By Bob McDonald, Quirks and Quarks

On the first anniversary of the worst oil spill in history,  beaches and marshes along the Gulf of Mexico appear relatively clean.  It would seem that the monumental efforts to clean up the oily mess from the sinking of the Deep Water Horizon oil rig have been successful.  But scientists are more concerned about the large amount of oil and dispersant that still remains hidden beneath the waves.

Most of the five million barrels of oil that gushed from the damaged well for three months did not make it to the shoreline.  It either floated out to sea on the surface, or more seriously, remained submerged within the water and sank to the bottom, thanks to 18,379 barrels of chemical dispersant added to the oil at the wellhead. This chemical sludge has not gone away.

According to the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling report, which is worth a read in its entirety, there is some good news and a lot of uncertainty about the impact of the spill on the environment.

First, the good news.  Thanks to warm temperatures in the Gulf, prevailing winds, ocean currents and the action of oil-eating bacteria in the water, the impact was less severe than early estimates predicted.

Lighter elements in the oil floating on the surface evaporated quickly, thanks to the hot summer sun in the Gulf.  Wind and currents directed much of the remaining surface oil offshore, while bacteria feasting on their newfound treasure ate it up more quickly than expected.

But a lot of oil still remains in the sea; and the scientists are having a hard time figuring out how much is down there and where it's located.

Part of the uncertainty lies in the fact that environmental studies in the area, prior to the spill, are sadly lacking.  Oil drilling has been happening in the Gulf for decades, so it's difficult to pick out the effects of the Deepwater Horizon from oil already on the bottom from other spills, and from natural oil seeps coming from the ocean floor.

Without a baseline to work from, it's difficult to see the overall effect.

One of the tactics used by BP to prevent the oil from reaching beaches was to inject chemical dispersants right at the wellhead.  Acting like a detergent, the dispersants turn the thick crude oil into tiny droplets that do not rise to the surface and are more easily dispersed by ocean currents.

While this strategy did reduce the amount of oil soiling beaches, it is actually an out-of-sight, out-of-mind approach to pollution. The submerged plume of this oil/ chemical mixture affects everything, from corals on the seafloor, through to fish in the water column, to sea mammals near the surface and birds flying above. It can be easily ingested by oysters and other bottom-dwellers, accumulate in fish flesh, and generally contaminate most living things that come in contact with it.

Even the shoreline cleanup efforts had subtle negative effects. Oil washing ashore tended to stick to the line of seaweed, driftwood and other debris found on most beaches at the high water level. This was convenient for cleanup crews because the oil-soaked material acted like a sponge, making it easier to scoop it into garbage bags and truck it away.  It made the beaches look clean afterwards, but that debris was a rich source of food for shorebirds.

Oil booms intended to protect mangrove swamps in the Mississippi Delta actually pushed some oil further into the swamps when winds blew them on shore. That forced people with shovels and garbage bags to stomp more deeply into the fragile landscape, causing further damage with their boots.

Today, the beaches look clean and the water is blue.  But the oil has not gone away.

The environmental impact of the spill penetrated every level of the food chain, from the sea floor on up, and it will continue to do so.  Scientists say it will take years, if not decades, to fully assess the impacts of the spill.  The area is huge, gathering samples of water, seafloor sediments, swamps, and mudflats is slow, and it will take time to see the long-term effects on plants, fish and wildlife. (You can follow the cleanup efforts here.)

One month before the leaking well was capped, President Obama said, "This oil spill is the worst environmental disaster America has ever faced."  But in one sense, they got off easy. The accident happened in warm weather on calm seas, in an area surrounded by the oil industry. Response was swift; tens of thousands of people were mobilized to battle it.

Now, imagine a similar spill in the dead of winter in the High Arctic ....