Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

Alexandra N Loginova,Anja Engel,David Clemens,Klaus Wallmann,Andrew W Dale,Stefan Sommer,Sören Thomsen,Frédéric A C Le Moigne

doi:10.5194/bg-17-4663-2020

Abstract

Abstract. The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterised by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilised by microbial communities, and the degradation of POM is associated with the production and reworking of dissolved organic matter (DOM). The release of DOM to the overlying waters may, therefore, represent an important organic matter escape mechanism from remineralisation within sediments but has received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion-driven fluxes and net in situ fluxes of DOC and DON to investigate processes affecting DOM cycling at the sediment–water interface along a transect at 12∘ S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore water DOC accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralisation within sediments. High DON accumulation resulted in very low pore water DOC ∕ DON ratios (≤1) which could be caused by an “uncoupling” in DOC and DON remineralisation. Diffusion-driven fluxes of DOC and DON exhibited high spatial variability and ranged from 0.2±0.1 to 2.5±1.3 mmolm-2d-1 and from -0.04±0.02 to 3.3±1.7 mmolm-2d-1, respectively. Generally low net in situ DOC and DON fluxes, as well as a steepening of spectral inclination (S) of CDOM and an increase in humic-like DOM at the sediment–water interface over time, indicated active microbial DOM utilisation. The latter may potentially be stimulated by the presence of nitrate (NO3-) and nitrite (NO2-) in the water column. The microbial DOC utilisation rates, estimated in our study, are potentially sufficient to support denitrification rates of 0.2–1.4 mmolm-2d-1, suggesting that the sediment release of DOM may on occasion contribute to nitrogen loss processes in the ETSP off Peru.

Highlights

The eastern tropical South Pacific (ETSP) is one of the most productive areas of the world ocean (Pennington et al, 2006)
Pore water dissolved organic carbon (DOC) concentrations and gradients decreased gradually towards station 4 (St. 4), where DOC concentrations ranged from 122 μmol L−1 at 0.5 cm to 544 μmol L−1 at 22.5 cm of sediment depth
Pore water DOC concentrations and gradients increased at station 5 (St. 5) and station 6 (St. 6), ranging from 177 μmol L−1 at 0.5 cm to 823 μmol L−1 at 22.5 cm and from 210 μmol L−1 at 1.5 cm to 702 μmol L−1 at 19.5 cm, respectively

Summary

Introduction

The eastern tropical South Pacific (ETSP) is one of the most productive areas of the world ocean (Pennington et al, 2006). Loginova et al.: DOM sediment Peru limited by the absence of oxygen (Demaison and Moore, 1980), recent studies report similar abilities of marine microbes to degrade organic matter in oxygenated surface waters and within OMZs (Pantoja et al, 2009; Maßmig et al, 2019, 2020), suggesting that other factors such as the quality of organic matter may regulate microbial activity within OMZs (Pantoja et al, 2009; Le Moigne et al, 2017). Similar to water column studies, extensive fieldwork campaigns conducted on sediments off Peru suggested intensive particulate organic matter (POM) remineralisation under full anoxia (Dale et al, 2015)

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