Hypoxia is a phenomenon that occurs in both freshwater and saltwater aquatic systems where the dissolved oxygen concentration drops below a certain point and begins to adversely affect respiring organisms. Generally speaking, the dissolved oxygen concentration during an episode of hypoxia is below 2ppm. Anoxia occurs when dissolved oxygen levels drop to zero.
Low dissolved O2 concentrations have been recorded in inland freshwater ponds as well as coastal areas around the world. According to the United Nations Environment Programme, in 2003 there were 146 instances of coastal hypoxia worldwide. These areas are shown on the map below.
Source: United Nations Environment Programme, GEO Yearbook 2003 (Nairobi: 2004), compiled from Boesch 2002, Caddy 2000, Diaz et al. (in press), Green and Short 2003, Rabalais 2002.
As the map indicates, hypoxia can occur with varying frequency. Most of these cases happen on an annual basis, usually in the warmer months of the year.