SCUBA sampling and acoustic survey coupling in shallow sandy bottoms

Coastal shallow sandy bottoms are subject to various anthropogenic disturbances, including coastal urban development and the construction of coastal defence structures. These increase habitat fragmentation and loss, and decrease biodiversity, negatively affecting ecosystem functions and services. For this reason, they are in need of well-informed ecosystem-based management. However, since these habitats are characterised by high dynamism and shallow depth, they are challenging to investigate and regularly monitor using research vessel-based instruments and samplers. The sandy coasts of the Emilia-Romagna region (NW Adriatic Sea) represent an ideal area for developing integrated monitoring approaches for shallow bottoms, the aim of the present study. They feature a variety of urbanisation, including bathing facilities, coastal defence structures, as well as river estuaries and small protected areas. For this purpose, 3 study sites with different levels of naturalness, tourist use, protection, and coastal defence interventions were selected in this region. For each site, sampling of sediment and macrobenthic organisms was manually performed by scientific divers on random points, spread to cover the entire area of interest. Geophysical data were acquired along a grid of routes via acoustic instruments on board the OpenSWAP, a fully autonomous surface vehicle designed and equipped for shallow waters. Its ability to follow the planned routes with great precision (<30 cm) allows accurate replication over a long time. To make the data acquired with different methods and resolutions comparable, all measured variables (e.g., depth, sediment features, species abundances) were interpolated within the areas of interest using the geostatistics Kriging method, obtaining the corresponding grid maps. Depth and sediment features are considered among the main drivers in structuring the benthic communities of these seabeds. However, the presence of both artificial structures and river mouths can strongly affect the species local distribution. Linear spatial correlations between species abundances, diversity indices, and environmental variables were used to highlight these effects, and the resulting residuals were mapped and analysed. Residuals distribution maps can be regarded as species distribution anomalies related to natural sedimentological and ecological processes and alterations induced by anthropogenic disturbances. Quantifying these anomalies allows us to better understand the effects of anthropogenic disturbances and interventions in these areas of tourist interest, thus forming the basis for integrated coastal management and the conservation of coastal habitats. This correlative spatial data analysis approach allows the integration of data collected by scuba divers and other sources. Although tested in sandy bottoms, it could be effective in other coastal habitats like seagrass meadows and rocky bottoms.