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  • Primary productivity and N2-fixation in the eastern Indian ocean: Bottom-up support for an ecologically and economically important ecosystem
Title: Primary productivity and N2-fixation in the eastern Indian ocean: Bottom-up support for an ecologically and economically important ecosystem
Oligotrophic ocean regions are characterized by strong nutrient limitation, low standing phytoplankton biomass, and highly efficient nutrient recycling. During the BLOOFINZ-IO expedition (February 2022), we quantified nutrient inventories, primary productivity and N2 fixation in the Argo Basin of the eastern Indian Ocean, the sole known spawning ground for Southern Bluefin Tuna. Low concentrations of surface nitrate (<0.02 μmol L−1) and persistent residual phosphate indicated N as the limiting macronutrient, with photophysiological indices consistent with iron colimitation. Depth-integrated net primary production (NPP), from 14C-based in-situ incubations during 4 Lagrangian experiments, averaged ~460 mg C m−2d−1, generally agreeing with mean satellite-based NPP estimates (459 mg C m−2d−1) but with spatial discrepancies. Nitrogen fixation provided a consistent new nitrogen source, contributing ~16 % to local NPP in the upper euphotic zone. Gross primary production (GPP), from fast-repetition-rate-fluorometry-based estimates of electron transport, revealed significant autotrophic respiration losses, with mean GPP:NPP ratios of ~1.8 consistent with metabolic costs under nutrient limitation. Net community production (NCP) from O2/Ar ratios averaged ~20 % of NPP in the upper 30 m. This result, in combination with N2 fixation measurement, indicates that N2 fixation supports most of the export production in the region. Together, the multimethod approach revealed a recycling-dominated ecosystem affected by episodic mixing events, where primary productivity is maintained primarily through efficient nitrogen recycling and physiological photoacclimation. These results provide a comprehensive baseline of bottom-up support of ecosystem productivity for the Argo Basin for assessing future climate-driven changes in stratification, nutrient cycling, and food-web dynamics.  more » « less
Award ID(s):
1851347 2404504
PAR ID:
10661679
Author(s) / Creator(s):
; ; ; ; ;
Publisher / Repository:
Deep-Sea Research Part II
Date Published:
Journal Name:
Deep Sea Research Part II: Topical Studies in Oceanography
Volume:
225
Issue:
C
ISSN:
0967-0645
Page Range / eLocation ID:
105570
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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