tidal channel of the northern part of the Venice Lagoon (water depths from 15m to less than 1 m). On the high-resolution backscatter data collected, we carried out a two-dimensional (2D) textural analysis with the TexAn software. We then performed an unsupervised classification of the backscatter data. As a result, we were able to identify different backscatter areas where several grab samples were collected for ground truthing. With the help of this sampling, we calibrated the textural analyses and obtained a classification of the different kinds of substrate. The characterization of the substrate was then tested with a set of bottom photographs. In addition, the first results of combined bathymetric, backscatter and water column data analysis for bottom vegetation detection in the shallow lagoonal channels will be also shown.