Laboratory production of bromoform, methylene bromide, and methyl iodide by macroalgae and distribution in nearshore southern California waters

Production rates of bromoform (CHBr 3 ), methylene bromide (CH 2 Br 2 ), and methyl iodide (CH 3 I) were measured in the laboratory for 11 species of marine macroalgae. Production rates of the volatile bromomethanes extrapolated to a global scale suggest that marine macroalgae produce 2 × 10 11 g Br yr −1 (1 × 10 9 mol Br yr −1 ), 98% of which is bromoform. Laminarians (kelps) produce 61% of this organic Br. These calculations suggest that marine macroalgae are important in the biogeochemical cycling of Br. Seawater concentrations of CHBr 3 , CH 2 Br 2 , and CH 3 I were determined from various southern California coastal locales. High concentrations were measured in seawater from the canopy and the bottom of a dense bed of Macrocystis as compared to other sites. Surface seawater concentrations of these halomethanes showed a strong cross‐shore gradient with the highest concentration in the kelp canopy and the lowest at 5 km offshore. Seawater adjacent to decaying macroalgae on the bottom of a submarine canyon was not enriched in halomethanes relative to surface water. Water exiting a productive estuary was enriched only with CH 2 Br 2 , although two algal species that are abundant there ( Ulva and Enteromorpha ) showed high laboratory production rates of both CHBr 3 and CH 2 Br 2 .