Nitrogen loss processes in response to upwelling in a Peruvian coastal setting dominated by denitrification – a mesocosm approach

Schulz, Kai G.; Achterberg, Eric P.; Arístegui, Javier; Bach, Lennart T.; Baños, Isabel; Boxhammer, Tim; Erler, Dirk; Igarza, Maricarmen; Kalter, Verena; Ludwig, Andrea; Löscher, Carolin; Meyer, Jana; Meyer, Judith; Minutolo, Fabrizio; von der Esch, Elisabeth; Ward, Bess B.; Riebesell, Ulf

Upwelling of nutrient-rich deep waters make eastern boundary upwelling systems (EBUSs), such as the Humboldt Current system, hot spots of marine productivity. Associated settling of organic matter to depth and consecutive aerobic decomposition results in large subsurface water volumes being oxygen depleted. Under these circumstances, organic matter remineralisation can continue via denitrification, which represents a major loss pathway for bioavailable nitrogen. Additionally, anaerobic ammonium oxidation can remove significant amounts of nitrogen in these areas. Here we assess the interplay of suboxic water upwelling and nitrogen cycling in a manipulative offshore mesocosm experiment. Measured denitrification rates in incubations with water from the oxygen-depleted bottom layer of the mesocosms (via inline-formula15N label incubations) mostly ranged between 5.5 and 20 nmol Ninline-formula2 Linline-formula−1 hinline-formula−1 (interquartile range), reaching up to 80 nmol Ninline-formula2 Linline-formula−1 hinline-formula−1. However, actual in situ rates in the mesocosms, estimated via Michaelis–Menten kinetic scaling, did most likely not exceed 0.2–4.2 nmol Ninline-formula2 Linline-formula−1 hinline-formula−1 (interquartile range) due to substrate limitation. In the surrounding Pacific, measured denitrification rates were similar, although indications of substrate limitation were detected only once. In contrast, anammox (anaerobic ammonium oxidation) made only a minor contribution to the overall nitrogen loss when encountered in both the mesocosms and the Pacific Ocean. This was potentially related to organic matter C inline-formula M11inlinescrollmathml / 8pt14ptsvg-formulamathimg165b352473919034209a9d51d0eaf41d bg-18-4305-2021-ie00001.svg8pt14ptbg-18-4305-2021-ie00001.png  N stoichiometry and/or process-specific oxygen and hydrogen sulfide sensitivities. Over the first 38 d of the experiment, total nitrogen loss calculated from in situ rates of denitrification and anammox was comparable to estimates from a full nitrogen budget in the mesocosms and ranged between inline-formula∼ 1 and 5.5 inline-formulaµmol N Linline-formula−1. This represents up to inline-formula∼  20 % of the initially bioavailable inorganic and organic nitrogen standing stocks. Interestingly, this loss is comparable to the total amount of particulate organic nitrogen that was exported into the sediment traps at the bottom of the mesocosms at about 20 mpage4306 depth. Altogether, this suggests that a significant portion, if not the majority of nitrogen that could be exported to depth, is already lost, i.e. converted to Ninline-formula2 in a relatively shallow layer of the surface ocean, provided that there are oxygen-deficient conditions like those during coastal upwelling in our study. Published data for primary productivity and nitrogen loss in all EBUSs reinforce such conclusion.

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Schulz, Kai G. / Achterberg, Eric P. / Arístegui, Javier / et al: Nitrogen loss processes in response to upwelling in a Peruvian coastal setting dominated by denitrification – a mesocosm approach. 2021. Copernicus Publications.

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