Gas accumulation in Lake Bosumtwi deep waters and its potential to contribute to fish kills

Lake Bosumtwi in tropical Ghana has been known for its recurrent fish kills, but they have recently been reported to happen less frequently. The lake formed in a meteorite impact crater in Ghana, West Africa. It plays an important role for the local inhabitants for recreation and for fisheries. The lake is deep, and recent observations indicate that recirculation is incomplete. In general, the deep water is anoxic. Fish kills have been associated with the mixing events in the slightly colder rainy season. As unpleasant smells from the water during deep mixing had been reported, the question arose whether toxic gases that had accumulated in the deep water could be responsible; namely, hydrogen sulphide or large amounts of carbon dioxide were considered the most probable candidates. The analysis of the water properties, however, did not detect any hydrogen sulphide nor immensely large concentrations of carbon dioxide. On the contrary, the presence of large amounts of bound nitrogen could be substantiated. We hence concluded that most probably bound nitrogen was responsible for the fish kills on two paths (1) as bound nitrogen as ammonium forms toxic ammonia when mixed into high pH surface water and (2) depletes oxygen when it is oxidized in the surface waters.