Improved bromide measurements using chloramine-T shows, bromide depletion in the Gulf of Maine

The assay for bromide (Br−) based on bromination of phenol red using chloramine-T was tested with small volumes (0.3 ml or less) of seawater and had two confounding issues. High blanks (with no added bromide) occasionally occurred, and the reaction-derived color frequently was destroyed by coupled reactions within the assay. The method was modified by lowering the assay pH to 3.9, by reducing the assay temperature to 13C, and by keeping the reaction short (10 s). As a result, blank absorbances were very low, and reaction-produced color was not destroyed in routine assays. In addition, no interferences by ionic seawater constituents were found. The assay was tested on natural water samples from the Antarctic continental shelf and from the Gulf of Maine. The results were highly precise; duplicates of 32 samples from the Gulf of Maine had an average standard error of the mean of ± 2.26 μmol kg−1. Antarctic bottom waters had bromide concentrations of 834.1 and 831.9 μmol kg−1 and the Br−/Chl ratio, where Chl is the chlorinity in μmol chloride equivalents (kg of seawater)−1, statistically matched the previously determined ocean mean of 0.15410 mol %. In the Gulf of Maine, vertical profiles from the sea surface to the sea floor showed lower concentrations inshore near the surface (minimum of 762.2 μmol kg−1) and greater concentrations at depth (maximum of 837.3 μmol kg−1 in a basin containing continental slope water). At the offshore basin with slope water, the Br−/Chl ratio of all waters below 15 m matched the ocean mean. Sites near the Maine and New Hampshire coast had lower concentrations of bromide, and the station-averaged Br−/Chl ratios reached a minimum of 0.15283 mol %. A previous explanation for reduced Br−/Chl ratios in shallow seas was that the fresh water Br−/Chl ratio could be much less than the ocean mean. A two end-member mixing model of the Gulf of Maine showed that this mechanism could explain a reduction in bromide concentrations in coastal Maine of about 0.6 μmol kg−1, but this depletion is 10-fold low of the observed bromide reduction. The lowered bromide content in coastal waters could be due to 1) significant macroalgal uptake along the coastline, and 2) in situ bromination of dissolved organics and planktonic detritus via photochemical or peroxide oxidation.