Oceanic distribution and sources of bromoform and dibromomethane in the Mauritanian upwelling

The tropical oceans are a source of reactive bromine to the atmosphere in the form of short‐lived brominated methanes as bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ). 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 CHBr 3 and 4.7 (range 3.1–7.0) pmol L −1 CH 2 Br 2 , values more typical of open ocean concentrations. The aqueous CHBr 3 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. CHBr 3 concentrations were maximized in warm and nitrogen‐depleted surface waters, while CH 2 Br 2 was maximized in colder and nitrogen‐enriched deeper waters, suggesting that both compounds, at least in part, have different sources and fates.