This year, however, 267 tons of marine life, including thousands of small fish, 72 Goliath groupers, and even a 21-ft whale shark have washed up on the beach since July – thanks to a a disastrous “red tide” of toxic algae.

The algae, called Karenia brevis, began in November and has affected beaches along about 150 miles of Florida’s Gulf Coast from Anna Maria Island to Naples. In Sarasota, two hours north of Sanibel, wildlife scientists recovered nine dead bottlenose dolphins last week.

“We had groupers probably four feet and five feet up here, and all kinds of fish wash up,” said Andrew Stone, who was taking a sunset walk on Sanibel’s Lighthouse beach with his wife, Joyce Hillman. The couple comes here from Bonita Springs every year.

“We had the binoculars and we were looking at fish bobbing up and down way out there, bigger than those buoys,” he said, pointing about 100 meters offshore. “And the bugs were horrific.”

While algal blooms are common here, they are usually constrained to a few months in late summer or early fall, and are mainly noticeable for the dark, greenish-red color they give the water. But this bloom has lasted from one season into the next without reprieve, and achieved the unusually high densities believed to be responsible for killing so much wildlife.

Sanibel Island is usually bustling at this time of year, but the sight and smell of scores of dead fish on beaches, and reports on the bloom in the news, have kept many tourists away.

The algae also causes mild respiratory irritation, so those who brave the smell can spend their time on the beach – if they can put up with a cough and watery eyes. People with asthma or other respiratory diseases are at risk of more serious complications, however.

“There’s definitely been many, many, fewer people,” said Hillman, the sunset reflecting on the reddish tint in the water behind her last week.

Clean-up crews had removed the dead fish from the sand earlier in the day, but the water was still dark, and the beach was sparsely populated. “This is the cleanest the beaches have been in two weeks,” she added.

Though Sanibel’s beaches have now been cleaned up – the bloom is continuing.

In a recently released survey, businesses in Sarasota County reported losses of up to 6% compared to last year, which they have attributed to the bloom.

“It’s scary, and it’s weird and it’s devastating really,” said Morgan Combs, who grew up on Sanibel and works at a local inn. “Just observing not having any traffic, and not seeing anyone on the beach other than cleaners.”

Scientists are still trying to determine what is responsible for this bloom, but the consensus so far seems to be that a combination of factors – including heavy rainfall, ocean temperature and salinity, unlucky wind patterns and pollution – could have aligned to create just the right conditions for Karenia brevis to thrive and to stay put.

“There’s a lot of different things that can be involved,” said Kate Hubbard, who leads the Florida Fish and Wildlife Conservation Commission’s harmful algal bloom research group. “We do know that we haven’t had the physical conditions that sometimes help with bloom termination, that help push the bloom offshore.”

At the same time that the Karenia brevis bloom is casting a pall on south-western Florida’s beaches, a record-breaking bloom of blue-green “algae” is also spreading through its freshwater.

Governor Rick Scott declared a state of emergency in July to respond to the blue-green algae (which is actually a species of cyanobacteria, not an algae) and combat pollution. But the foul-smelling algae are clogging canals, preventing fishing and similarlyaffecting tourism business.

There is more clarity on the causes of the cyanobacteria bloom: scientists believe that it is likely caused by warmer waters from climate change and polluted runoff from agriculture and urban areas surrounding Lake Okeechobee.

The pollution is made worse because many wetlands in Florida have been developed, so water moves quickly into the rivers and lakes, without natural filtration through the estuaries.

Some people are concerned the same pollution exacerbating the blue-green algae may be reaching the sea and feeding the Karenia brevis as well, but there is disagreement among experts on the issue.

Rick Bartleson, an environmental scientist at the non-profit Sanibel-Captiva Conservation Foundation, thinks it’s clear that nutrients from the land are at least a part of the problem.

“There are other factors that had to work together [to create] this perfect storm for the developing red tide population … [but], it’s obvious that you can’t have dense blooms like this, and long lasting blooms, without having a high nutrient supply rate to the algae.”

Hubbard is much more circumspect.

“It’s a naturally occurring event, it’s been documented centuries ago,” Hubbard said.

One idea to tackle the blooms that Hubbard and collaborators are working on involves field and lab studies to inform whether installing shellfish in some of the areas in Florida often impacted by blooms of toxic algae might one day help, because shellfish are natural “grazers” that filter out and eat the algae.

As Florida prepares for November elections, gubernatorial and senate candidates have been forced to address the crises caused by algae. Senate candidate Bill Nelson, running against Scott, held a roundtable on the blue-green algae on Friday, and candidates in the Republican and Democratic primaries for governor addressed the issue in their debates.

Activists in areas affected by “red tides” held a peaceful protest called “Hands along the water” on beaches along Florida’s coast on Sunday, to “show that we do not, and will not, stand for our beautiful beaches, wildlife, homes and livelihoods to continuously be destroyed and impacted by the water released from Lake Okeechobee.”