The relationship between volatile halocarbons and phytoplankton pigments during a Trichodesmium bloom in the coastal eastern Arabian Sea

Eukaryotic phytoplankton such as diatoms and prymnesiophytes produce biogenic halocarbons in the ocean that serve as important sources of chlorine and bromine to the atmosphere, but the role of cyanobacteria in halocarbon production is not well established. We studied distributions of chloroform (CHCl3), carbon tetrachloride (CCl4), methylene bromide (CH2Br2) and bromoform (CHBr3) in relation to phytoplankton composition, determined from pigment analysis complemented by microscopic examination, for one month in coastal waters of the eastern Arabian that experienced a Trichodesmium bloom that typically occurs during the Spring Intermonsoon season. High concentrations of zeaxanthin (23 μg l−1), alpha beta betacarotene (6 μg l−1) and chlorophyll a (67 μg l−1) were found within the bloom whereas the marker pigment concentrations were low outside the bloom. CHCl3 and CCl4 occurred in relatively high concentrations in surface waters whereas CH2Br2 and CHBr3 were restricted to the subsurface layer. Chlorinated halocarbons were positively inter-correlated and with CHBr3. The observed spatial and temporal trends in brominated compounds appear to be related to the abundance of Trichodesmium although correlations between concentrations of brominated compounds with various marker pigments were poor and statistically non-significant. The results support the existence of multiple sources and sinks of halogenated compounds, which might obscure the relationship between halocarbons and phytoplankton composition.