Jan Lüdke

The intraseasonal evolution of physical and biogeochemical properties during a coastal trapped wave event off central Peru is analysed using data from an extensive shipboard observational programme conducted between April and June 2017, and remote sensing data. The poleward velocities in the Peru–Chile Undercurrent were highly variable and strongly intensified to above 0.5 m s⁻¹ between the middle and end of May. This intensification was likely caused by a first-baroclinic-mode downwelling coastal trapped wave, excited by a westerly wind anomaly at the Equator and originating at about 95^∘ W. Local winds along the South American coast did not impact the wave. Although there is general agreement between the observed cross-shore-depth velocity structure of the coastal trapped wave and the velocity structure of first vertical mode solution of a linear wave model, there are differences in the details of the two flow distributions. The enhanced poleward flow increased water mass advection from the equatorial current system to the study site. The resulting shorter alongshore transit times between the Equator and the coast off central Peru led to a strong increase in nitrate concentrations, less anoxic water, likely less fixed nitrogen loss to N₂ and a decrease of the nitrogen deficit compared to the situation before the poleward flow intensification. This study highlights the role of changes in the alongshore advection due to coastal trapped waves for the nutrient budget and the cumulative strength of N cycling in the Peruvian oxygen minimum zone. Enhanced availability of nitrate may impact a range of pelagic and benthic elemental cycles, as it represents a major electron acceptor for organic carbon degradation during denitrification and is involved in sulfide oxidation in sediments.

The eastern boundary region of the southeastern Pacific Ocean hosts one of the world's most dynamic and productive upwelling systems with an associated oxygen minimum zone (OMZ). The variability in downward export fluxes in this region, with strongly varying surface productivity, upwelling intensities and water column oxygen content, is however poorly understood. Thorium-234 (²³⁴Th) is a powerful tracer to study the dynamics of export fluxes of carbon and other elements, yet intense advection and diffusion in nearshore environments impact the assessment of depth-integrated ²³⁴Th fluxes when not properly evaluated. Here we use vessel-mounted acoustic Doppler current profiler (VmADCP) current velocities, satellite wind speed and in situ microstructure measurements to determine the magnitude of advective and diffusive fluxes over the entire ²³⁴Th flux budget at 25 stations from 11 to 16^∘ S in the Peruvian OMZ. Contrary to findings along the GEOTRACES P16 eastern section, our results showed that weak surface wind speed during our cruises induced low upwelling rates and minimal upwelled ²³⁴Th fluxes, whereas vertical diffusive ²³⁴Th fluxes were important only at a few shallow shelf stations. Horizontal advective and diffusive ²³⁴Th fluxes were negligible because of small alongshore ²³⁴Th gradients. Our data indicated a poor correlation between seawater ²³⁸U activity and salinity. Assuming a linear relationship between the two would lead to significant underestimations of the total ²³⁴Th flux by up to 40 % in our study. Proper evaluation of both physical transport and variability in ²³⁸U activity is thus crucial in coastal ²³⁴Th flux studies. Finally, we showed large temporal variations on ²³⁴Th residence times across the Peruvian upwelling zone and cautioned future carbon export studies to take these temporal variabilities into consideration while evaluating carbon export efficiency.

In the framework of the Peruvian Oxygen minimum zone System Tracer Release Experiment (POSTRE) about 70 kg of trifluoromethyl sulfur pentafluoride (SF5CF3) was injected into the bottom boundary layer of the upper Peruvian continental slope at 250m depth in October 2015. Three different injection sites, at 10°45’S, 12°20’S and 14°S were selected. At the tracer release sites and due to tide-topography interaction, mixing above the upper continental slope of Peru was intensified. Turbulent dissipation rates increase by about an order of magnitude in lower 50 to 100m above the bottom. During previous tracer release experiments, where tracer was injected into the stratified mixing layer above the bottom boundary layer, a change of the center of mass toward higher densities resulted. Newer theories suggest that this diapycnal downwelling is balanced by a diapycnal upwelling within the bottom boundary layer. Indeed, during the tracer survey it was found that the density of tracer’s center of mass had decreased by 0.13 kg m-3. This corresponds to an upward displacement of 70-100m. Using microsctructure shear data from 8 cruises, we obtain a diapycnal velocity of about 0.5 m day-1 within the bottom boundary layer. This suggests that on average, the tracer was trapped within the bottom boundary layer for a period between 1.5 and 3 month. Overall, our tracer study provides the first observational evidence of diapycnal upwelling occurring within the bottom boundary layer of a bottom enhanced mixing environment and supports recent ideas of a vigorous global overturning circulation.

The eastern boundary region of the southeast Pacific Ocean hosts one of the world's most dynamic and productive upwelling systems with an associated oxygen minimum zone (OMZ). The variability in downward export fluxes in this region, with strongly varying surface productivity, upwelling intensities and water column oxygen content, is however poorly understood. Thorium-234 (²³⁴Th) is a powerful tracer to study the dynamics of export fluxes of carbon and other elements, yet intense advection and diffusion in nearshore environments impact the assessment of depth-integrated ²³⁴Th fluxes when not properly evaluated. Here we use ADCP current velocities, satellite wind stress and in situ microstructure measurements to determine the magnitude of advective and diffusive fluxes over the entire ²³⁴Th flux budget at 25 stations from 11° S to 16° S in the Peruvian OMZ. Contrary to findings along the GEOTRACES P16 eastern section, our results showed that weak surface wind stress during our cruises induced low upwelling rates and minimal upwelling ²³⁴Th fluxes, whereas vertical diffusive ²³⁴Th fluxes were important only at a few shallow shelf stations. Similarly, horizontal advective and diffusive ²³⁴Th fluxes were negligible due to small alongshore ²³⁴Th gradients. Our data indicated a poor correlation between seawater ²³⁸U activity and salinity. Assuming a linear relationship between the two would lead to significant underestimations of the total ²³⁴Th flux by up to 40 % in our study. Proper evaluation of both physical transport and variability in ²³⁸U activity is thus crucial in coastal ²³⁴Th flux studies. Finally, we showed large temporal variations on ²³⁴Th residence times across the Peruvian upwelling zone, and cautioned future carbon export studies to take these temporal changes into consideration while evaluating carbon export efficiency.

Oxygen minimum zones (OMZs) show distinct biogeochemical processes that relate to microorganisms being able to thrive under low or even absent oxygen. Microbial degradation of organic matter is expected to be reduced in OMZs, although quantitative evidence is low. Here, we present heterotrophic bacterial production (³H leucine incorporation), extracellular enzyme rates (leucine aminopeptidase/β-glucosidase) and bacterial cell abundance for various in situ oxygen concentrations in the water column, including the upper and lower oxycline, of the eastern tropical South Pacific off Peru. Bacterial heterotrophic activity in the suboxic core of the OMZ (at in situ ≤ 5 µmol O₂ kg⁻¹) ranged from 0.3 to 281 µmol C m⁻³ d⁻¹ and was not significantly lower than in waters of 5–60 µmol O₂ kg⁻¹. Moreover, bacterial abundance in the OMZ and leucine aminopeptidase activity were significantly higher in suboxic waters compared to waters of 5–60 µmol O₂ kg⁻¹, suggesting no impairment of bacterial organic-matter degradation in the core of the OMZ. Nevertheless, high cell-specific bacterial production was observed in samples from oxyclines, and cell-specific extracellular enzyme rates were especially high at the lower oxycline, corroborating earlier findings of highly active and distinct micro-aerobic bacterial communities. To assess the impact of bacterial degradation of dissolved organic matter (DOM) for oxygen loss in the Peruvian OMZ, we compared diapycnal fluxes of oxygen and dissolved organic carbon (DOC) and their microbial uptake within the upper 60 m of the water column. Our data indicate low bacterial growth efficiencies of 1 %–21 % at the upper oxycline, resulting in a high bacterial oxygen demand that can explain up to 33 % of the observed average oxygen loss over depth. Our study therewith shows that microbial degradation of DOM has a considerable share in sustaining the OMZ off Peru.

The availability of the micronutrient iron (Fe) in surface waters determines primary production, N₂ fixation, and microbial community structure in large parts of the world's ocean, and thus it plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g., Fe, manganese (Mn), and cobalt (Co)), with shelf sediments typically forming a key source. Over the last 5 decades, an expansion and intensification of OMZs has been observed and this trend is likely to proceed. However, it is unclear how trace-metal (TM) distributions and transport are influenced by decreasing oxygen (O₂) concentrations. Here we present dissolved (d; 0.2 µm) TM data collected at seven stations along a 50 km transect in the Mauritanian shelf region. We observed enhanced concentrations of Fe, Co, and Mn corresponding with low O₂ concentrations (−1), which were decoupled from major nutrients and nutrient-like and scavenged TMs (cadmium (Cd), lead (Pb), nickel (Ni), and copper (Cu)). Additionally, data from repeated station occupations indicated a direct link between dissolved and leachable particulate Fe, Co, Mn, and O₂. An observed dFe (dissolved iron) decrease from 10 to 5 nmol L⁻¹ coincided with an O₂ increase from 30 to 50 µmol kg⁻¹ and with a concomitant decrease in turbidity. The changes in Fe (Co and Mn) were likely driven by variations in their release from sediment pore water, facilitated by lower O₂ concentrations and longer residence time of the water mass on the shelf. Variations in organic matter remineralization and lithogenic inputs (atmospheric deposition or sediment resuspension; assessed using Al as indicator for lithogenic inputs) only played a minor role in redox-sensitive TM variability. Vertical dFe fluxes from O₂-depleted subsurface-to-surface waters (0.08–13.5 µmol m⁻² d⁻¹) driven by turbulent mixing and vertical advection were an order of magnitude larger than atmospheric deposition fluxes (0.63–1.43 µmol m⁻² d⁻¹; estimated using dAl inventories in the surface mixed layer) in the continental slope and shelf region. Benthic fluxes are therefore the dominant dFe supply to surface waters on the continental margins of the Mauritanian upwelling region. Overall, our results indicated that the projected future decrease in O₂ concentrations in OMZs may result in increases in Fe, Mn, and Co concentrations.

The Peruvian Upwelling System is characterized by high primary productivity fuelled by the supply of nutrients in a highly dynamic boundary circulation. The intraseasonal evolution of the physical and biogeochemical properties is analysed based on shipboard observations and remote sensing conducted between April and June 2017 off central Peru. The poleward transport in the subsurface Peru Chile Undercurrent was highly variable and strongly intensified between mid and end of May. This intensification was likely caused by a first baroclinic mode downwelling coastal trapped wave excited at the equator at about 95° W that propagated poleward along the South American coast. The intensified poleward flow shortens the time of water mass advection from the equatorial current system to the study site. The impact of the anomalous advection is mostly noticed in the nitrogen cycle because during the shorter time needed for poleward advection less fixed nitrogen loss occurs within the waters. This causes a strong increase of nitrate concentrations and a decrease in the nitrogen deficit. These changes suggest that the advection caused by the coastal trapped wave supersedes the simultaneous effect of anomalous downwelling in terms of nutrient response.