Spatial and temporal distributions of bromoform and dibromomethane in the Atlantic Ocean and their relationship with photosynthetic biomass
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abstract
Atmospheric mixing ratios and seawater concentrations of bromoform (CHBr3), dibromomethane (CH2Br2), and other brominated very shortlived substances (BrVSLS) were measured during five cruises from 1994 to 2010. These cruises were conducted over large latitudinal (62N60S) and longitudinal transects (11W86W) in the Atlantic Ocean. Elevated seawater concentrations of CHBr3 and CH2Br2 were often observed in regions where chlorophyll a concentrations were also elevated, which suggests biogeochemical processes associated with photosynthetic biomass may be related to CHBr3 and CH2Br2 production. Our results suggest that, at least in the open ocean, several phytoplankton taxa may contribute to the production of these trace gases. While observed correlations between CHBr3 and CH2Br2 in different regions are usually interpreted as common sources for these compounds, results in this study suggest different biogeochemical processes may contribute separately to the production of these trace gases. Heterotrophic bacterial abundance was significantly correlated with CH2Br2, but not with CHBr3, which suggests the biogeochemical processes associated with heterotrophic bacteria may be related to CH2Br2 in seawater but probably not to CHBr3. In general, the Atlantic Ocean is a net source for CHBr3 and CH2Br2, except for a few locations where these trace gases were undersaturated in seawater. Assuming fluxes measured in the Atlantic open ocean are globally representative, the resulting extrapolated, global openocean annual net seatoair fluxes calculated from data from the five cruises was estimated at 0.243.80 Gmol Br yr1 for CHBr3 and 0.110.77 Gmol Br yr1 for CH2Br2.
