MEROMIXIS IN HYPERSALINE MONO LAKE, CALIFORNIA .2. NITROGEN FLUXES

Vertical fluxes of nitrogen were examined in hypersaline Mono Lake ove r a 9-yr period which encompassed the onset, persistence, and breakdow n of meromixis. Under monomictic conditions, ammonia, which accumulate s in the hypolimnion, is mixed into the euphotic region during autumn overturn. Following the onset of meromixis in 1983 and elimination of the winter period of holomixis, ammonia was depleted in the mixolimnio n and accumulated beneath the chemocline. The mean rate of particulate nitrogen deposition, as measured by sediment traps over a 2-yr period during meromixis, was 2.0 mmol m-2 d-1. Until meromixis weakened in 1 988, ammonia concentrations in the euphotic zone remained below 5 muM and increased to approximately 500 muM beneath the chemocline. Meromix is ended in November 1988 and a large pulse of ammonia was injected in to surface waters, resulting in surface ammonia concentrations of appr oximately 45 muM. Because the pH of Mono Lake is high (9.8) the NH3 : NH4+ ratio is approximately 5, and elevated surface concentrations of ammonia during the 2 yr following breakdown of meromixis resulted in h igh losses of nitrogen via ammonia volatilization (mean, approximately 10 mmol m-2 d-1). High release rates of ammonia from the sediments we re estimated from both the ammonia gradients in pore-water profiles (3 -10 mmol m-2 d-1) and the balance of mixolimnetic nitrogen fluxes (4-1 0 mmol m-2 d-1). The monimolimnetic balance suggested fluxes of ammoni a out of the sediments below the chemocline were reduced during meromi xis