Meromixis in hypersaline Mono Lake, California. 2. Nitrogen fluxes

Vertical fluxes of nitrogen were examined in hypersaline Mono Lake over a 9-yr period which encompassed the onset, persistence, and breakdown of meromixis. Under monomictic conditions, ammonia, which accumulates 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 mixolimnion 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 1988, ammonia concentrations in the euphotic zone remained below 5 µM and increased to ~500 µM beneath the chemocline. Meromixis ended in November 1988 and a large pulse of ammonia was injected into surface waters, resulting in surface ammonia concentrations of ~45 µM. Because the pH of Mono Lake is high (9.8) the : ratio is ~5, and elevated surface concentrations of ammonia during the 2 yr following breakdown of meromixis resulted in high losses of nitrogen via ammonia volatilization (mean, ~10 mmol m^{− 2} d⁻¹). High release rates of ammonia from the sediments were estimated from both the ammonia gradients in pore-water profiles (3–10 mmol m⁻² d⁻¹) and the balance of mixolimnetic nitrogen fluxes (4–10 mmol m⁻² d⁻¹). The monimolimnetic balance suggested fluxes of ammonia out of the sediments below the chemocline were reduced during meromixis.