Dataset: Gulf of Mexico diel ammonium uptake

We conducted short term diel NH4+ uptake experiments in deckboard incubators.  Each incubator was uniformly shaded using clear or blue-tinted acrylic sheets to achieve three light levels as determined by simultaneously using a 2-π LI-COR photosynthetically active radiation (PAR) sensor to measure downwelling irradiance and a 4-π water-proof LI-COR PAR sensor to measure ambient irradiance within the incubator.  These calibrations were done on the ship to account for light reflection or ship shading. Incubation light levels for the NF17 cruise were determined to be 145% (clear, surface), 79% (mixed layer), and 21% (lower mixed layer) of surface irradiance.  For the NF1802 cruise, the clear incubator was replaced with one screened to 1.7% surface irradiance to mimic deep chlorophyll maximum light. All incubators were cooled with mixed-layer seawater. Samples were drawn from depths near these light levels (as determined from noon CTD casts with CTD-mounted PAR sensor).  

Nine to twelve 2.7-L bottles were filled at dusk. Two or three “control” bottles (24-h incubation) and an additional “time point” bottle (4-h incubation) were immediately spiked with 15NH4+ (the remainder of the bottles were not immediately spiked). All bottles were then placed in a single deckboard incubator.  After ~4 h, the first experimental bottle was removed from the incubator and filtered as described above and a second experimental bottle was spiked.  This process continued for 24h to produce six sequential 4-h incubations from which diurnal patterns of nutrient uptake could be determined.  At the end of 24h, the control bottles were also removed and filtered.  At the end of the incubation bottles were immediately vacuum filtered onto pre-combusted 25-mm GF/F filters in the dark. Filters were rinsed with filtered seawater, wrapped in foil and stored at  80°C. On land, samples were fumigated with HCl vapor to remove inorganic carbon, dried, and placed inside a tin cup for C/N and isotopic analysis at the UC Davis stable isotope facility. NH4+ uptake rates in each incubation bottle (and associated uncertainties) were determined using equations in Stukel (2020).  For additional details see Yingling et al. (submitted).