Bromine biogeodynamics in the NE Atlantic: A perspective from natural wetlands of western Portugal

Abstract

Bromine (Br) cycling in natural wetlands is highly complex, including abiotic/biotic processes and multiphase inorganic/organic Br-species. Wetland ecosystems receive Br primarily from the ocean, functioning as either sinks or sources of Br, with the overall imbalance largely decided by the prevailing climate. Aiming to trace the present-day transport of oceanogenic Br (i.e., derived from salt-water spray-droplets) and its uptake and storage in brackish and freshwater wetlands, we surveyed waters, autochthonous plants, and soils/sediments from coastal marshes and mountain peatlands in the westernmost fringe of northern Portugal. The calculated enrichment factors of bromide (Br^-) relative to chloride in rainfall (EF_sea = 16.8-75.3), rivers (EF_sea = 1.3-13.9) and wetland waters, superficial (EF_sea = 5.8-13.1) and interstitial (EF_sea = 2.1-8.9), increased towards the inland highlands. We hypothesized that these values derived mostly from a known Br autocatalytic (heterogeneous) chemical cycle, starting at the seawater-aqueous interface and progressing in altitude. Br-bearing air masses are carried far from the Atlantic Ocean by moist westerlies, with Br^- rainout from the atmosphere supplying the neighbouring mountain peatlands. Average [Br] in sampled wetland soils/sediments (111-253 mg/kg) agreed with values from other coastal regions, and they were directly correlated with the abundance of organic matter, varying irrespective the [Br^-] of interstitial waters (129 μg/L-79 mg/L). According to the computed bioconcentration factors, the aqueous component was the major source of Br for all plant species investigated (BF_plant/water = 2.1-508.0), as described elsewhere. However, Br contents in plants (14-173 mg/kg) evidenced interspecific differences, also suggesting a divergence from the acknowledged halophytic-glycophytic "model". As plants are recognized producers of Br volatile molecules (e.g., methyl bromide, CH₃Br), we interpreted translocation factors less than one in vascular species as explanatory of phytovolatilization rather than restriction of Br^- upward movement in plants. Further investigation is needed, since considerable intrinsic plant variations in CH₃Br emissions are mentioned in the literature.

Keywords: (De)bromination; Autochthonous plants; Climate; Peatlands; Phytovolatilization; Salt marshes.