Factors controlling hypolimnetic ammonia accumulation in a lake

Abstract : The factors controlling the degree of hypolimnetic ammonia accumulation in Lake Onogawa are discussed based on periodic observations since 1993. The standing stock of ammonia in the bottom 9m of the water column was a good measurement for determining the extent of the hypolimnetic ammonia accumulation. It varied threefold from 144mmolm~-2 in 1998 to 429mmolm~-2 in 1996. The correlation between the annual maxima of the ammonia standing stocks and the annual maxima of the thickness of anoxic layers was significant at P = 0.01. This fact suggests that the degree of development of the anoxic layer is the primary factor controlling the extent of hypolimnetic ammonia accumulation. Sporadic local heavy rainfalls in 1998 perturbed the water column, and the formation of the anoxic layer was postponed more than one month, resulting in a lower level of hypolimnetic ammonia accumulation in 1998. A thick mineral deposit apparently formed during the local heavy rainfall and seemed to enclose the freshly deposited organic matter, which might be an effective source material of the hypolimnetic ammonia, and resulted in a low level of ammonia accumulation in 1999. By 2000, the lake seems to have recovered from the perturbation, suggesting that the major part of the hypolimnetic ammonia is derived from fresh organic matter deposited within a year. Key words : Lake Onogawa・Anoxic layer・Lake perturbation・Ammonia sourc