Deep Scattering Layers of the Northern Gulf of Mexico Observed With a Shipboard 38-kHz Acoustic Doppler Current Profiler

Abstract

Midwater sound-scattering layers containing aggregations of zooplankton and micronekton prey form in response to a trade-off between predator avoidance at depth and optimal foraging near the surface. Although the volume backscatter strength of zooplankton aggregations have been extensively studied in the past, fewer studies have specifically examined other descriptive characteristics of these layers such as depth of layers, timing of migrations, and the presence of secondary scattering layers below the main scattering layer. In the present study, patterns of deep scattering layers (DSLs) were characterized using relative acoustic backscatter from a ship-mounted 38-kHz phased-array, acoustic Doppler current profiler (ADCP) in the northern Gulf of Mexico in summers 2002 and 2003. Temporal patterns of scattering layers were analyzed with respect to the timing of the daytime and nighttime diel vertical migrations, and spatial patterns of scattering layers were analyzed with respect to their proximity to mesoscale circulation features associated with upwelling, downwelling, and water depth. The most prominent main scattering layer was consistently found at daytime depth of 450 to 550 m below the surface except during an unusual shoaling event in which a significant shallowing of the layer was observed at 200 to 300 m below the surface. This event coincided with the crossing of a strong frontal boundary between high salinity, blue water and low salinity, green water from the Mississippi River plume. Less prominent secondary scattering layers found deeper than the main scattering layer showed regional variability and appear to be more frequently associated with shallower shelf depths than in the deepwater basin. Variability among deep scattering layers in this region may have important implications for the behavior and interactions of higher trophic levels dependent on these prey layers.