Mesozooplankton community variability in the Seychelles-Chagos Thermocline Ridge of the western Indian Ocean

Abstract

The Seychelles-Chagos Thermocline Ridge (SCTR), which is formed by strong air-ocean that causes open-ocean upwelling in the western Indian Ocean, is known to enhance upper-ocean nutrients which promotes growth of phytoplankton leading to high abundance of zooplankton and consequentially linking to nursery grounds for fish. To evaluate the evidence for significant upwelling of SCTR in the western Indian Ocean, the open-ocean upwelling ecosystem in the SCTR and non-SCTR, which were still uncertain, were compared to identify the upwelling indicators with variability in community of mesozooplankton and the supporting environmental measurements. We investigated the epipelagic (0-200 m) mesozooplankton and physico-chemical factors with MOCNESS at three depth strata along 67°E (16°S–5°N) across central and western Indian Ocean. Surveyed stations were clustered into five environmental regions, SCTR regions (I and II) and non-SCTR regions including East African Costal Current (EACC), South Equatorial Current (SEC), and Indonesian Throughflow (ITF) based on cluster analysis. Although higher abundance of mesozooplankton were observed in the surface mixed layer (SML) (avg. 1,112 inds. m-3), in which Noctiluca scintillans was distributed ubiquitously, there were no significant difference between SCTR and non-SCTR. We found significant peaks (p<0.05) of abundance (962 inds. m-3) with significantly lower temperature (avg. 17.7°C) at SCTR-II of the middle layer(ML), in which N. scintillans and Oncaea copepodites were dominant, and significantly lower abundance with dominance in copepodites of Scolecithricella and Scolecithrix at non-SCTR. The mesozooplankton was significantly variable in the bottom layer (BL) dominated by either ostracods or Scolecithricella copepodites. The spatial difference in abundance of Oncaea spp. affected the higher ratio of cyclopoid-to-calanoid (cy:ca) in ML of SCTR, in which opportunistic copepods peaked as result of enhanced nutrients and the related increased concentration of chl-a in the upwelled waters. Principal component analysis with environmental factors and dominant mesozooplankton groups showed that dominant groups were affected by significantly higher temperature (25.3~29.8°C, p<0.05) in SML and by more saline water (34.8~35.7) in ML, and no significant association was found in BL. We suggest that the middle layer of SCTR hosts as a hotspot with opportunistic feeders of Oncaea spp., that is regulated by bottom-up effects of environmental factors found in the open-ocean upwelling of western Indian Ocean.