Hunter, William

van Oevelen, Dick

Witte, Ursula

Over 1 million km2 of seafloor experience permanent low-oxygen conditions within oxygen minimum zones (OMZs). OMZs are predicted to grow as a consequence of climate change, potentially affecting oceanic biogeochemical cycles. The Arabian Sea OMZ impinges upon the western Indian continental margin at bathyal depths (150 - 1500 m) producing a strong depth dependent oxygen gradient at the sea floor. The influence of the OMZ upon the short term processing of organic matter by sediment ecosystems was investigated using in situ stable isotope pulse chase experiments. These deployed doses of 13C:15N labeled organic matter onto the sediment surface at four stations from across the OMZ (water depth 540 - 1100 m; [O2] = 0.35 - 15 μM). In order to prevent experimentally anoxia, the mesocosms were not sealed. 13C and 15N labels were traced into sediment, bacteria, fauna and 13C into sediment porewater DIC and DOC. However, the DIC and DOC flux to the water column could not be measured, limiting our capacity to obtain mass-balance for C in each experimental mesocosm. Linear Inverse Modeling (LIM) provides a method to obtain a mass-balanced model of carbon flow that integrates stable-isotope tracer data with community biomass and biogeochemical flux data from a range of sources. Here we present an adaptation of the LIM methodology used to investigate how ecosystem structure influenced carbon flow across the Indian margin OMZ. We demonstrate how oxygen conditions affect food-web complexity, affecting the linkages between the bacteria, foraminifera and metazoan fauna, and their contributions to benthic respiration. The food-web models demonstrate how changes in ecosystem complexity are associated with oxygen availability across the OMZ and allow us to obtain a complete carbon budget for the stationa where stable-isotope labelling experiments were conducted

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Balancing the (carbon) budget: Using linear inverse models toestimate carbon flows and mass-balance 13C:15N labellingexperiments in low oxygen sediments.Hunter, W., van Oevelen, D., & Witte, U. (2013). Balancing the (carbon) budget: Using linear inverse models toestimate carbon flows and mass-balance 13C:15N labelling experiments in low oxygen sediments.. Postersession presented at European Geoscience Union 2013, Vienna, Austria.Queen's University Belfast - Research Portal:Link to publication record in Queen's University Belfast Research PortalPublisher rightsCopyright the authors 2013.General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or othercopyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associatedwith these rights.Take down policyThe Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made toensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in theResearch Portal that you believe breaches copyright or violates any law, please contact openaccess@qub.ac.uk.Download date:09. Sep. 2018Approximately 6 % of the continental margin sea floor experiences persistent dysoxia within oxygen minimum zones (OMZs) (Helly & Levin, 2004). OMZs are predicted to grow as a consequence of climate change, with implications for marine biogeochemical cycles (Stramma et al., 2008). The Arabian Sea OMZ impinges upon the Indian continental margin at bathyal depths (150 – 1500 m) generating a depth-dependent oxygen gradient. In 2008 a multi-national expedition led by Prof Hiroshi Kitazato (JAMSTEC, Japan) investigated the effects of oxygen availability, sediment geochemistry and community structure upon carbon & nitrogen cycling pathways at the Indian margin OMZ. In situ 13C-tracer experiments were conducted to quantify OM processing by sediment bacteria and fauna (Witte et al., 2011; Hunter et al., 2012 a; b). However, no empirical data on 13C fluxes from the sediment were available. We closed the carbon budget for each 13C-tracer experiment using linear inverse modelling (LIM) to reconstruct sediment carbon fluxes. William R Hunter1,3, Dick Van Oevelen2, Ursula, Witte1 1Oceanlab, University of Aberdeen AB41 6AA, UK.  2Royal Netherlands Institute for Sea Research, Yerseke, Netherlands. 3Present Address: Department for Limnology, Universität Wien, Austria. Email: william.hunter@univie.ac.at References Helly and Levin (2004). Deep-Sea Res Part I 51 (9): 1159-1168. Hunter et al. (2012a). Biogeosciences 9: 993-1006. Hunter et al. (2012b). ISME Journal 6 (11): 2140-2151.   Stramma et al. (2008). Science 320 (5876): 655-658. Van Oevelen et al. (2010). Ecosystems 13: 32-45. Witte et a., (2011). Geophys Res Abs. Vol. 13, EGU2011-3089.  Acknowledgements Figure 2 images collected by the submersible Shinkai 6500, courtesy of JAMSTEC. This study was funded by The Carnegie Trust, Scotland. WRH was funded under a NERC doctoral training grant. The authors wish to thank Prof. H. Kitazato, the YK08-11 science team, and the captain and crew of the RV Yokosuka. Without their unwavering dedication and enthusiasm this work would not have been possible. Mass Spectroscopy was carried out at the Royal Netherlands Institute for Sea Research, UC Davis Stable Isotope Facility and Macaulay Land Use Research Institute. 2 3 Linear Inverse Modelling Solutions Experimental Design & Model Formulation Fig 3. LIM solutions for carbon flows across the Indian margin OMZ. Barcharts reveal contributions of detrital, bacterial and faunal OM sources to organismal carbon budgets.      Abbreviations: lDet, labile detrius; rDet, refractory detritus; Bac, Bacteria; Nem, Nematodes, SDFPol / pSDF Surface Deposit Feeding polychaetes, SSDFPol / pSSDF, subsurface deposit feeding polychaetes; Othpol / pOth, other polychaetes Crust, macrofaunal crustacea; Moll, molluscs; Sip, sipunculans; Asc, ascidians; Oph, megafaunal ophiuroids; Dec, megafaunal decapods  1.  Food-web model complexity increases concomitantly with oxygen availability. 2.  Across the Indian margin OMZ labile phytodetritus was primarily processed by foraminifera and metazoan macrofauna. 3.  LIM estimates community respiration to be greatest at the 800 m stations, driven by higher metazoan faunal contributions.  4.  Bacterial contributions to sediment respiration increased concomitantly with the depth-dependent oxygen gradient. 5.  At present, foraminiferal data are only available for station T1 540 m. Foraminifera must be integrated into all models to accurately reconstruct sediment carbon fluxes. Balancing the (Carbon) Budget: Using linear inverse models to estimate carbon flows and mass-balance 13C:15N labelling experiments in low oxygen sediments. T1 540 m T1 800 m T2 800 m T2 1100 m < 1.5  mmol C m-2 d-1 <  0.15  mmol C m-2 d-1 Taxa Proportion of orgC Uptake Fig. 3 Summary Fig 4. Sediment community respiration and the relative contributions of bacteria and major faunal groups estimated from LIM solutions.    T1 540 m T1 800 m T2 800 m T2 1100 m Sed. Respiration 358.40 1089.00 1088.00 923.30  (µmol C. m-2. d-1) Standard Deviation 3.63  4.55  7.73  20.69 ! Experimental Design & Data Acquisition Semi-enclosed mesocosms deployed fixed doses of 13C-labelled diatoms (Thallassiosira weissflogii).at the seafloor across the Indian margin OMZ (540 – 1100 m, Figs 1, 2). The 13C label was traced into sediment OM, bacteria, foraminifera and metazoan fauna (A. Enge & P. Heinz, unpub. data; Hunter et al., 2012a; b; Witte et al., 2011). Linear Inverse Models Food web models were constructed following                 Van Oevelen et al. (2010).  Food web components and flow linkages were fixed         a priori.  Flow magnitudes were constrained using data from the mesocosm experiments.  Final model solutions were obtained by Bayesian sampling for best fit from 25,000 iterations of each model. Fig 1. Bathymetric Map of the Indian Continental margin with experimental stations marked (left) & corresponding O2 profiles (right). Fig. 3 Introduction Fig 2. Deployment of semi-enclosed mesocosms at the Indian margin seafloor. Proportion of Sediment Respiration Station T1 540 m T1 800 m T2 800 m T2 1100 m 

Balancing the (carbon) budget: Using linear inverse models to estimate carbon flows and mass-balance <sup>13</sup>C:<sup>15</sup>N labelling experiments in low oxygen sediments.

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Balancing the (carbon) budget: Using linear inverse models to estimate carbon flows and mass-balance <sup>13</sup>C:<sup>15</sup>N labelling experiments in low oxygen sediments.

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