Pollution Leads to Greater Number of Dead Zones in Ocean

By Linda Hunter
Published: October 2006

Sewage spills into the ocean.

The number of “dead zones” in the world’s oceans may have increased by a third in just two years, threatening fish stocks and the people who depend on them, according to a report released by the United Nations. Fertilizers, sewage, fossil fuel burning and other pollutants have led to a doubling in the number of oxygen-deficient coastal areas every decade since the 1960s.

Scientists have found 200 "dead zones" in the world's oceans. A dead zone is an area of the ocean that doesn’t have enough oxygen to support most marine life. They are caused by the over-enrichment of a water body (by runoff, sewage spills, etc.) with nutrients, resulting in the excessive growth of algae. These algae eventually die and sink to the sea floor where microbes decompose them, consuming oxygen in the process.

These dead zones are occurring in many areas along the coasts of major continents, and they are spreading over larger areas of the sea floor. Because very few organisms can tolerate the lack of oxygen in these areas, they can destroy the habitat in which numerous organisms make their home.

The cause of anoxic bottom waters is fairly simple: the organic matter produced by phytoplankton at the surface of the ocean (in the euphotic zone) sinks to the bottom (the benthic zone), where it is subject to breakdown by the action of bacteria, a process known as bacterial respiration.

The problem is, while phytoplankton use carbon dioxide and produce oxygen during photosynthesis, bacteria use oxygen and give off carbon dioxide during respiration. The oxygen used by bacteria is the oxygen dissolved in the water, and that’s the same oxygen that all of the other oxygen-respiring animals on the bottom (crabs, clams, shrimp, and a host of mud-loving creatures) and swimming in the water (zooplankton, fish) require for life to continue.

The first "dead zones" -- where pollution-fed algae remove oxygen from the water -- were found in northern latitudes like the Chesapeake Bay on the U.S. East Coast and the Scandinavian fjords. Today, the best known is in the Gulf of Mexico, where fertilizers and other algae-multiplying nutrients are dumped by the Mississippi River. Others have been appearing off South America, Ghana, China, Japan, Australia, New Zealand, Portugal and Britain.

Growing global populations, mainly concentrated along coastlines, and the resulting increase in untreated sewage are endangering human health and wildlife, as well as livelihoods from fisheries to tourism, according to the "State of the Marine Environment" report. The report is compiled from a wide variety of governments, academic and other sources by UNEP's Global Programme of Action for the Protection of the Marine Environment from Land-Based Sources.

"An estimated 80 percent of marine pollution originates from the land," said Achim Steiner, United Nations Undersecretary-General. "And this could rise significantly by 2050 if, as expected, coastal populations double in just over 40 years time and action to combat pollution is not accelerated," Steiner said in a statement.

In many developing countries, between 80 and 90 percent of sewage entering the coastal zones is estimated to be raw and untreated. These wastes contain bacteria and viruses that can contaminate marine species such as shellfish that are consumed by people.

It is time that we take steps to reduce runoff, improve water quality and restore natural ecosystems so that watersheds empty only clean water into the ocean to eliminate these dead zones and ensure a healthy ocean for future generations.