Mapping subtropical and tropical rhodolith seabeds using Side Scan Sonar technology

Abstract

The cartography of seabeds, using acoustic technologies for mapping marine benthic communities is increasing and highly demanded in the management of subtropical and tropical environments where rapid anthropogenic development often occurring in coastal zones. The side-scan sonar (SSS) is an effective tool with greater use in several nautical disciplines (e.g. seaweed monitoring). In this study, the SSS was successfully employed to collect data from subtropical (Canary Islands, Spain) and tropical (Principe Island, Republic of Sao Tome and Principe) rhodolith seabeds from 3-40m in depth. A list of pre-characterization habitats was generated in each studied area, using post-processing software and then corroborated by ground-truthing methods (submarine video and diving). Rhodolith seabeds were distinguished from bare sediment, rocky and coral reefs and seagrass meadows. In Gando bay (Canary Islands), two main areas were surveyed. The northern part showed extensive rhodolith beds up to 40m deep interspersed with rocky areas and sand ripples. Otherwise , sand stretches appeared combined with rhodoliths and mixed patches of macroalgae and marine seaweeds (e.g. Caulerpa prolifera and Cymodocea nodosa, respectively) from 15 to 40m. Otherwise, in Principe Island four areas were analyzed with rhodolith beds mostly concentrated at the north and west part of the island. These biogenic habitats appeared mixed with sandy ripples and colonies of Scleractinian corals in shallow waters (from 3 to 10m) becoming more homogeneous in deeper areas (30m). The created Geo-referenced habitat maps denoted the efficiency of SSS methods (combined with ground-truthing) for similar studies involving rhodolith seabeds.