ALBERT

Description: We measured the vertical water column distribution of nitrous oxide (N2O) during the European Iron Fertilization Experiment (EIFEX) in the subpolar South Atlantic Ocean during February/March 2004 (R/V Polarstern cruise ANT XXI/3). Despite a huge build‐up and sedimentation of a phytoplankton bloom, a comparison of the N2O concentrations within the fertilized patch with concentrations measured outside the fertilized patch revealed no N2O accumulation within 33 days. This is in contrast to a previous study in the Southern Ocean, where enhanced N2O accumulation occurred in the pycnocline. Thus, we conclude that Fe fertilization does not necessarily trigger additional N2O formation and we caution that a predicted radiative offset due to a Fe‐induced additional release of oceanic N2O might be overestimated. Rapid sedimentation events during EIFEX might have hindered the build‐up of N2O and suggest, that not only the production of phytoplankton biomass but also its pathway in the water column needs to be considered if N2O radiative offset is modeled.

Description: H2O2 was measured in the upper water column (0–200 m) along a west-east transect through the Equatorial Atlantic as part of the German SOLAS (Surface Ocean Lower Atmosphere) cruise Meteor 55 (M55). Vertical profiles of H2O2 showed characteristic exponential decay consistent with light profiles and rainwater inputs. Integrated (0–100 m) water column H2O2 inventories ranged from 1.1–8.9 mmol m−2 with the highest values in the Amazon Plume. H2O2 inventories were also higher at the Equatorial Upwelling and after heavy rain showers in the region of the Inter Tropical Convergence Zone (ITCZ). Analysis of rain water samples collected during the cruise gave a volume weighted mean of 10.8 μmol L−1 (range 1.5–22.3 μmol L−1). This work highlights the importance of rainwater as a major source for H2O2 in the surface waters under the ITCZ.

Description: Nitrogen fixation supports new production in the oligotrophic oceans and removes dinitrogen and carbon dioxide from mixed layer waters. N‐fixation rates have been estimated in various ways but measurements are still too rare and factors limiting N‐fixation are not yet fully understood. Here we present data from a transect along 10°N through the tropical Atlantic on the Meteor Cruise 55 where N‐fixation rates between 3.7 and 255 μmol N*m−2*d−1 were recorded. The highest rates occurred off Africa in the eastern tropical North Atlantic (ETNA), and in the Amazon River plume in the West and contributed to 1–12.2% of the N‐demand of primary production. N‐fixation rates correlated with dissolved Fe concentrations, which were 20–280 times greater than the estimated demand. High atmospheric Fe inputs combined with the shallow nutricline make the ETNA a favourable environment for N‐fixers.

Description: The tropical oceans are a source of reactive bromine to the atmosphere in the form of short-lived brominated methanes as bromoform (CHBr3) and dibromomethane (CH2Br2). Elevated atmospheric concentrations above the tropical oceans are related to oceanic supersaturations of the compounds and especially to upwelling regimes. Although the sources of these brominated gases in the open ocean are not well understood, they have been habitually linked to phytoplankton, especially diatom abundance. Thus according to common assumptions, high concentrations of the brominated trace gases were expected to occur in the biologically active and diatom-rich Mauritanian upwelling. However, contrary to expectations, only low levels were encountered in the upwelling waters, 10.7 (range 5.2–23.8) pmol L−1 CHBr3 and 4.7 (range 3.1–7.0) pmol L−1 CH2Br2, values more typical of open ocean concentrations. The aqueous CHBr3 concentrations were not correlated to high chlorophyll a values or diatom abundances. However, significant correlations existed with low concentrations of marker pigments for diatoms, cyanobacteria, and degradation, suggesting miscellaneous small biological sources of the compound in the upwelling. Air-sea exchange could additionally account for an oceanic source in fresh upwelled waters, while advection of different water masses also influenced the distribution. CHBr3 concentrations were maximized in warm and nitrogen-depleted surface waters, while CH2Br2 was maximized in colder and nitrogen-enriched deeper waters, suggesting that both compounds, at least in part, have different sources and fates.