In situ fluxes of solutes were measured in benthic chambers along an E-W transect at 18.1 oN using autonomous Biogeochemical Observatories (BIGO). A total of nine BIGO deployments were made at nine stations along a latitudinal transect across the shelf/slope covering a horizontal distance of 50 km from ~50 to ~1110 m water depth. The design and implementation of the BIGO landers has been discussed in detail previously (Krahmann et al. 2021). Each BIGO (I and II) contained two circular flux chambers (K1 and K2) with an internal diameter of 28.8 cm where the sediment was incubated for ~30 h. Discrete samples were taken using glass syringes (eight per chamber) at pre-programmed time intervals for chemical analysis. After BIGO recovery, the syringes were immediately transferred to the onboard cool room (4°C) for filtering (0.2 µm) and sub-sampling. Benthic lander samples were analyzed for the isotopic composition (in per mil units, ‰) of nitrate (δ15N-NO3- and δ18O-NO3-) and ammonium (δ15N-NH4+). Most of the samples were analyzed for δ15N-NO3- and δ18O-NO3-. Nitrate dual isotopes were analyzed using the denitrifier method Samples for δ15N-NH4+ were analyzed using the hypobromite/azide-method where NH4+ concentrations were above the detection limit for a reliable isotope analysis (1 µM). In the denitrifier method, NO3- and NO2- are quantitatively converted to N2O by Pseudomonas aureofaciens (ATTC 13985). The hypobromite/azide-method is based on the chemical conversion of NH4+ to N2O by a subsequent addition of a hypobromite and azide solution. For both methods, the sample volume was adjusted to a sample size of 10 nmol of N2O. N2O was extracted from the sample vials by purging with helium and measured with a GasBench II, coupled to an isotope ratio mass spectrometer (Delta Plus, Thermo Fisher Scientific, Germany). All samples were measured with international standards. NO2- comprised on average 2 % of the combined NO2- + NO3- pool and the contribution of NO2- interference to the reported δ15N and δ18O nitrate values was considered negligible. N and O isotope ratios are reported in per mil (‰), relative to the analytical standards (N2 in air for δ15N, and Vienna Standard Mean Ocean Water (VSMOW) for O).
