Vertical Structure of the Water Column at the Virgin Islands Shelf Break and Trough

The steep shelf break and trough between St Thomas and St Croix in the Eastern Caribbean Sea comprise a region with complex shelf break frontal circulation and cross-frontal exchanges. Also, the region is biologically unique because it includes two productive spawning aggregation areas 12 km south of St. Thomas: the Red Hind Conservation District and the Grammanik Bank. Little is known about the exchanges of water masses between the trough and these two spawning aggregation sites. The flow in this area and its variability has the potential to control the manner in which offshore waters move onto the shelf break and beyond, therefore affecting the patterns of recruitment along the islands located between Vieques (Puerto Rico) and Tortola (British Virgin Islands).

In-situ oceanographic data collected in the region during April, 2017 were analyzed to examine the characteristics of the water mass. Vertical profiles of temperature, salinity, density, chlorophyll-a and oxygen obtained with a Conductivity-Temperature-Depth were used to identify different water masses, specifically Caribbean Surface Water, Subtropical Underwater, Sargasso Sea Water, Tropical Atlantic Central Water, Antarctic Intermediate Water and North Atlantic Deep Water. In the upper 300 m, comparison of deep and shallow stations in the shelf break region show that differences exist among temperature, salinity and density profiles. Nevertheless, the most notable differences in the vertical structure are seen in the along-shelf direction and not in the cross-shelf direction. It is our hope that this detailed description of water structure along the shelfbreak will contribute to a more accurate understanding of physical-biological interactions and drivers in the region.