Oxygen consumption and ammonia accumulation in the hypolimnion of Walker Lake, Nevada

Abstract

Walker Lake (area = 140 km², Zmean = 19.3 m) is a large, terminal lake in western Nevada. As a result of anthropogenic desiccation, the lake has decreased in volume by 75% since the 1880s. The hypolimnion of the lake, now too small to meet the oxygen demand exerted by decaying matter, rapidly goes anoxic after thermal stratification. Field and laboratory studies were conducted to examine the feasibility of using oxygenation to avoid hypolimnetic anoxia and subsequent accumulation of ammonia in the hypolimnion, and to estimate the required DO capacity of an oxygenation system for the lake. The accumulation of inorganic nitrogen in water overlaying sediment was measured in laboratory chambers under various DO levels. Rates of ammonia accumulation ranged from 16.8 to 23.5 mg-N m⁻² d⁻¹ in chambers with 0, 2.5 and 4.8 mg L⁻¹ DO, and ammonia release was not significantly different between treatments. Beggiatoa sp. on the sediment surface of the moderately aerated chambers (2.5 and 4.8 mg L⁻¹ DO) indicated that oxygen penetration into sediment was minimal. In contrast, ammonia accumulation was reversed in chambers with 10 mg L⁻¹ DO, where oxygen penetration into sediment stimulated nitrification and denitrification. Ammonia accumulation in anoxic chambers (18.1 and 20.6 mg-N m⁻² d⁻¹) was similar to ammonia accumulation in the hypolimnion from July through September of 1998 (16.5 mg-N m⁻² d⁻¹). Areal hypolimnetic oxygen demand averaged 1.2 g O₂ m⁻² d⁻¹ for 1994–1996 and 1998. Sediment oxygen demand (SOD) determined in experimental chambers averaged approximately 0.14 g O₂ m⁻² d⁻¹. Continuous water currents at the sediment-water interface of 5–6 cm s⁻¹ resulted in a substantial increase in SOD (0.38 g O₂ m⁻² d⁻¹). The recommended oxygen delivery capacity of an oxygenation system, taking into account increased SOD due to mixing in the hypolimnion after system start-up, is 215 Mg d⁻¹. Experimental results suggest that the system should maintain high levels of DO at the sediment-water interface (~10 mg L⁻¹) to insure adequate oxygen penetration into the sediments, and a subsequent inhibition of ammonia accumulation in the hypolimnion of the lake.