Harms, Natalie C., Lahajnar, Niko, Gaye, Birgit, Rixen, Tim ORCID: https://orcid.org/0000-0001-8376-891X and Freitag, Ralf (2024) Chapter 12 - Biogeochemistry of the South Indian Ocean—Water Masses, Nutrient Distribution, and Sinking Particulate Matter. In: Deep-Sea Mining and the Water Column. , ed. by Sharma, Rahul. Springer Nature Switzerland AG, Cham, pp. 377-413. DOI https://doi.org/10.1007/978-3-031-59060-3_12.

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Abstract

The South Indian Ocean, one of the least explored ocean regions, is dominated by the Indian Ocean subtropical gyre (IOSG), one of the five extensive oligotrophic areas in the world’s oceans. Here, we present results from a comprehensive study on physical water column data (e.g., temperature, salinity and sigma-theta), nutrient and stable isotope data, and long-time sinking particulate matter collection retrieved between 2014 and 2019 by sediment trap moorings in the INDEX (Indian Ocean Exploration) area. Our research contributes to the main understanding of key processes of the N and C cycle and provides the first multidisciplinary approach of water mass interfingering, N sources and transformation processes and the quantity, quality and variability of sediment trap-based sinking particulate matter fluxes. With this knowledge, future impacts from deep-sea mining activities on persisting biogeochemical cycles in the South Indian Ocean will be better understood and evaluated. Numerous water samples along 3°S to 28°S allow detailed water mass analyses accompanied by a comprehensive study of nitrogen sources and cycling processes. We demonstrate the convergence and mixing of water masses of Antarctic and Subantarctic origin with water masses from the North Indian Ocean and track the changes in nutrient distribution and signatures of stable isotopes (N and O of nitrate) along these diverse water masses. This provides evidence for the injection of preformed nutrients from the Subantarctic and the Arabian Sea. Furthermore, dinitrogen (N2) fixation evidently accounts for a significant proportion of primary production, and we calculate that ~32–34% of the nitrified nitrate is contributed by N2 fixation in surface waters of the IOSG at ~20–24°S. Nevertheless, the IOSG is a strongly nutrient-limited ocean realm and is characterised by low primary production rates and hence very low sinking particulate matter fluxes. Sinking particulate matter considered here was collected during the 5-year sediment trap deployment experiment between 2014 and 2019 to provide new information on the major factors that control the particle export out of the biologically active zone, its transfer into the ocean’s interior and its potential storage in pelagic sediments of the South Indian Ocean. Comparing particulate organic carbon (POC) fluxes to global data, the IOSG reveals the lowest fluxes worldwide (0.52 ± 0.18 mg m−2 d−1 at 500 m.a.b.), even compared to other oligotrophic areas. Preliminary estimates indicate an average POC export efficiency of ε ≈ 0.02. Based on net primary production (NPP) data in surface waters of the IOSG only 0.17% of the POC produced at the sea surface reaches the sea floor. Due to intense degradation at the sediment–water interface, less than 0.01% (~0.02 mg POC m−2 d−1) accumulates in surface sediments. Assuming that the IOSG, as well as comparable ocean regions, will expand under climate warming, it is of major importance to investigate POC export fluxes and its final carbon storage in the sediments in order to study the organic carbon pump and potential changes in the global C cycle. Furthermore, we present the seasonal variability of particle fluxes in the IOSG that shows a decoupling between NPP and the amount of exported particles. In the winter season, the external nutrient supply via gyre boundaries and/or subsurface waters elevate NPP and favour a complex zooplankton-dominated food web. In a first step, this enhances the organic matter production but, on the other hand, reduces the export of particles by increasing their fragmentation into free/unprotected particles, vulnerable to remineralisation/degradation. In comparison, during the low productive summer season, a foraminifera-dominated zooplankton community increases the particle export efficiency by ballast-minerals (here CaCO3). Consequently, the final export of particulate matter is anti-correlated to NPP rates. Various factors, such as variations in the ocean mixed layer depth, impacts of physical mixing (surface wind stress, cyclonic eddies), changes in the plankton community structure and concomitant changes, impact the vertical transfer of particles to the sea floor. Understanding these complex mechanisms is challenging, but they have to be studied in detail to make robust statements about global climate predictions.

Document Type:	Book chapter
Programme Area:	PA2
Research affiliation:	Biogeochemistry and Geology > Carbon and Nutrient Cycling
DOI:	https://doi.org/10.1007/978-3-031-59060-3_12
Date Deposited:	14 Aug 2024 13:22
Last Modified:	14 Aug 2024 13:22
URI:	http://cris.leibniz-zmt.de/id/eprint/5454

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