Stagnant lakes can spawn disaster. In Africa, more than 1,800 unsuspecting villagers living around Lake Nyos suffocated when a huge cloud of carbon dioxide erupted from the stagnant waters in 1986. Geoff Schladow, UC Davis assistant professor of civil and environmental engineering, has modeled water conditions such as those found in Lake Nyos to help devise new ways to prevent similar disasters. The model provides information for field studies looking to improve water circulation by piping air to the bottom of stagnant lakes. As the air bubbles rise, they generate circulation, just as boiling water is mixed when bubbles rise from the bottom of the pan. Stagnant lakes are a seasonal problem. In summer, lake surface water warms but the deep water stays cold, creating conditions for potentially toxic blooms of algae. As the algae die and sink, their ensuing decomposition reduces the supply of oxygen at the bottom. Carbon dioxide builds up in place of oxygen, ripening conditions for a disaster such as occurred at Lake Nyos. Without oxygen, heavy metals such as mercury convert to a form that can easily enter the food chain, a problem Schladow is investigating at Clear Lake, Calif. Similar circulation problems can happen in winter, when ice-covered lakes lose oxygen, resulting in a condition known as "winterkill." A recent paper by Schladow published in the journal Limnology and Oceanography outlines the effects of artificial mixing.