Anoxic Dead Zones

A dead zone is an area of depleted oxygen, whether hypoxic (low oxygen) or anoxic (no oxygen).  Dead zones result from excess nutrients in the water, such as nitrogen and phosphorus, which in turn lead to increased levels of harmful bacteria.  This process is called eutrophication. The reduction of oxygen also fosters the growth of toxic algae blooms. These blooms block sunlight, further preventing oxygen from reaching marine life beneath them.

A variety of causes can lead to the excess nutrients that result in oxygen level changes, including agricultural runoff from fields, burning of fossil fuels, wastewater from sewage facilities, and, in the case of certain types of fish farming, an overabundance of feed and fish feces beneath pens. Specifically, open-net carnivorous fish aquaculture contributes to nutrient loading through uneaten feed, fish waste, and the discharge of cleaning chemicals. These discharges trigger algal blooms that consume oxygen and smother aquatic ecosystems. Over time, the accumulation of waste can create persistent dead zones around fish farms, making the surrounding water uninhabitable for all marine life.

A lack of sufficient oxygen can alter the habits of creatures as they attempt to circumvent these conditions, and further, can lead to mass die-offs.