Diapycnal dissolved organic matter supply into the upper Peruvian oxycline

Alexandra N. Loginova, Sören Thomsen, Marcus Dengler, Jan Lüdke, Anja Engel
2018 Biogeosciences Discussions  
<p><strong>Abstract.</strong> 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 (OMZ) in the world ocean where dissolved oxygen (O<sub>2</sub>) concentrations reach well below 1<span class="thinspace"></span>µmol<span
more » ... "thinspace"></span>kg<sup>&amp;minus;1</sup>. 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 O<sub>2</sub> respiration within the Peruvian OMZ remains unclear so far. Here, we evaluate the diapycnal fluxes of O<sub>2</sub>, dissolved organic carbon (DOC), dissolved organic nitrogen, dissolved hydrolysable amino acids (DHAA) and dissolved combined carbohydrates (DCCHO) and the composition of DOM in the ETSP off Peru to learn, whether labile DOM is reaching into the core of the OMZ and how important DOM utilization might be for O<sub>2</sub> attenuation. The observed diapycnal <sub>2</sub> flux (50<span class="thinspace"></span>mmol<span class="thinspace"></span>O<sub>2</sub><span class="thinspace"></span>m<sup>&amp;minus;2</sup><span class="thinspace"></span>day<sup>&amp;minus;1</sup> at max) was limited to the upper 80<span class="thinspace"></span>m of the water column, the flux attenuation of ~1<span class="thinspace"></span>µmol<span class="thinspace"></span>L<sup>&amp;minus;1</sup>day<sup>&amp;minus;1</sup>, was comparable to previously published O<sub>2</sub> consumption rates for the North and South Pacific OMZs. The diapycnal DOM flux (31<span class="thinspace"></span>mmol<span class="thinspace"></span>C<span class="thinspace"></span>m<sup>&amp;minus;2</sup><span class="thinspace"></span>day<sup>&amp;minus;1</sup> at max) was limited to ~30<span class="thinspace"></span>m water depth, suggesting that the labile DOM is already utilized 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 already 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<span class="thinspace"></span>µmol<span class="thinspace"></span>L<sup>&amp;minus;1</sup><span class="thinspace"></span>day<sup>&amp;minus;1</sup> at max), could account for up to 38<span class="thinspace"></span>% of the diapycnal O<sub>2</sub> supply in the upper oxycline, which suggests that DOM utilization may play a significant role for shape of the upper Peruvian oxycline.</p>
doi:10.5194/bg-2018-284 fatcat:ltvtdnumefezfdpffjqjpnunxi