ALBERT

Description: Anaerobic oxidation of ammonium (anammox) in oxygen minimum zones (OMZs) is a major pathway of oceanic nitrogen loss. Ammonium released from sinking particles has been suggested to fuel this process. During cruises to the Peruvian OMZ in April–June 2017 we found that anammox rates are strongly correlated with the volume of small particles (128–512 µm), even though anammox bacteria were not directly associated with particles. This suggests that the relationship between anammox rates and particles is related to the ammonium released from particles by remineralization. To investigate this, ammonium release from particles was modelled and theoretical encounters of free-living anammox bacteria with ammonium in the particle boundary layer were calculated. These results indicated that small sinking particles could be responsible for ~75% of ammonium release in anoxic waters and that free-living anammox bacteria frequently encounter ammonium in the vicinity of smaller particles. This indicates a so far underestimated role of abundant, slow-sinking small particles in controlling oceanic nutrient budgets, and furthermore implies that observations of the volume of small particles could be used to estimate N-loss across large areas.

Description: Filaments and fronts play a crucial role for a net offshore and downward nutrient transport in Eastern Boundary Upwelling Regions (EBUS) and thereby reduce primary production. Often studies are either based on observations or model simulations but seldom both approaches are combined quantitatively to assess the importance of filaments for primary production and nutrient transport. Here we combine targeted interdisciplinary shipboard observations of a cold filament off Peru with submesoscale-permitting (1/45°) coupled physical (CROCO) and biogeochemical (PISCES) model simulations to (i) evaluate the model simulations in detail, including the timescales of biogeochemical modification of the newly upwelled water and (ii) quantify the net effect of submesoscale fronts and filaments on primary production of the Peruvian upwelling system. The observed filament contains relatively cold, fresh and nutrient-rich waters originating in the coastal upwelling. Enhanced nitrate concentrations and offshore velocities of up to 0.5 m s−1 within the filament suggest an offshore transport of nutrients. Surface chlorophyll in the filament is a factor 4 lower than at the upwelling front while surface primary production is a factor 2 higher, highlighting the additional value of direct rate measurements for model validation. The simulation exhibits filaments that are similar in horizontal and vertical scale compared to the observed filament. Nitrate concentrations and primary pro- duction within filaments in the model are comparable to observations as well, justifying further analysis of nitrate uptake and subduction using the model. Virtual Lagrangian floats were released in the subsurface waters along the shelf and biogeochemical variables tracked along the trajectories of floats upwelled near the coast. In the submesoscale-permitting (1/45°) simulation 43.0 % of upwelled floats and 40.6 % of upwelled nitrate is subducted within 20 days after upwelling, which corresponds to an increase of nitrate subduction compared to a mesoscale-resolving (1/9°) simulation by 13.9 %. This suggests that submesoscale processes further reduce primary production by amplifying the downward and offshore export of nutrients found in previous mesoscale studies, which are thus likely to underestimate the reduction in primary production due to eddy-fluxes. Moreover, this downward and offshore transport could also enhance the export of fresh organic matter below the photic zone and thereby potentially stimulate microbial activity in the upper offshore oxygen minimum zone.

Description: We collected sediment cores from the Janssand intertidal sand flat in Germany in April 2016, August 2016, and February 2017. All measurements were made in laboratory experiments using NOx (nitrate + nitrite) microbiosensors and oxygen microelectrodes. Cores were kept at in situ temperatures with a simulated day/night cycle and regular flushing with site seawater to simulate natural porewater advection during periods of inundation. We determined the depth of maximum microphytobenthos photosynthetic activity from steady-state oxygen profiles. We then flushed site seawater down through the core to measure net NOx and oxygen consumption rates at that depth. We measured potential denitrification rates at high spatial resolution by flushing acetylene- and nitrate-amended site seawater down through the core and measuring the accumulation of nitrous oxide using a microelectrode. Finally, we applied a range of realistic downward advective flows of seawater into sediment, in both the light and the dark, in both the spring and summer. We measured the depth to which NOx and oxygen penetrated over the course of several hours under these steady flow conditions using NOx microbiosensors and oxygen microelectrodes. from our study site, NOx always reached the depths of maximum denitrification potential, regardless of light availability or season.

Description: Multibeam bathymetry raw data was recorded in the Atlantic during cruise MSM89 that took place between 2020-01-17 and 2020-02-20. The data was collected using the ship's own Kongsberg EM 122. This data is part of the DAM (German Marine Research Alliance) underway research data project.