Why Is Hypoxia Bad?



Zones of low oxygen cause devastating effects on the fauna that depend on oxygen. Some animals like fish and crabs can move to a more-suitable location. Animals like bivalves (clams and oysters) can not always escape the deadly effects of hypoxia. These animals often perish in such conditions.


In closed systems like ponds, the animals may have no way out of a low oxygen zone. Some species in deep water situations may not be able to move from the lower layers of the water where the low oxygen areas tend to occur. This is because of the temperature or pressure needs of the organism. These animals are effectively trapped in this location and may die.


Fish Kills

In some cases, hypoxia may cause a fish kill. When hypoxia takes hold on an entire pond or on a vast area of open ocean, the animals can not escape and many of them die. Fish kills are detected by seeing dead, floating fish and other animals. The observer may also notice stressed fish at the water surface gulping for life-saving oxygen. These fish often perish as well. Bacteria will begin to decompose the fish and cause an even lower dissolved oxygen value. Eventually the fish will decompose or will be eaten by scavengers and the oxygen will slowly return to a more acceptable level. Unfortunately a fish kill may lead to an unbalanced fish population and other environmental problems.


Recently, there was a fish kill near LUMCON. Floating fish were observed by LUMCON employees on Monday August 23, 2004. Upon further inspection, we concluded that the pond's dissolved oxygen dropped to dangerously low levels and caused a fish kill. Our findings are in the chart below.


August 19, 2004 August 23, 2004 August 25, 2004
Bottom DO Reading 1.8 0.4 2.8
Top DO Reading 3.4 0.6 5.8
This chart shows how the oxygen dropped on August 23 to dangerous levels causing a fish kill. The DO measurements before and after this event are given for comparison

These images were taken after the fish kill at LUMCON.



The Dead Zone

Every summer a very large area of hypoxia forms off the coast of Louisiana. It is commonly known as The Dead Zone. LUMCON researcher Nancy Rabalais maps this area and compares it to data she has been collecting since the mid-1980's.


The mechanics of the Gulf of Mexico hypoxia is somewhat complex. Fortunately it can be simplified in to just a few processes. The story of the dead zone does not start anywhere near Louisiana; it starts in the largest watershed in the United States - the Mississippi River watershed. Excess nitrogen and other nutrients used by midwest farmers washes away from their fields and into the tributaries of the Mississippi River. Over time, all of these nutrients flush down the Mississippi and are deposited into the gulf. Once the nutrients get to the gulf, the phytoplankton devour the nutrients and multiply. Remember that phytoplankton use photosynthesis which produces oxygen, so up to this point, the oxygen is not depleted yet. Once the nutrients begin to run out for the phytoplankton, there is a mass die-off of the tiny plants and they begin to fall to the gulf floor. This is where hypoxia begins. Bacteria decompose the falling phytoplankton and use enormous amounts of oxygen. If you couple this cycle along with the warm temperatures and salinity effects mentioned earlier, it creates a huge area of depleted oxygen concentrations.


This is the Mississippi River watershed. Source: National Source for Appropriate Technology This map indicates how widespread the area of hypoxia was in the summer of 2001. It was more than 7722 square miles in area.
Source: NOAA

The topic of hypoxia is actually quite controversial. Agriculturalists in the midwest do not believe that their industry is the sole culprit for the existence of the dead zone. In fact, they sometimes say the term 'dead zone' is misleading and misrepresents the the truth. Scientist, on the other hand, tend to blame the nitrates that empty into the gulf from farmers. There has been legislation regarding this topic (see The Harmful Algal Bloom and Hypoxia Research Amendments Act of 2003 (H.R. 1856 IH)).