Dissipation Processes in the Tongue of the Ocean

The Tongue of the Ocean (TOTO) region located within the Bahamas archipelago is a relatively under-studied region in terms of both its biological and physical oceanographic characteristics. This region is comprised of a deep trough along with steep boundaries that are commonly associated with elevated turbulent dissipation rates, and #949;, when impacted by currents and internal waves. A prey-field mapping cruise took place in the fall between 9/15/2008 and 10/01/2008, consisting of a series of transects and 'clovers' to study the spatial and temporal variability. The region is characterized by a deep scattering layer (DSL), which is preyed on by nekton that serves as the food for beaked whale and other whale species. This study marks the first of its kind where concurrent measurements of acoustic backscatter and turbulence have been conducted for a nekton scattering layer well below the euphotic zone. In this novel study, turbulence data collected from a deep microstructure profiler are compared to biological and shear data collected by a 38 kHz Simrad EK 60 echosounder and a hydrographic Doppler sonar system, respectively. From these measurements, the primary processes responsible for the turbulent production in the TOTO region are assessed. The DSL around 500 m and a surface scattering layer (SSL) are investigated for raised and #949; values. Strong correlation between turbulence levels and scattering intensity of prey is generally found in the SSL with dissipation levels as large as ~10-7 W kg-1, three orders of magnitude above background levels. In the deep scattering layer, however, the correlations are relatively weak, but exhibit dissipation levels ~10-8 W kg-1. The absence of turbulence bursts of O(10-5 W kg-1) proposed to occur within dense biomass aggregations suggests biologically generated turbulence is not efficient by the marine biosphere. Areas of elevated turbulence have many implications from sustaining the abyssal stratification to transporting of nutrients and gases to and from the surface. We present an examination of the TOTO mixing levels with those from a typical open ocean site.