Description: 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 15N label incubations) mostly ranged between 5.5 and 20 nmol N2 L−1 h−1 (interquartile range), reaching up to 80 nmol N2 L−1 h−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 N2 L−1 h−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 / 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 ∼ 1 and 5.5 µmol N L−1. This represents up to ∼  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 m 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 N2 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.

Description: Oxygen minimum zones (OMZs) in the ocean are characterized by enhanced carbon dioxide (CO2) levels and are being further acidified by increasing anthropogenic atmospheric CO2. To investigate how on-going ocean deoxygenation will impact biogeochemical processes, a large-scale mesocosm experiment was conducted offshore Peru in austral summer (Feb-Apr) 2017, coinciding with a rare coastal El Niño event. We deployed eight mesocosms, each with a volume of 55 m3 and a length of 19 m, at the surface water in the coastal area of Callao (12.06° S, 77.23° W). The mesocosm bags were filled by surrounding surface water with daily or every-2nd-day nutrient and CO2 measurements for 10 days to monitor the initial conditions. Deep water masses from two different locations in the nearby OMZs were collected (at a depth of 30 and 70 m, respectively) and added to the mesocosms to simulate upwelling events on day 13 (see Bach et al., 2020 for details). Here we report every-2nd-day measurements of carbonate chemistry parameters in the individual mesocosms and the surrounding Pacific waters over 50 days. Depth-integrated seawater samples were taken from the surface (0-10 m for day 3-28; 0-12.5 m for day 29-50) and bottom layer (10-17 m for day 3-28; 12.5-17 m for day 29-50) of the mesocosms and the surrounding coastal water (named “Pacific”) using a 5-L integrating water sampler. Total alkalinity (TA) was measured by a two-stage open-cell potentiometric titration using a Metrohm 862 Compact Titrosampler, Aquatrode Plus (Pt1000) and a 907 Titrando unit, and pH (total scale) was measured spectrophotometrically by measuring the absorbance ratios after adding the indicator dye m-cresol purple (mCP) on a Varian-Cary 100 double-beam spectrophotometer (Varian). With inputs of the measured TA and pH, other CO2 parameters, such as dissolved inorganic carbon, pCO2, calcite and aragonite saturation state, and CO2 fluxes (FCO2), were calculated using the Excel version of CO2SYS. The performance of pH and TA measurements were also evaluated by examining the standard deviations and range controls of triplicate measurements of samples or reference materials. Our observations showed an acidification of surface water in the mesocosms by the OMZ water addition, followed by a rapid drop in pCO2 to near or below the atmospheric level due to enhance phytoplankton production. The positive CO2 fluxes in the surrounding Pacific waters indicated our study site was a local CO2 source during our study. Nevertheless, our mesocosm experiment suggests this CO2 export to the atmosphere can be largely dampened by biological processes. As a unique dataset that characterized near-shore carbonate chemistry with a high temporal resolution during a rare coastal El Niño event, our study gives important insights into the carbonate chemistry responses to extreme climate events in the Peruvian upwelling system.