Physical and Biological Assessment of the Deep Scattering Layer Along a Central Pacific Transect

The taxa of the mesopelagic zone constitute one of the most vital biological features in
oceans around the globe. From the Arctic to the Southern ocean, these organisms aggregate at depth to form a Deep Scattering Layer (DSL) that commonly undertakes a vertical migration to the surface each night. We sought to examine changes in the depth and composition of the Deep Scattering Layer along a transect of relatively unstudied open-ocean in the central Pacific (Stanford @ Sea S-235). We sampled the DSL at various depths within the Oxygen Minimum Layer (OML), using discrete net tows and concurrently determined the presence and density of biomass in the water column using an Acoustic Doppler Current Profiler (ADCP). We examined physical parameters of the water column including oxygen concentration, temperature and fluorescence, taking measurements each day along the transect.
We observed a dramatic increase in the amplitude of Diel Vertical Migration (DVM) as we headed southward along the transect and considered the physical and biological influences that might be inducing this behavior. We found the depth of the DSL to be closely associated with the depth of the Oxygen Minimum Layer, as well as with surface productivity, measured in terms of Chlorophyll-A Fluorescence. A significant increase in zooplankton density, consistent with ADCP readings, was also observed as we headed south. Finally, our data showed a higher proportion of nekton in our deepest tows and of gelatinous organisms in our shallowest tows. These findings present an interesting area for further research into the adaptations and ecological niches of certain mesopelagic organisms.