Vertical Compression of the Mesopelagic Deep Scattering Layer Associated With the Oxygen Minimum Zone in the Eastern Arabian Sea

Abstract

The Deep Scattering Layer (DSL), formed as an antipredator refuge, shapes the ecology of the mesopelagic system. Despite its ecological importance and distinctive hydrography of the Indian Ocean, a comprehensive understanding of the determinants of its daytime residence depth is still lacking. We identify hydrographic and biological controls on the daytime vertical position of DSL by integrating shipborne ADCP acoustic backscatter, Multiple Plankton Net (MPN) tows, CTD profiles, and satellite‐derived products. The DSL became progressively thinner within the OMZ, with a deeper upper boundary and a shallower lower boundary, indicating compression of available daytime habitat under low‐oxygen conditions. Among the environmental variables examined, dissolved oxygen exhibited the strongest relationship with DSL structure, whereas indirect indicators suggested a secondary influence of ambient light conditions. Taxonomic and isotopic analyses revealed reduced abundance of several migratory taxa within the OMZ and indicated trophic variability among dominant migrators such as euphausiids, myctophid larvae, and Pleuromamma indica . These findings demonstrate that dissolved oxygen is a major control on DSL formation in the Arabian Sea and that OMZ conditions influence both the physical structure of the DSL and the ecological characteristics of its constituent organisms. The study provides a regional baseline for evaluating how future ocean deoxygenation and OMZ expansion may affect mesopelagic ecosystems and their contribution to carbon cycling.