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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Aug 2018

Research article | 06 Aug 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Biogeosciences (BG).

Composition and Vertical Flux of Particulate Organic Matter to the Oxygen Minimum Zone of the Central Baltic Sea: Impact of a sporadic North Sea inflow

Carolina Cisternas-Novoa, Frédéric A. C. Le Moigne, and Anja Engel Carolina Cisternas-Novoa et al.
  • Cisternas-Novoa, GEOMAR, Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, D-24105 Kiel, Germany

Abstract. Sinking particles are the main form to transport photosynthetically fixed carbon from the euphotic zone to the ocean interior. Oxygen (O2) depletion may improve the efficiency of the biological carbon pump. However, how the lack of O2 mechanistically enhances particulate organic matter (POM) fluxes is not well understood. In the Baltic Sea, the Gotland Basin (GB) and the Landsort Deep (LD) exhibit permanent bottom-water hypoxia, this is on occasions alleviated by Major Baltic Inflow (MBI), such as the one that occurred in 2014/2015 which oxygenated the bottom waters of the GB (but not of the LD). Here, we investigate the distribution and fluxes of POM in the GB and the LD in June 2015 and how they were affected by the 2015 MBI.

Fluxes and composition of sinking particles were different in the GB and the LD. In the GB, POC flux was 18% lower at 40m than at 180m. Particulate nitrogen (PN) and Coomassie stainable particles (CSP) fluxes decreased with depth, and particulate organic phosphorous (POP), biogenic silicate (BSi), Chl a, and transparent exopolymeric particles (TEP) clearly peaked within the core of the oxygen minimum zone (OMZ), which coincided with a high flux of manganese oxide (MnOx)-like particles. Contrastingly, in the LD, POC, PN, and CSP fluxes decreased 28, 42 and 56% respectively from 40 to 180m. POP, BSi, and TEP fluxes, however, did not decrease with depth and only a slightly higher flux was measured at 110m. MnOx-like particle flux was two orders of magnitude higher in the GB relative to the LD.

MnOx-like particles formed after the inflow of oxygenated water into the deep GB may form aggregates with POM. Our results suggest, that when the deep waters of GB were oxygenated (2014/2015 North Sea inflow), not only transparent exopolymeric particles, as indicated previously, but also POC, POP, BSi, and Chl a may bind to MnOx-like particles. POM associated with MnOx-like particles may accumulate in the redoxcline, where they formed larger particles that eventually sank to the seafloor. We propose that this mechanism would alter the vertical distribution and the flux of POM, and it may contribute to the higher transfer efficiency of POC in the GB. This is consistent with the fact that the OM reaching the seafloor was fresher and less degraded in the GB than in the LD.

Carolina Cisternas-Novoa et al.
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Status: final response (author comments only)
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Carolina Cisternas-Novoa et al.
Carolina Cisternas-Novoa et al.
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Short summary
We investigated the composition and vertical flux of particulate organic matter (POM) in the Gotland Basin affected by Major Baltic Inflow (MBI) that increased the O2 concentration in the deep waters and the Landsort Deep that was not affected by the MBI, exhibited low O2 concentration and sulfidic conditions. In the Gotland Basin, O2 intrusions lead to a high abundance of MnOx-POM aggregates that increased the export efficiency of POM with implications for the O2 and POM cycles.
We investigated the composition and vertical flux of particulate organic matter (POM) in the...