Out on the water, it starts as a slight rainbow shimmer, then turns to wide orange streamers of oil whipping through the waves. Later, on the beach, we witness a vast, Olympic-sized swimming pool of dark chocolatey syrup left behind at low tide, and thick dark patches of crude bubbling on the sand.

The smell of the oil on the beach is so strong it burns your nostrils, and leaves you feeling dizzy and headachey even after a few minutes away from it.

According to marine biologist Rick Steiner, my companion on a boat ride through the slick, this is the most volatile and toxic form of crude oil in the waters and lapping on to the beaches of Grand Isle, the area at the heart of the slowly unfolding environmental apocalypse that has engulfed Louisiana, and is now moving eastwards, threatening Mississippi, Alabama, and the Florida Panhandle.

Fifty-three days after BP's ruptured well began spewing crude oil from 5,000ft below the sea, the wholesale slaughter of dolphins, pelicans, hermit crab and other marine life is only now becoming readily visible to humans.

So too is the futility of the Obama administration's response effort, with protective boom left to float uselessly at sea or – in the case of the Queen Bess pelican sanctuary which we visit – trapping the oil in vulnerable nesting grounds.

Steiner, 57, a marine biologist from the University of Alaska and a veteran of America's last oil spill disaster, the Exxon Valdez, says he is in the Gulf of Mexico "to bear witness", and for days he has been taking to the beaches and the waters in a Greenpeace boat gathering evidence.

The first casualties on Steiner's tour appear minutes after our boat leaves the marina and moves through Barataria Pass, prime feeding ground for bottlenose dolphins. Several appear, swimming, eating, even mating in waters criss-crossed by wide burnt-orange streamers of oil. All are at risk of absorbing toxins, from the original spill and from more than 1.2m gallons of chemicals dumped into the Gulf to try to break up the slick, says Steiner.

"They get it in their eyes. They get it in the fish they eat and it is also possible when they come to the surface and open their blowhole to breathe that they are inhaling some of it," he says.

The Greenpeace crew turn up the throttle and the boat pulls up to the orange and yellow protective boom around Queen Bess island, which was intended as a haven for the brown pelican. These birds, until recently, were on the federal government's list of endangered species and were doing OK – but now that recovery appears to have been abruptly reversed.

A dark tideline of oil encircles the island, and has crept into the marsh grasses, where the pelican nest. Many, if not most, of the adult birds had patches of oil on their chest feathers. Nearly all are doomed, says Steiner, if not now, then at some point in the future. "The risks in here to birds are not just acute mortality right here right now," he says. "There is mortality we won't see for a month or two months, or even a year."

He points out a pelican standing so still it looks like it's been made out of a slab of chocolate, another frantically flapping its spread wings to try to shake off the oil, and then another manically pecking at the spots on its chest. "He could be a candidate for cleaning, and he may survive," Steiner says. "He obviously won't if he's not cleaned."

Rescue teams have plucked hundreds of birds from the muck. But stripping oil from the feathers of stricken birds is a slow and delicate operation, and there is no assurance of the birds' survival. About a third of the rescued birds have died so far.

As we pull up to Queen Bess island, two crew boats are at work shoring up the two lines of defence for the island: an outer ring of orange and yellow protective boom intended to push the oil back out to sea, as well as an inner ring of white absorbent material that is supposed to suck up any of the crude that gets through.

Since oil began lapping at the Louisiana coast, the government has set down 2.25m ft of containment boom and 2.55m ft of absorbent material. But local sports fishermen on Grand Isle complain response crews bungled the protection zone for Queen Bess because they only put a portion of the island behind the orange and yellow barrier boom. That turned the boom into traps which pushed even greater quantities of oil onshore. Steiner agrees: "I would say 70% or 80% of the booms are doing absolutely nothing at all."

The efforts on the beaches seem equally futile. By day workers in white protective suits march along the sands of the state park on the eastern end of Grand Isle, trying to suck up the oil. But as the tide goes out there is only more oil to be found, and dozens of dead hermit crab that have struggled to flee to shore.

Steiner says he has seen it all before, after the Exxon Valdez went aground in 1989, and then in other oil spills he has monitored around the world from Lebanon to Pakistan. There is, he says, a drearily familiar pattern. "Industry always habitually understate the size of a spill and impact as well as habitually overstate the effectiveness of the response."

In the case of the Exxon Valdez, he says, the environmental impacts persisted for months or years after the tanker went aground. That catastrophe, which saw 11m gallons of crude dumped into the pristine waters of Alaska, occurred within the space of six hours.

This spill is much worse. BP's well on the ocean floor has been spewing greater volumes of crude oil into the water for 53 days. Even by the administration's most optimistic forecasts, it will keep gushing until August, and the clean-up could last well into the autumn.

"This is just the start. It is going to keep coming in even if they shut the damn thing off today," says Steiner.