Oxidation of ammonia and methane in an alkaline, saline lake

The oxidation of ammonia (NH3) and methane (CH4) was investigated in an alk aline saline lake. Mono Lake, California (U.S.A.). Ammonia oxidation was ex amined in April and July 1995 by comparing dark (CO2)-C-14 fixation rates i n the presence or absence of methyl fluoride (MeF), an inhibitor of NH3 oxi dation. Ammonia oxidizer-mediated dark (CO2)-C-14 fixation rates were simil ar in surface (5-7 m) and oxycline (11-15 m) waters, ranging between 70-340 and 89-186 nM d(-1), respectively, or 1-7% of primary production by phytop lankton. Ammonia oxidation rates ranged between 580-2,830 nM d(-1) in surfa ce waters and 732-1,548 nM d(-1) in oxycline waters. Methane oxidation was examined using a (CH4)-C-14 tracer technique in July 1994, April 1995, and July 1995. Methane oxidation rates were consistently higher in July, and ra tes in oxycline and anaerobic bottom waters (0.5-37 and 7-48 nM d(-1), resp ectively) were 10-fold higher than those in aerobic surface waters (0.04-3. 8 nM d(-1)). The majority of CH4 oxidation, in terms of integrated activity , occurred within anoxic bottom waters. Water column oxidation reduced the potential lake-atmosphere CH4 flux by a factor of two to three. Measured ox idation rates and water column concentrations were used to estimate the bio logical turnover times of NH3 and CH4. The NH3 pool turns over rapidly, on time scales of 0.8 d in surface waters and 10 d within the oxycline, while CH4 is cycled on 10(2)-d time scales in surface waters and 10(2)-d time sca les within oxycline and bottom waters. Our data suggest an important role f or NH3 oxidation in alkaline, saline lakes since the process converts volat ile NH, to soluble NO2, thereby reducing loss via lake-atmosphere exchange and maintaining nitrogen in a form that is readily available to phytoplankt on.