What is Hypoxia?
Hypoxia occurs when the amount of dissolved oxygen in water becomes too low to support most aquatic life (typically below 2 mg/l). Most organisms avoid waters that are hypoxic — sometimes referred to as a dead zone — because it can cause physiological stress and can impact an organism’s growth, reproduction and survival. If organisms cannot escape, hypoxia can be fatal, reducing overall populations and, in some cases, negatively affecting commercial and recreational harvests of fish and shellfish.
Hypoxia occurs in both fresh and salt water and have been increasing in frequency throughout the country. Dead zones can occur naturally, but they can also be created or enhanced by human activity. There are many factors that combine to create dead zones, but excess nutrients entering the water are a primary cause of dead zones in the Great Lakes.
The nutrients from these sources can stimulate algae growth, more than the lake would naturally support, which then sinks and decomposes in the water. The decomposition process consumes oxygen and there is none to replace it because of the temperature and water density gradient during the summer. The dissolved oxygen supply available for aquatic life is depleted until the lake “turns over” or the stratified layers of water mix.
Invasive species like zebra and quagga mussels are also thought to contribute to the development of dead zones, specifically in Lake Erie. The mussels filter out nutrients and green algae early in the year and release fecal pellets (excess nutrients) late into the summer. They also reject the blue green algae, which contributes to harmful algal blooms (which in turn can contribute to dead zones).
Lake Erie Dead Zone
Lake Erie’s hypoxic zone is very large — about the size of Rhode Island and Delaware combined. And while the Gulf of Mexico’s dead zone has received more attention, during some years Lake Erie’s zone may be greater in volume.