Hypoxia (Low Oxygen) and Anoxia (No Oxygen)

Hypoxia (Low Oxygen) and Anoxia (No Oxygen)

Oxygen is essential for life, but the supply in coastal waters around the world is decreasing - dropping more frequently, over larger spans of time and space. When oxygen levels become alarmingly low (less than 2-3 milligrams of oxygen per liter of water) the condition is called Hypoxia. When oxygen levels are at zero, the condition is called Anoxia.

A majority of sea life cannot handle these poor conditions. Fish, crabs, shrimp, and shellfish unable to flee these conditions stress or die, causing a ripple effect felt by their entire ecosystem. Consequently, the remaining environment becomes flush with anaerobic microbes that possess their own negative environmental impacts.

Many conditions contribute to hypoxia. Warm water (which can be present during the summer, in shallows, near factory effluent, etc.) holds less oxygen than cold. Sheltered coves with weak winds and currents allow water to stratify, forming a persistent, low-oxygen band. In stagnant waters, hypoxia or anoxia is a natural condition. In Narragansett Bay, however, the conditions were naturally rare until humans significantly altered its patterns.

Acute incidents are usually part of a chain reaction, beginning with an excess of nutrients in the watershed. Nitrogen can come from sewage treatment plant effluent, run-off organic materials on fertilized grounds, or bird and animal waste. Under these conditions (amongst many others), the resulting over-enrichment feeds an algal bloom. The bloom deprives water underneath it from sunlight, sinks, and then decomposes, drawing oxygen out of the water below. The only short-term solution is a change in weather or currents, which in turn mixes surface (oxygenated) and subsurface (hypoxia or anoxia) waters. The long-term solution is controlling the supply of nutrients entering the watershed.

What should I do?

  • Check for updates from the Bay Line: (401) 222-8888 (May 15 – October 15)
  • In the meantime, help prevent hypoxia by supporting efforts to improve water quality in the Bay. (See "Introduction to Narragansett Bay")

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