For nearly 40 years, biologists have been measuring Tampa Bay's water quality and hoping it will get cleaner.
Last year, for only the fourth time, the state's largest estuary met all of its water quality targets, the experts from the Tampa Bay Estuary Program announced Tuesday.
They're just not sure why.
Since 1974 the estuary program's scientists have checked the quality in the four sections of the bay — Old Tampa Bay, Hillsborough Bay, Middle Tampa Bay and Lower Tampa Bay — to see whether the water is clean enough to promote the natural recovery of sea grass.
Sea grasses are crucial to the health of the bay, but decades ago they declined sharply and by 1982 there were only 22,000 acres left.
By 2011, though, strenuous local government efforts to clean up pollution flowing into the bay had encouraged the spread of sea grass until it covered nearly 33,000 acres of the bay, leading to a rebound of fish population, particularly redfish and spotted sea trout.
The bay's water quality has fluctuated, as measured by the amount of microscopic algae in the water (as indicated by chlorophyll, a plant pigment) and the amount of visible sunlight penetrating the water. Both are important indicators for the growth of sea grass.
Not until 2006 did every corner of the bay register as clean enough to grow sea grass, and the bay repeated that feat in 2007 and 2010.
In other years, though, the water quality in Old Tampa Bay, which encompasses the part of the bay near Clearwater, Safety Harbor and Oldsmar, slipped. That area has been repeatedly plagued by algae blooms that can kill fish. Estuary program experts blamed pollution-laden runoff they called "urban slobber."
But last summer there was no big algae bloom, said Nanette O'Hara of the Tampa Bay Estuary Program. When biologists checked the pollution levels, they found that for the first time since 2010 the whole bay was good enough to foster sea grass recovery.
O'Hara said scientists are hopeful that the next check of the spread of sea grass through the bay shows another increase, thanks to the water quality.
Craig Pittman can be reached at firstname.lastname@example.org