Sound Scattering Layers Within and Beyond the Seychelles-Chagos Thermocline Ridge in the Southwest Indian Ocean

Abstract

In global oceans, ubiquitous and persistent sound scattering layers (SL) are frequently detected with echosounders. The southwest Indian Ocean has a unique feature, a region of significant upwelling known as the Seychelles-Chagos Thermocline Ridge (SCTR), which affects sea surface temperature and marine ecosystems. Despite their importance, sound SL within and beyond the SCTR are poorly understood. This study aimed to compare the characteristics of the sound SL within and beyond the SCTR in connection with environmental properties, and dominant zooplankton. To this end, the region north of the 12°S latitude in the survey area was defined as SCTR, and the region south of 12°S was defined as non-SCTR. The results indicated contrasting oceanographic properties based on the depth layers between SCTR and non-SCTR regions. Distribution dynamics of the sound SL differed between the two regions. In particular, the diel vertical migration pattern, acoustic scattering values, metrics, and positional properties of acoustic scatterers showed two distinct features. In addition, the density of zooplankton sampled was higher in SCTR than in the non-SCTR region. This is the first study to present bioacoustic and hydrographic water properties within and beyond the SCTR in the southwest Indian Ocean.

Characterizing the Sound-Scattering Layer and Its Environmental Drivers in the North Equatorial Current of the Central and Western Pacific Ocean

“…The variation in NASC values generally follows the DVM pattern of the scattering layer. The DVM of the scattering layer is a behavioral mechanism typically characterized by plankton and micronekton ascending to feed at night and descending to deeper layers during the day to avoid higher trophic-level predators [7]. Biological factors and abiotic factors, such as light intensity, water transparency, and water temperature, influence the DVM of the scattering layer.…”

“…The sound-scattering layer (SSL) refers to a water layer with high scattering strength, which is produced by the aggregation of marine organisms [7]. The primary components of the scattering layer are zooplankton and fish [8].…”

Contribution of aged organic carbon to suspended particulate organic carbon in the western equatorial Indian Ocean

“…The equatorial Indian Ocean is influenced by the Wyrtki jet, a zonal current system characterized by a narrow, fast-flowing eastward current at the surface, which makes the equatorial Indian Ocean less productive than the equatorial Pacific or Atlantic (Wyrtki, 1973;George et al, 2013). The tropical Indian Ocean also includes the Seychelles Chagos Thermocline Ridge (SCTR), an area between 10°S and 5°S characterized by open ocean upwelling that brings nutrient-rich waters to the surface, resulting in high biological productivity (Hermes and Reason, 2008;Dilmahamod et al, 2016;Kang et al, 2021;Kim et al, 2022;Lee et al, 2022). However, only a limited number of investigations have utilized carbon isotopes for carbon cycle studies in the Indian Ocean.…”