The explosion and subsequent fire injured 16 others and caused the Deepwater Horizon to sink, triggering the largest accidental marine oil spill in petroleum industry history.
Around the gulf, scientists are saying the environmental impact of the spill was subtle and may not be understood for years.
A rookery for brown pelicans called Cat Island appears to be crowded with birds, but the once lush undergrowth and a stand of black mangroves on the island are decimated, said Todd Baker, a biologist with the Louisiana Department of Wildlife and Fisheries.
"This was a lush green island; you couldn't see the ground," the Los Angeles Times quoted Baker as saying.
As much of the gulf looks to have recovered, Florida State University oceanography Professor Ian MacDonald said scientists should look for "a marginal reduction in the productivity and biodiversity" of the ecosystem.
Reports of higher fatalities among dolphins and sea turtles have yet to be fully explained. Dolphin deaths rose before the spill, likely due to cold temperatures, but then spiked again after the spill, the newspaper said.
President Barack Obama in a statement called the explosion and oil spill a "catastrophic event (that) deeply affected the lives of millions of Americans." In a separate statement, Interior Secretary Ken Salazar said, "Our thoughts and prayers are with their loved ones on this solemn anniversary."
Wednesday's moment of silence comes a day after BP Chairman Carl-Henric Svanberg defended the global oil and gas giant's widely criticized handling of the crisis, which sent an estimated 4.9 million barrels into the gulf over three months, devastating marine and wildlife habitats, as well as the gulf's fishing and tourism industries.
BP, of London, has learned from the accident and taken steps to become safer, Svanberg told media in his native Sweden.
"But we weren't unsafe before either -- 50,000 holes had been drilled in the Gulf of Mexico before [the disaster] happened," he said in an interview with business daily Dagens Industri.
"The whole industry has learned from the accident and we are doing everything to ensure that it doesn't happen again," said Svanberg, who caused an uproar after meeting with Obama about the crisis last June when he said, "We care about the small people" along the U.S. Gulf Coast.
He later apologized, saying he made a "slip in translation" of a Swedish phrase, "den lilla manniskan," which should have been translated as "the common man."
He told Stockholm's Svenska Dagbladet in an interview published Tuesday the translation blunder "became a much bigger deal in Sweden than in England or the United States," although "it is clear that was unfortunate," he said.
A year after the disaster, assessments of the damage to the gulf, its people and their livelihoods vary.
The last 1,041 square miles of water nearest the sunken rig's wellhead were reopened to commercial and recreational fishing Tuesday, a day before the anniversary, the National Oceanic and Atmospheric Administration said, making all federal waters in the gulf formerly closed now open to fishing.
The agency said it found "no detectable oil or dispersant odors or flavors, and results of chemical analysis for oil-related compounds and dispersants [were] well below the levels of concern."
Some scientists still question the effect of the vast underwater plumes that spread across the gulf. Hundreds of miles of beaches are reclaimed, with only a fraction still soiled.
The explosion anniversary is the last day for filing claims to be included in a trial next year over the liability of Deepwater Horizon owner Transocean Ltd., one of the world's largest offshore-drilling contractors.
The trial, set to begin in February, will seek to determine whether Transocean of Geneva, Switzerland, which leased the rig to BP, can limit what it pays to people making claims under maritime law and assign percentages of fault to other companies in the disaster.
On Tuesday, Florida Gov. Rick Scott said the state would not join the lawsuit against Transocean but would go directly after BP for damages, a decision that has been met with praise and criticism, The Miami Herald said.
U.S. District Judge Carl Barbier in New Orleans is overseeing about 60,000 claims stemming from the disaster.
Also Wednesday, BP and other companies involved in the litigation are expected to file counterclaims against Transocean and cross-claims against each other, lawyers said.
Separately, BP ended a moratorium on political giving, making contributions to House Speaker John Boehner, R-Ohio, and other GOP leaders, the Federal Election Commission said Tuesday.
BP Corp. North America Inc. gave $5,000 to Boehner and House Majority Whip Kevin McCarthy, R-Calif., through its political action committee, an FEC campaign finance report indicated.
House Ways and Means Committee Chairman Dave Camp, R-Mich., received $1,000 and House Energy and Commerce Committee Chairman Fred Upton, R-Mich., got $5,000.
Upton told The Hill Tuesday he would return BP's contribution.
Other recipients were the National Republican Congressional Committee and the National Republican Senatorial Committee, which each accepted $5,000, the FEC report said.
The only Democrat getting a donation was Rep. Pete Visclosky of Indiana, who received $3,000, the report said. Visclosky is a ranking member of the House Appropriations Committee's Subcommittee on Energy and Water Development.