rhodoliths

A small submersible remotely operated vehicle (ROV) was used to explore mid-depth habitats of Saba Bank – a submerged carbonate platform in the northeastern Caribbean Sea. The ROV enabled observation and specimen collection to greater depths (40 to > 150 m) than possible with conventional scuba methods. The topography, substrate, benthic communities and fish assemblages of a prominent front reef system at Overall Bank were examined in some detail. Observations made from ROV indicate that substrate and benthic communities show consistent zonation patterns along the depth gradient of the front reef slope. A transition in the reef fish assemblage was also evident, though less pronounced, along this same depth gradient. Fish diversity (number of species observed per survey) was greatest in the reef crest zone and declined with depth, however sightings of commercially important lutjanids species such as silk and blackfin snapper increased with depth. Significant cover by reef-building corals (i.e. constructional reef development) was only evident in the reef crest zone, and terminated at 38 to 42 m depth. Substrate of the transition slope zone, from 42 m to 75 m depth, was primarily a hard bottom consisting of consolidated reef structures and rubble with sand interspersed. Sponges, gorgonians and macro algae dominated the benthic community there. In the deep slope zone (> 75 m depth), a soft bottom substrate predominated that was composed of fine, readily resuspended sediments together with scattered rubble fragments. Benthic invertebrates were very sparse in the deep slope zone. In addition to the surveys at Overall Bank, four ROV surveys were made at two other Saba Bank areas: Poison Bank and Grapplers Bank. The substrate at Poison Bank was comprised of coralline algal nodules or “rhodoliths” which formed extensive rhodoliths beds. At Grapplers Bank, a steep rocky escarpment was explored. The near-vertical rocky scarp began at 120 m depth and extended down slope beyond the limits of the ROV survey (157 m depth). Observations made from ROV at Overall Bank suggested a continuous reef system that is relatively uniform and predictable at mid-depths in terms of its structure, substrate composition, and community zonation patterns. In contrast, the few observations made by ROV at Poison Bank and Grapplers Bank revealed habitats that were quite different from those at Overall Bank. This implies that future ROV explorations to new areas of Saba Bank are likely to reveal still greater diversity in mid-depth habitat types.