Diapycnal dissolved organic matter supply into the upper Peruvian oxycline

Alexandra N Loginova,Marcus Dengler,Anja Engel,Sören Thomsen,Jan Lüdke

doi:10.5194/bg-16-2033-2019

Abstract

Abstract. The eastern tropical South Pacific (ETSP) hosts the Peruvian upwelling system, which represents one of the most productive areas in the world ocean. High primary production followed by rapid heterotrophic utilization of organic matter supports the formation of one of the most intense oxygen minimum zones (OMZs) in the world ocean, where dissolved oxygen (O2) concentrations reach less than 1 µmol kg−1. The high productivity leads to an accumulation of dissolved organic matter (DOM) in the surface layers that may serve as a substrate for heterotrophic respiration. However, the importance of DOM utilization for O2 respiration in the Peruvian upwelling system in general and for shaping the upper oxycline in particular remains unclear so far. This study reports the first estimates of diapycnal fluxes and supply of O2, dissolved organic carbon (DOC), dissolved organic nitrogen, dissolved hydrolysable amino acids (DHAA) and dissolved combined carbohydrates (DCCHO) for the ETSP off Peru. Diapycnal flux and supply estimates were obtained by combining measured vertical diffusivities and solute concentration gradients. They were analysed together with the molecular composition of DCCHO and DHAA to infer the transport of labile DOM into the upper OMZ and the potential role of DOM utilization for the attenuation of the diapycnal O2 flux that ventilates the OMZ. The observed diapycnal O2 flux (50 mmol O2 m−2 d−1 at maximum) was limited to the upper 80 m of the water column; the O2 supply of ∼1 µmol kg−1 d−1 was comparable to previously published O2 consumption rates for the North and South Pacific OMZs. The diapycnal DOM flux (31 mmol C m−2 d−1 at maximum) was limited to ∼30 m water depth, suggesting that the labile DOM is extensively consumed within the upper part of the shallow oxycline off Peru. The analyses of DCCHO and DHAA composition support this finding, suggesting that DOM undergoes comprehensive remineralization within the upper part of the oxycline, as the DOM within the core of the OMZ was found to be largely altered. Estimated by a simple equation for carbon combustion, aerobic respiration of DCCHO and DHAA, supplied by diapycnal mixing (0.46 µmol kg−1 d−1 at maximum), could account for up to 38 % of the diapycnal O2 supply in the upper oxycline, which suggests that DOM utilization plays a significant role for shaping the upper oxycline in the ETSP.

Highlights

Dissolved oxygen (O2) plays a key role for biological production and cycling of elements in marine ecosystems as well as for the spatial distribution of marine organisms (Ekau et al, 2010; Gilly et al, 2013)
The core of the Peruvian oxygen minimum zones (OMZs) is considered to be fully anoxic (e.g. Ulloa et al, 2012), as O2 concentrations below the detection limit (DL) of ∼ 0.01 μmol kg−1 were observed between 20 and 400 m depth by high-precision STOX sensor measurements (Revsbech et al, 2009; Kalvelage et al, 2013; Thomsen et al, 2016a). Those low-O2 concentrations are due to a sluggish ventilation by ocean currents, carrying low-O2 waters to the eastern tropical South Pacific (ETSP), and microbial respiration attributed to utilization of organic matter (OM) originating from the upper water column
The main vertical dissolved organic carbon (DOC) gradient was found at shallow depth compared to the oxycline

Summary

Introduction

Dissolved oxygen (O2) plays a key role for biological production and cycling of elements in marine ecosystems as well as for the spatial distribution of marine organisms (Ekau et al, 2010; Gilly et al, 2013). Ulloa et al, 2012), as O2 concentrations below the detection limit (DL) of ∼ 0.01 μmol kg−1 were observed between 20 and 400 m depth by high-precision STOX sensor measurements (Revsbech et al, 2009; Kalvelage et al, 2013; Thomsen et al, 2016a). Those low-O2 concentrations are due to a sluggish ventilation by ocean currents, carrying low-O2 waters to the ETSP, and microbial respiration attributed to utilization of organic matter (OM) originating from the upper water column. Loginova et al.: DOM supply into the oxycline off Peru (e.g. Czeschel et al, 2011; Brandt et al, 2015; Kalvelage et al, 2015)

Methods

Results

Discussion

Conclusion