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

Abstract

<p><strong>Abstract.</strong> Sinking particles are the main form to transport photosynthetically fixed carbon from the euphotic zone to the ocean interior. Oxygen (O<sub>2</sub>) depletion may improve the efficiency of the biological carbon pump. However, how the lack of O<sub>2</sub> 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. <br><br> Fluxes and composition of sinking particles were different in the GB and the LD. In the GB, POC flux was 18 % lower at 40 m than at 180 m. Particulate nitrogen (PN) and Coomassie stainable particles (CSP) fluxes decreased with depth, and particulate organic phosphorous (POP), biogenic silicate (BSi), Chl <i>a</i>, 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 180 m. POP, BSi, and TEP fluxes, however, did not decrease with depth and only a slightly higher flux was measured at 110 m. MnOx-like particle flux was two orders of magnitude higher in the GB relative to the LD. <br><br> 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 <i>a</i> 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.</p>

Reply on RC1

“…Enhanced transport of organic matter to the sediment via Mn-oxides could thus have led to a higher deposition and increased benthic organic matter degradation rates. According to the numbers presented by Cisternas-Novoa et al (2019), however, the increased vertical POC flux after oxygenation would still have remained relatively low. In contrast, lateral transport could have been a substantial source of POC and associated elements after the inflow.…”

Reply on RC2

“…CC BY 4.0 License. increased POC transport in the EGB was thought to be the formation of manganese oxides (Mn-oxides) in the oxygenated water column, scavenging organic matter while sinking (Cisternas-Novoa et al, 2019). Several other studies have confirmed that the MBI led to a considerable formation and sedimentation of Mn-oxides in the water column of the EGB (Dellwig et al, 2018;Hermans et al, 2019;van de Velde et al, 2020b).…”

“…In oxygen minimum zones, particle fluxes are normally higher than in surrounding oxic waters, possibly because of lower particle mineralisation rates as a result of less energetic electron acceptors or loss of zooplankton that consume particles (Le Moigne et al, 2017). However, a study conducted in the EGB in the summer of 2015 suggested that the particle sedimentation rate and freshness of the organic material reaching the sediment increased due to the MBI (Cisternas-Novoa et al, 2019). In the newly oxygenated EGB, the flux of particulate organic carbon (POC) increased by 18 % between 40 m and 180 m depth, while it decreased by 28 % at a permanently anoxic site nearby (Cisternas-Novoa et al, 2019).…”

“…However, a study conducted in the EGB in the summer of 2015 suggested that the particle sedimentation rate and freshness of the organic material reaching the sediment increased due to the MBI (Cisternas-Novoa et al, 2019). In the newly oxygenated EGB, the flux of particulate organic carbon (POC) increased by 18 % between 40 m and 180 m depth, while it decreased by 28 % at a permanently anoxic site nearby (Cisternas-Novoa et al, 2019). The cause of the https://doi.org/10.5194/bg-2021-5 Preprint.…”