Dissolved organic carbon contribution to oxygen respiration in the central Red Sea

Abstract

In oligotrophic waters, dissolved organic carbon (DOC) is mostly produced in the surface layers by phytoplankton and remineralized by heterotrophic prokaryotes throughout the water column. DOC surface excess is subducted and exported to deeper layers where a semi-labile fraction is further processed contributing to oxygen consumption. How this cycling of DOC occurs in the Red Sea, one of the warmest oligotrophic marine basins, is virtually unknown. We examined DOC vertical and seasonal variability in a mesopelagic station (ca. 700 m depth) of the central Red Sea performing monthly profile samplings over a two-year period. Together with DOC vertical and seasonal distribution we evaluated the interaction with heterotrophic prokaryotes and contribution to oxygen respiration. DOC values ranged from 41.4 to 95.4 µmol C L−1, with concentrations in the epipelagic (70.0 ± 7.5 µmol C L−1) 40% higher on average than in the mesopelagic (50.7 ± 4.1 µmol C L−1). Subduction of seasonally accumulated semi-labile DOC was estimated to be responsible for ∼20% of the oxygen consumption mostly occurring at the low epipelagic-upper mesopelagic boundary layer. Variability in mesopelagic waters was higher than expected (ca. 20 µmol C L−1) evidencing a more active realm than previously thought, with consequences for carbon sequestration.

Peer Review #3 of "Seasonal variability and vertical distribution of autotrophic and heterotrophic picoplankton in the Central Red Sea (v0.1)"

“…Regarding the vertical distribution of heterotrophic prokaryotes, we confirm the findings of two recent studies conducted at the same site as ours, focused on the interactions of bacteria with DOC stocks at the diel (García et al 2018) and seasonal scales (Calleja et al 2019). As previously reported (Calleja et al 2019;García et al 2018), LNA bacteria dominated in the epipelagic zone while HNA bacteria prevailed in the mesopelagic zone.…”

“…Regarding the vertical distribution of heterotrophic prokaryotes, we confirm the findings of two recent studies conducted at the same site as ours, focused on the interactions of bacteria with DOC stocks at the diel (García et al 2018) and seasonal scales (Calleja et al 2019). As previously reported (Calleja et al 2019;García et al 2018), LNA bacteria dominated in the epipelagic zone while HNA bacteria prevailed in the mesopelagic zone. The lower relative numbers of HNA cells in the upper 100 m (usually below 51%) could be explained by the presence of protistan grazers with a preference for the larger HNA cells (Gonzalez et al 1990;Lefort & Gasol 2014).…”

“…Accordingly, DOC concentrations did not exceed 95 µmol L -1 . More information on the hydrological features and DOC dynamics of the study site can be found in Calleja et al (2019). In the nearby shallow waters of KAUST Harbor, although conditions were still oligotrophic year-round, higher concentrations of DIN and DOC were occasionally observed (Silva et al 2019).…”

“…For heterotrophic prokaryotes, although our knowledge about their diversity is increasing (Ngugi et al 2012;Pearman et al 2017;Thompson et al 2017), only a few studies have investigated their vertical distribution in Red Sea waters (Calbet et al 2015;Qian et al 2011). A recent report using data collected from the same site as this study has shown that the abundance of heterotrophic bacteria can change temporally up to 3-fold within the same depth in the upper epipelagic (Calleja et al 2019;García et al 2018). Other studies conducted at that site have shown that LNA bacteria dominated in the epipelagic layer, while HNA cells were more abundant in the mesopelagic layer, indicating that each group seems to prefer different environmental conditions Calleja et al 2018).…”

Electron transport and electron density inside quasi-one-dimensional disordered conductors

“…In addition to studies of the conductance and transmission, which refer to quantities evaluated outside the system, the statistics of transport inside random systems has also been studied for many years [12][13][14][15][16]. Just as the conductance problem, the problem of electron transport inside random systems is also of interest in various other branches of physics, as it is representative of a more general wave-scattering problem in a quasi-one-dimensional (q1D) disordered system: e.g., an electromagnetic wave traveling in a disordered waveguide [17] the interest being in the energy density inside the structure-, or an elastic wave propagating in a disordered elastic waveguide [18,19].…”