Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

Abstract

Abstract. Recent modeling results suggest that oceanic oxygen levels will decrease significantly over the next decades to centuries in response to climate change and altered ocean circulation. Hence, the future ocean may experience major shifts in nutrient cycling triggered by the expansion and intensification of tropical oxygen minimum zones (OMZs), which are connected to the most productive upwelling systems in the ocean. There are numerous feedbacks among oxygen concentrations, nutrient cycling and biological productivity; however, existing knowledge is insufficient to understand physical, chemical and biological interactions in order to adequately assess past and potential future changes.In the following, we summarize one decade of research performed in the framework of the Collaborative Research Center 754 (SFB754) focusing on climate-biogeochemistry interactions in tropical OMZs. We investigated the influence of low environmental oxygen conditions on biogeochemical cycles, organic matter formation and remineralization, greenhouse gas production and the ecology in OMZ regions of the eastern tropical South Pacific compared to the weaker OMZ of the eastern tropical North Atlantic. Based on our findings, a coupling of primary production and organic matter export via the nitrogen cycle is proposed, which may, however, be impacted by several additional factors, e.g., micronutrients, particles acting as microniches, vertical and horizontal transport of organic material and the role of zooplankton and viruses therein.

Particle export fluxes to the oxygen minimum zone of the eastern tropical North Atlantic

“…Our data on PHAA do not suggest preferential amino acid loss due to components of sinking POM degradation in the ETNA OMZ. This is in accordance with the absence of microbial N-loss processes and absence of denitrifying bacteria in ETNA oxygen-deficient waters (Löscher et al, 2016). Instead, a slight increase of [PHAA-C] : [POC] in the OMZ may point to higher protein production by bacterial growth as previously observed for mesopelagic waters Cronin, 1982, 1984) and may be related to increased growth efficiency of bacteria experiencing low-oxygen condition as suggested by Keil et al (2016).…”

Microstructure and composition of marine aggregates as co-determinants for vertical particulate organic carbon transfer in the global ocean

“…The higher NPP in M 4 AGO is due to the enhanced remineralization rates in surface waters, which also lead to the lower export efficiencies in the equatorial and subtropical regions. Estimates of the export efficiency from satellite data, in situ observations or models lead to partly contrasting patterns (e.g., Lutz et al, 2002;DeVries and Weber, 2017;Henson et al, 2011Henson et al, , 2012Buesseler, 1998;Neuer et al, 2002;Siegel et al, 2014;Cram et al, 2018). In contrast to our two model runs, the highest p ratios are suggested to be found in the North Pacific and Antarctic Ocean (e.g., Dunne et al, 2005;Henson et al, 2011;DeVries and Weber, 2017).…”

N2O Emissions From the Northern Benguela Upwelling System

“…Recently, Buitenhuis et al (2018) suggested the contribution of coastal oceans to be much lower than assumed by Nevison et al (2004;2.5 Tg-N per year). Löscher et al (2016) reported extreme N 2 O concentrations at the fringes of bottom H 2 S-containing waters on the Peruvian shelf. Furthermore, our estimate is conservative since our measurements do not cover the southern Benguela region (Lamont et al, 2015) where low-O 2 waters and upwelling potentially foster high N 2 O production and fluxes to the atmosphere.…”

“…Furthermore, our estimate is conservative since our measurements do not cover the southern Benguela region (Lamont et al, 2015) where low-O 2 waters and upwelling potentially foster high N 2 O production and fluxes to the atmosphere. Löscher et al (2016) reported extreme N 2 O concentrations at the fringes of bottom H 2 S-containing waters on the Peruvian shelf. Given the widespread occurrence of sulphidic events in the BUS (Ohde & Dadou, 2018), it could be that such conditions enhance surface fluxes of N 2 O as bottom waters are advected/mixed toward the surface.…”