Global Warming and Ocean Deserts

Reader Contribution by Richard Hilderman and Ph.D.

A desert is a barren region that can support little or no natural vegetation.  Death Valley in the United States and the Sahara in Africa are examples of land deserts. However, deserts can also be found in the oceans.  Examples of ocean deserts are tropical waters and dead zones.  To understand how ocean deserts are created we must discuss how the various life forms in the ocean interact to create ecosystems. The four main groups of organisms that make up ocean ecosystems are primary producers, grazers, predators and scavengers.  The interaction of these organisms is essential for their survival.  If organisms in one group become endangered this will have a negative impact on life forms found in other groups.

The primary producers utilize carbon dioxide by photosynthesis to create plant tissue and oxygen.  Ninety percent of the primary produces are phytoplankton which are single cell algae that live near the surface of the ocean.  Grazers are animals which lunch on the phytoplankton.  The grazers utilize oxygen to convert plant tissue into animal tissue and carbon dioxide. Some predators feast on grazers while other predators eat smaller predators all the way up the food chain.  Predators also use oxygen and release carbon dioxide when they convert the tissue of the animals they eat into their own tissue.  Scavengers are bacteria that utilize oxygen to convert dead/decaying tissue into nutrients for the phytoplankton and carbon dioxide.  The scavengers do not live near the surface of the ocean but live at much greater depth.  Thus there must be upwelling mechanisms to transport the nutrients to the surface otherwise the phytoplankton would die.  If the phytoplankton density decreases, this would affect the survival of other ocean inhabitants and create ocean deserts. 

Upwelling is created along continents and by ocean storms and hurricanes. However, the amount of upwelling that occurs is limited by how deeply warm water can penetrate the ocean.  There is an inverse relationship between the depth of warm water and upwelling.  In other words, the deeper the warm water the less upwelling.  The tropical oceans are an example of this inverse relationship. The warm water restricts the upwelling of nutrients for the phytoplankton.  The crystal clear blue waters of the tropics are due to a lack of phytoplankton.  This lack of efficient upwelling in tropical waters means ocean inhabitant densities are less in the tropics than in northern latitudes.  Does global warming generate ocean temperatures that inhibit upwelling?  Since 1980 the tropics have expanded more than 2 degrees latitude which is an additional 8.5 million square miles of tropical ocean.

Ocean dead zones are created through the over use of fertilizers. The excess fertilizer enters rivers and eventually the ocean.  In the ocean excess fertilizer are nutrients for the phytoplankton and triggers excessive algal blooms. Eventually the blooms die and sink.  The excess dead algae stimulate an exponential increase in bacterial scavengers which utilize oxygen to digest the algae.  The end result is that this portion of the ocean becomes void of oxygen that ocean inhabitants need to survive.  Mobile animals can leave the area while immobile or slow moving animals can’t escape and die. This area of the ocean containing no oxygen or life is called a dead zone.  Due to excess fertilizer runoff from the Mississippi River the Gulf of Mexico contains the largest area of dead zones in all the world’s oceans.

Clearly, the current global warming trend is creating more tropical oceans and the overuse of fertilizers is triggering ocean dead zones.  Both of these will have negative impact on the ocean’s ecological systems.  What will be the impact on the health of the oceans and the ability of humans to use the ocean as a food source?