Fish kills, like this one in a lake at the Chautauqua National Wildlife Refuge, can result from hypoxia, or very low levels of oxygen in the water. Hypoxia can occur when algae grow excessively at the water's surface. As these organisms die, they sink to the bottom and are decomposed by bacteria, which use up oxygen in the water. Throughout most of the year, oxygen mixes completely throughout the water body. However, during the warmer months, or when ice forms on the surface, temperature differences cause the water to become stratified. This layering prevents the complete mixing of the oxygen, leading to hypoxia. Hypoxia can also occur due to winds forcing water with low dissolved oxygen into a water body (like an estuary). This can occur during a storm event like a hurricane.