Production and consumption of methyl halides in a freshwater lake

The concentrations of the methyl halides CH 3 Cl and CH 3 Br in Lake Washington and its tributary streams were measured during the period 1994–1998. Strong seasonal variation was observed. In summer, the surface mixed layer in the middle of the lake was found to be generally undersaturated for CH 3 Cl but consistently supersaturated for CH 3 Br relative to air‐water gas equilibrium; meanwhile, the deep waters (z max ‐ 65 m) of the summertime lake were found to be depleted to near zero levels for both compounds. In winter, the methyl halides were observed to be vertically well mixed throughout the water column and strongly undersaturated in both species. A simple mass‐balance box model indicates that the wintertime lake ( T = 8°C) is a net sink for atmospheric methyl halides with estimated loss rates of ~2.6–3.2 pM d −1 for CH 3 Cl (1 pM = 10 −12 mol L −1 ) and 0.09–0.13 pM d −1 for CH 3 Br, probably because of unspecified microbiological processes. In summer, the mass balance model indicates that there is a net source of CH 3 Br in the surface mixed layer (0.17–0.25 pM d −1 ), as required to maintain the observed strong supersaturation. However, the summer surface waters are apparently a sink for CH 3 Cl, (1.9–2.9 pM d −1 ), with enhanced biological production compensated by larger combined microbial and chemical sinks terms. In summer, methyl halide concentrations in the deep waters continue to decrease exponentially at the observed wintertime rate, while both production and loss rates in the surface waters are likely much faster than in winter, resulting in much shorter turnover timescales. An order‐of‐magnitude extrapolation of these Lake Washington methyl halide fluxes to a global source/sink from freshwater lakes implies a negligible contribution (<0.05%) with respect to the currently estimated global budgets of these gases.