Diapycnal dissolved organic matter supply into the upper Peruvian oxycline

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

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 (inline-formulaO2) concentrations reach less than 1 inline-formulaµmol kginline-formula−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 inline-formulaO2 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 inline-formulaO2, 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 inline-formulaO2 flux that ventilates the OMZ. The observed diapycnal inline-formulaO2 flux (50 mmol inline-formulaO2 minline-formula−2 dinline-formula−1 at maximum) was limited to the upper 80 m of the water column; the inline-formulaO2 supply of inline-formula∼1inline-formulaµmol kginline-formula−1 dinline-formula−1 was comparable to previously published inline-formulaO2 consumption rates for the North and South Pacific OMZs. The diapycnal DOM flux (31 mmol C minline-formula−2 dinline-formula−1 at maximum) was limited to inline-formula∼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 inline-formulaµmol kginline-formula−1 dinline-formula−1 at maximum), could account for up to 38 % of the diapycnal inline-formulaO2 supply in the upper oxycline, which suggests that DOM utilization plays a significant role for shaping the upper oxycline in the ETSP.



Loginova, Alexandra N. / Thomsen, Sören / Dengler, Marcus / et al: Diapycnal dissolved organic matter supply into the upper Peruvian oxycline. 2019. Copernicus Publications.


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