Vertical biochemical composition of particulate organic matter in the Seychelles-Chagos Thermocline Ridge (SCTR), southwestern Indian Ocean

The Seychelles-Chagos Thermocline Ridge (SCTR) in the southwestern Indian Ocean exhibits unique physical characteristics and heightened biological activity relative to non-SCTR areas. This study analyzed the vertical distribution and biochemical composition of particulate organic matter (POM) in these contrasting regions. The SCTR displayed a shallower thermocline and cooler surface temperatures, consistent with upwelling, correlating with significantly higher chlorophyll-a levels (t-test, p < 0.05). Micro-sized phytoplankton dominated in the SCTR, accounting for 13.6 ± 3.0% of the total phytoplankton biomass, compared to 5.6 ± 1.6% in non-SCTR regions. CHEMTAX analysis showed distinct phytoplankton communities, with diatoms four times more abundant in the SCTR (16.5 ± 4.5%) than in non-SCTR areas (4.2 ± 1.2%). Prochlorococcus dominated both regions but contributed less in the SCTR (25.2 ± 3.9%) than in non-SCTR (46.8 ± 7.1%). The macromolecular composition of POM highlighted regional contrasts, with lipid concentrations in the SCTR’s photic layer significantly higher (61.3 μg L⁻¹) than in non-SCTR (23.4 μg L⁻¹). The total food material concentration in the SCTR’s photic zone was over double that of non-SCTR (96.3 μg L⁻¹ vs. 40.9 μg L⁻¹), indicating enhanced productivity. Elevated macromolecule levels extended into the SCTR’s aphotic layer, with a less marked decline in the protein-to-carbohydrate ratio from photic to aphotic layers, suggesting fresher POM sinking to the deep ocean. These findings underscore the SCTR's role in enhanced biological productivity and efficient carbon sequestration, with notable implications for the biological carbon pump in the Indian Ocean.

Presentation Preference: Poster

Primary Presenter: Sanghoon Park, Pusan national university (mossinp@pusan.ac.kr)

Authors:

Sang Heon Lee, Pusan National University  (sanglee@pusan.ac.kr)

Sanghoon Park, Pusan National University  (mossinp@pusan.ac.kr)