SOUTH FLORIDA -- With fears subsiding that the Deepwater Horizon oil spill will darken the waters of the Keys, Florida Bay advocates have the chance to focus on some welcome good news.

Water quality in the bay, which was darkened by two distinct algae blooms between 2005 and 2008, has returned to normal. And no bloom has been detected in the shallow 850-square-mile estuary over the past two years.

"It's been very quiet," said David Rudnick, Florida Bay section leader for the South Florida Water Management District.

The district took over Florida Bay water quality monitoring from Florida International University in late 2008, when the bloom in the eastern bay was finally dissipating after lingering for three years.

Since then chlorophyll readings, an indicator of algae concentrations, have steadily declined, Rudnick said.

Scientists said the catalyst for that eastern bay bloom was the active 2005 hurricane season, in which Katrina, Rita and Wilma all swept through the area, delivering heavy rainfall that clogged the regional canal system and forced water managers to release large pulses of nutrient-laden water into Barnes Sound, near the eastern bay.

But the bloom was also a source of much controversy because SFWMD scientists concluded that another cause was work that was going on at the time along the southern portion of the 18-Mile Stretch.

In 2007, a much larger bloom -- the worst to rock the estuary since the early 1990s -- developed in the southern bay. The pea-green water virtually wiped out the sponge community over a 25-square-mile area.

A task force of scientists from disparate agencies, including the National Park Service, the SFWMD, the Florida Fish and Wildlife Conservation Commission and the Florida Keys National Marine Sanctuary, never determined what caused that bloom.

It may have just been a natural event, said Joe Boyer, who at the time was in charge of the bay water quality monitoring program at FIU. Likewise, there's no way of knowing why the past few years have been bloom-free, Boyer said.

Work began in the past year on two major Everglades restoration projects expected to help Florida Bay, one along the Tamiami Trail and the other affecting the voluminous C-111 canal that runs through extreme south Miami-Dade County. But neither project is far enough along to have made a difference.

"It's a real tough case. We don't have a working model on what starts these things," Boyer said.

Rudnick said water managers have adjusted their strategies over the past two decades so the bay gets more regular inputs of vital freshwater. But he acknowledged that there haven't been any significant changes in that regard over the past few years.

Like Boyer, he isn't sure why Florida Bay is doing better these days.

"Answering the 'why' is always the most interesting and difficult question," Rudnick said. "We're very good at answering the 'what.' The 'why' takes a great deal of insight and often you don't have the answers until well after the fact."

rsilk@keysnews.com