Stratification governs the plankton community structure and trophic interaction in the Southwestern tropical Indian Ocean during boreal summer

Abstract

Seychelles–Chagosthermocline ridge is one of the major upwelling area in the tropical Indian Ocean, where planktons are known to have large impacts on ocean ecosystem and biogeochemical cycles. Time series assessments of surface zooplankton abundance, biovolume, and community composition were carried out in the Indian Ocean during June 2014. In this paper, we explore the impact of phytoplankton size class biomass on trophic interaction in the thermocline ridge of the southwestern tropical Indian Ocean. Relatively, nitrate was a dominant contributor than the nitrite in the dissolved inorganic nitrogen pool. Size fractionated chlorophyll-a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton biomass was dominated, accounted for approximately 72% of total Chl a. We assumed that picophytoplankton biomass was most likely influenced by dissolved inorganic nutrients (chiefly nitrite), that showed a strong linear relationship with picophytoplankton biomass (r2=0.41). These results support the key role of nitrite is not only supplying and promoting the growth of smaller phytoplankton biomass, but also controls the structure of zooplankton communities. As a result surface waters were dominated by picophytoplankton biomass, in this condition result in zooplankton being dominated by Poecilostomatoida, constituting approximately 68% of the total zooplankton count. These organisms were widely distributed and occurred in large numbers and displayed a strong linear relationship with picophytoplankton biomass (r2=0.87). These empirical results suggest that abundance of poecilostomatoids presumably supports tertiary trophic levels, suggesting they might play a key role as carbon drivers in the vicinity of the thermocline ridge of the Indian Ocean.