By: Jessica Wurzbacher, MSc
What is a Dead Zone?
A dead zone is an area of an ocean (or lake) that has too little oxygen to support marine life; it is hypoxic. This is a natural phenomenon that has been increasing in shallow coastal and estuarine areas as a result of human activities.
Why do Dead Zones Form?
Eutrophication is an increase in nutrients in the water (particularly phosphorus and nitrogen). Human activities have resulted in the near doubling of nitrogen and tripling of phosphorus flows to the environment when compared to natural values (The Millennium Ecosystem Assessment; WRI). Sources of nutrients in coastal waters include; lawn fertilizers, agricultural manure, sewage output and storm water. Dead zones are the most severe result of eutrophication.This dramatic increase in previously limited nutrients causes massive algal blooms. These "red tides" or Harmful Algal Blooms can cause fish kills, human illness through shellfish poisoning, and death of marine mammals and shore birds. This population explosion is unsustainable, and eventually dies off, as they block out the light and use up all the oxygen. The algae sink to the bottom, and bacterial decomposition uses the remaining oxygen from the water.
Fig 1. A menhaden (Brevoortia sp.) fish kill in August 2003 was caused by severe hypoxic conditions in Greenwich Bay, Narragansett Bay, Rhode Island, USA. Image Credit: Chris Deacutis (WRI)
Stratification of the water column due to density differences also contributes to the formation of hypoxic or "dead zones". In the Chesapeake Bay, cooler, saltier (denser) water from the ocean, flows underneath the warmer, fresher (less dense) water from the rivers. A boundary, called a pycnocline, forms and oxygen consumed below the pycnocline cannot be replenished from above. The pycnocline is typically strongest in spring and early summer when fresh water flows are usually at their highest.
Where are dead zones?
The has been a staggering increase in the number of dead zones worldwide over the past 60 years, from just 42 in 1950, to 405 in 2008 (Diaz & Rosenberg, 2008) (Fig 3). Dead zones now cover 95,000 square miles; this is the size of the United Kingdom.
Fig 3. Worldwide map representing 762 coastal areas impacted by eutrophication and/or hypoxia. Click here to see an interactive map of worldwide dead zones created by Dr. Robert Diaz of Virginia Institute of Marine Science and World Resources Institute (WRI).
The largest dead zone worldwide is the Baltic Sea. Overfishing of Baltic cod has greatly intensified the problem. Cod eat sprats, a small, herring-like species that eat microscopic marine creatures called zooplankton that in turn eat the algae. So, fewer cod and an explosion of zooplankton-eating sprats means more algae and less oxygen - a vicious cycle develops (Westman, 2010)
The second largest dead zone is the northern Gulf of Mexico, surrounding the outflow of the Mississippi River, in the summer of 2002 it covered 8,500 square miles (approximately the same size as the state of Massachusetts) (Gulf Hypoxia). The catchment area of the Mississippi River basin is vast, draining approximately 41% of the land area of the continental United States (Fig 4).
Fig 4. The drainage basin of the Mississippi River
The Chesapeake Bay also has a significant dead zone, each summer it occupies about 40% of the area of the Bay and 5% of its volume, stretching from Baltimore Harbor all the way down below the Potomac into Virginia waters (VIMS) (Fig 5).
Fig 5. The extent of hypoxia in the Chesapeake Bay (University of Michigan)
The first dead zone in the Chesapeake Bay was reported in the 1930's and it has been growing larger over the years. The dead zone this year (2011) may be one of the Bay's five worst ever, covering a massive 1,200 square miles (VIMS). The average volume of hypoxic water (Dissolved Oxygen ¡Â2.0 mg-L 1) in the Chesapeake Bay is predicted to be 6.8 km©ø for June to September 2011. This would be the 4th highest hypoxic volume for the period of record (1985-2010) (Eco-Check). On the Severn River there have been reports of oxygen levels as low as 0.02 mg-L 1, dangerously low for fish, crab and shellfish (Henkart, 2011; cited in Wood, 2011)
Hypoxic conditions can also develop naturally, and may be permanent or semi-permanent. They can form in areas of upwelling, or when there are changes in wind and circulation patterns. The Black Sea has been anoxic in its deepest parts for millennia, due to its shallow sill which prevents mixing and oxygen replenishment. There are also areas of the eastern tropical Pacific Ocean and Northern Indian Ocean where minimum circulation occurs creating Oxygen Minimum Zones (OMZ) (Fig 6).
Fig 6. Climatological mean dissolved oxygen concentrations at 400m depth (µmol kg-1)
What Problems are Associated with Dead Zones?As the name implies, Dead Zones create an area devoid of marine live, directly due to low levels of oxygen which suffocates bottom dwelling lobsters, clams and oysters. The fish can also quickly become unconscious and doomed without time to escape. Low oxygen levels also causes reproductive problems in fish involving decreased size of reproductive organs, low egg counts and lack of spawning. In the Chesapeake Bay dead zones are linked to chronic outbreaks of bacterial disease (mycobacteriosis) among striped bass as the fish are forced up from the cooler bottom water that is low in oxygen to the warmer surface waters. Over 75% of fish are infected in the Bay's summertime dead zone (VIMS).
What are the Solutions?The main goal in reducing dead zone is to keep the fertilizers on the land and out of coastal waters, fortunately this is a goal shared by both conservationist and farmers.
There has been a success story in the Black Sea, although largely unintentional. Previously holding the claim to the largest dead zone in the world, this significantly disappeared between 1991 and 2001. The reason; following the collapse of the Soviet Union and the demise of eastern European economies, the fertilizers became too expensive, and nutrient loads entering the sea were therefore considerably reduced. Fishing has now become a major economic activity in the region.
Our ocean ecosystems are fragile and the combined threats of climate change, overexploitation, pollution and habitat loss, all mostly caused by human activity, are undermining the sustainability. Increasing hypoxia and dead zones, along with warming oceans and increasing acidification are creating multiple stressors. Looking back on past events, multiple stressors have been a precondition for mass extinction.
What can you do?