All about “Dead Zones”

Zones of Oxygen Depletion

What is Hypoxia?  Waters that have a Dissolved Oxygen (DO) concentration less than 2mg/L (<3mg/L, some systems) are defined as Hypoxia  If DO= 0 mg/L, it is called Anoxia  Oxygen Depletion caused by – Excessive nutrients, intense biological productivity that depletes oxygen – Decomposition of organic matter consumes oxygen – Stratification prevents oxygen refreshment

Gulf of Mexico – Dead Zone Dead Zone > 7,000 sq. miles in the Gulf of Mexico with no marine life in 1999.

The Gulf of Mexico is the largest fishery in the US The Gulf of Mexico is the largest fishery in the US

Hypoxia: Low dissolved oxygen (< 3 mg L -1 ) “And the fish are the lucky ones…”

Number of fish kill events and number of fishkilled in Gulf of Mexico coastal waters, (NOAA/ORCA 1991; USEPA 1999). Millions of fish killed Number of fish kill events

Catch per unit effort in the Gulf of Mexico,

Gulf of Mexico – Dead Zone

The Size of the Dead Zone Correlates with Mississippi River Discharge Values

Nitrogen inputs to the Gulf since

Nitrogen loading in the Gulf triggers algal blooms, which decay and sink to the bottom. The degradation of this algal biomass results in oxygen depletion and the hypoxic / dead zones.

Sources of nutrients to estuaries  Agriculture: Fertilizers  Human waste: sewage treatment plants, septic tanks, landfills  Atmosphere: Industrial emissions

Annual Nitrogen Inputs to the Mississippi/ Atchafalaya River Basin

Fertilizer use estimates for the United States, 1991 (USEPA 1999).

Farm fertilizers and other nutrients flushed into Mississippi River Discharge of N into GOM stimulates algal blooms Decay of algal blooms removes bottom oxygen Without oxygen, fish and Shrimp die or leave the GOM Simplified Model of Gulf of Mexico Dead Zone Formation

Pacific Northwest Dead Zone

Primary Productivity July-August 2005 Upwelling Season

Jet Stream shifts to the North Upwelling of Nutrient-rich bottom water stimulates algal blooms Decay of algal blooms removes bottom oxygen Without oxygen, fish and crabs die or leave Simplified Model of Oregon Dead Zone Formation Northerly coastal winds persist days instead of 2-5 days Persistent winds trigger ‘super-charged’ upwelling

The End