Spatial Variation in Primary Production in the Eastern Indian Ocean

Haijiao Liu,Jun Sun,Xiaodong Zhang,Yuyao Song,Chao Wu,Guicheng Zhang,Xingzhou Wang,Ju Chen,Dongxiao Wang,Satheeswaran Thangaraj

doi:10.3389/fmars.2021.757529

Abstract

To examine the spatial pattern and controlling factors of the primary productivity (PP) of phytoplankton in the eastern Indian Ocean (EIO), deck-incubation carbon fixation (a 14C tracer technique) and the related hydrographic properties were measured at 15 locations during the pre-summer monsoon season (February–April 2017). There are knowledge gaps in the field observations of PP in the EIO. The estimated daily carbon production rates integrated over the photic zone ranged from 113 to 817 mgC m–2 d–1, with a mean of 522 mgC m–2 d–1. The mixed-layer integrated primary production (MLD-PP) ranged from 29.0 to 303.7 mgC m–2 d–1 (mean: 177.2 mgC m–2 d–1). The contribution of MLD-PP to the photic zone-integrated PP (PZI-PP) varied between 19 and 51% (mean: 36%). Strong spatial variability in the carbon fixation rates was found in the study region. Specifically, the surface primary production rates were relatively higher in the Bay of Bengal domain affected by riverine flux and lower in the equatorial domain owing to the presence of intermonsoonal Wyrtki jets, which were characterized by a depression of thermocline and nitracline. The PZI-PP exhibited a linear (positive) relationship with nutrient values, but with no significance, indicating a partial control of macronutrients and a light limitation of carbon fixation. As evident from the vertical profiles, the primary production process mainly occurred above the nitracline depth and at high photosynthetic efficiency. Phytoplankton (&gt;5 μm), including dinoflagellates, Trichodesmium, coccolithophores, and dissolved nutrients, are thought to have been correlated with primary production during the study period. The measured on-deck biological data of our study allow for a general understanding of the trends in PP in the survey area of the EIO and can be incorporated into global primary production models.

Highlights

Phytoplankton photosynthesis is a crucial and highly variable step of the global carbon cycle and marine ecosystems
The present in situ observations attempted to understand the spatial variability of Net primary productivity (NPP) and the associated hydrographic properties within the photic zone in the eastern Indian Ocean (EIO) during the spring intermonsoon period
Influenced by these factors, a remarkable difference in the primary production rate was observed among the survey areas, especially between the Bay of Bengal (BoB) domain and the other domains

Summary

Introduction

Phytoplankton photosynthesis is a crucial and highly variable step of the global carbon cycle and marine ecosystems. Primary marine primary production contributes to nearly 50% of the bulk photosynthetic carbon of the planet and is closely related to global fishery catches (Saba et al, 2011; Behrenfeld et al, 2001; Chassot et al, 2010). Marine NPP can be estimated using a variety of ocean color models (Behrenfeld et al, 2006; Chassot et al, 2010; Saba et al, 2011). These indirect productivity estimates show large uncertainties between various models (Friedrichs et al, 2009). Understanding the dynamics and controlling factors of the spatial–temporal distribution of primary production is a fundamental issue for the higher trophic levels in aquatic systems

Methods

Results

Discussion

Conclusion