<p><strong>In the depths of the ocean and on shore, science is only beginning to measure the long-term impact of the <a href="http://news.nationalgeographic.com/news/2010/05/100527-energy-nation-gulf-oil-spill-top-kill-obama/">worst offshore oil spill in U.S. history</a>.</strong></p><p>On the second anniversary of the <em>Deepwater Horizon</em> drilling rig explosion, a slew of new studies paint a complex picture of how the Gulf of Mexico's ecosystems absorbed the insult of 4.9 million barrels of crude oil.</p><p>The catastrophic failure of BP's Macondo well off the coast of Louisiana on April 20, 2010, triggered a blast and fire that took the lives of 11 rig workers and sent oil spewing from the deep sea bed for 87 days. Unprecedented steps were taken to minimize the amount of oil that reached shore, including the application of some 800,000 gallons (3,028,000 liters) of dispersants directly at the wellhead nearly a mile (1,500 meters) below the surface. Still, the oil left its mark, scientists now say, on marine mammals, salt marshes, corals, tiny organisms and coastal communities. The new studies track both lingering harm and recovery.</p><p>(Related Quiz: "<a href="http://environment.nationalgeographic.com/environment/energy/great-energy-challenge/gulf-oil-spill-anniversary-quiz/">How Much Do You Know About the Gulf Oil Spill?</a>")</p><p>Bottlenose dolphins in oil-contaminated Barataria Bay off the coast of Louisiana are showing <a href="http://www.gulfspillrestoration.noaa.gov/2012/03/study-shows-some-gulf-dolphins-severely-ill/">signs of serious illness</a>, including extremely low weight, anemia, low blood sugar, and some symptoms of liver and lung disease, according to a health assessment conducted by U.S. <a href="http://www.noaa.org/">National Oceanic and Atmospheric Administration</a> (NOAA) scientists and their partners.</p><p>The scientists, who performed comprehensive physicals last summer on 32 dolphins from the bay, also found that half of the tested dolphins showed abnormally low levels of hormones that regulate stress response, metabolism, and immune function, indicating adrenal insufficiency. One of the dolphins in the survey was found dead on Grand Isle in January.</p><p>Lori Schwacke of NOAA, the project's lead investigator, said the findings were preliminary and could not be conclusively linked to the oil spill. But she added that control groups of dolphins living along the Atlantic coast and in other areas that were not affected by the 2010 spill did not manifest those symptoms.</p><p>"The findings that we have are consistent with other studies that have looked at the effects of oil exposure in other mammals," she said.</p><p>The study is a part of an ongoing examination of the<a href="http://www.gulfspillrestoration.noaa.gov/assessment"> U.S. government-led Natural Resource Damage Assessment</a> process and the Gulf of Mexico Dolphin Unusual Mortality Event. Since February 2010, more than 675 dolphins have stranded in the northern Gulf of Mexico—a much higher rate than the usual average of 74 dolphins per year.</p><p>BP's Houston office did not respond to requests for comment on this research or the other scientific studies related to the Gulf Oil Spill.</p><p>(Related: "<a href="http://news.nationalgeographic.com/news/energy/2011/01/110111-five-mandates-gulf-oil-spill-commission/">The Next Oil Spill: Five Needed Mandates to Head it Off</a>" and "<a href="http://news.nationalgeographic.com/news/2010/11/110127-gulf-oil-spill-dispersants-environment-science/">Gulf Spill Dispersants Surprisingly Long-lasting</a>")</p><p><em></em></p><p>—<em>Barbara Mulligan</em></p><p><em>This story is part of a </em><a href="http://news.nationalgeographic.com/news/energy" target="_blank"><em>special series</em></a><em> that explores energy issues. For more, visit <a href="http://www.greatenergychallenge.com/" target="_blank">The Great Energy Challenge</a></em>.</p><p><em>Editor's note: A previous version of this story incorrectly called the&nbsp;Deepwater Horizon oil&nbsp;spill the worst&nbsp;in U.S. history. At&nbsp;5 million barrels, it was the worst U.S. offshore spill, but on land it was surpassed by the Lakeview gusher in 1910 and 1911, which spilled 9.4 million barrels of oil in California's San Joaquin valley.</em></p>

Dolphins: Signs of Serious Illness

In the depths of the ocean and on shore, science is only beginning to measure the long-term impact of the worst offshore oil spill in U.S. history.

On the second anniversary of the Deepwater Horizon drilling rig explosion, a slew of new studies paint a complex picture of how the Gulf of Mexico's ecosystems absorbed the insult of 4.9 million barrels of crude oil.

The catastrophic failure of BP's Macondo well off the coast of Louisiana on April 20, 2010, triggered a blast and fire that took the lives of 11 rig workers and sent oil spewing from the deep sea bed for 87 days. Unprecedented steps were taken to minimize the amount of oil that reached shore, including the application of some 800,000 gallons (3,028,000 liters) of dispersants directly at the wellhead nearly a mile (1,500 meters) below the surface. Still, the oil left its mark, scientists now say, on marine mammals, salt marshes, corals, tiny organisms and coastal communities. The new studies track both lingering harm and recovery.

(Related Quiz: "How Much Do You Know About the Gulf Oil Spill?")

Bottlenose dolphins in oil-contaminated Barataria Bay off the coast of Louisiana are showing signs of serious illness, including extremely low weight, anemia, low blood sugar, and some symptoms of liver and lung disease, according to a health assessment conducted by U.S. National Oceanic and Atmospheric Administration (NOAA) scientists and their partners.

The scientists, who performed comprehensive physicals last summer on 32 dolphins from the bay, also found that half of the tested dolphins showed abnormally low levels of hormones that regulate stress response, metabolism, and immune function, indicating adrenal insufficiency. One of the dolphins in the survey was found dead on Grand Isle in January.

Lori Schwacke of NOAA, the project's lead investigator, said the findings were preliminary and could not be conclusively linked to the oil spill. But she added that control groups of dolphins living along the Atlantic coast and in other areas that were not affected by the 2010 spill did not manifest those symptoms.

"The findings that we have are consistent with other studies that have looked at the effects of oil exposure in other mammals," she said.

The study is a part of an ongoing examination of the U.S. government-led Natural Resource Damage Assessment process and the Gulf of Mexico Dolphin Unusual Mortality Event. Since February 2010, more than 675 dolphins have stranded in the northern Gulf of Mexico—a much higher rate than the usual average of 74 dolphins per year.

BP's Houston office did not respond to requests for comment on this research or the other scientific studies related to the Gulf Oil Spill.

(Related: "The Next Oil Spill: Five Needed Mandates to Head it Off" and "Gulf Spill Dispersants Surprisingly Long-lasting")

Barbara Mulligan

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.

Editor's note: A previous version of this story incorrectly called the Deepwater Horizon oil spill the worst in U.S. history. At 5 million barrels, it was the worst U.S. offshore spill, but on land it was surpassed by the Lakeview gusher in 1910 and 1911, which spilled 9.4 million barrels of oil in California's San Joaquin valley.

Photograph by Paul Sutherland, National Geographic

Gulf Spill Pictures: Ten New Studies Show Impact on Coast

Bottlenose dolphins off Louisiana's coast are severely ill, while deep-sea corals show signs of tissue damage—just two findings in a slew of studies two years after the Gulf of Mexico oil spill.

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