When the Tampa Bay National Estuary Program was established in 1991, toxic pollution was not high on the list of what scientists believed were the bay's major problems.

But subsequent research has changed bay managers' perceptions. Toxic contaminants now rank among the more significant issues to be addressed in the TBEP's management plan for Tampa Bay.

A four-year study conducted by the National Oceanic and Atmospheric Administration (NOAA) triggered that change in thinking.

The good news is that, by and large, the problem is not severe. Overall, scientists report that less than one percent of Tampa Bay is highly contaminated, with the most tainted sediments found in the lower Hillsborough River, northern Hillsborough Bay near the Port of Tampa and St. Petersburg's Bayboro Harbor.

However, the research revealed the presence of a number of pesticides, hydrocarbons (a component of gasoline and other fuels) and trace metals - some in areas previously believed to be among the cleanest in the bay. And it has reinforced the knowledge that many of these substances linger in the environment for many decades.

South Hillsborough's Cockroach Bay, for instance, tested high for levels of the agricultural pesticide DDT, which was banned from use in the early 1970s. Oysters in Cockroach Bay and St. Petersburg's Weedon Island Preserve had elevated levels of chlordane - another banned pesticide. And leaded gasoline, also outlawed for years, is believed to be the source of much of the lead found at the bottom of the bay.

The chemicals, some naturally occurring and some manmade, tend to latch on to fine particles suspended in water and are frequently carried to Tampa Bay in wastewater or stormwater runoff. When they settle to the bay bottom, they may contaminate filter- or bottom-feeding creatures such as oysters and catfish, traveling up the food chain to larger fish, birds and even humans.

The amounts of these contaminants in the bay generally are not lethal, but the NOAA research found that they are impairing the health of some bay creatures. Liver lesions were discovered on some catfish, for instance, while reproductive and growth problems were identified in some bay oysters.

Contamination in Tampa Bay is in the middle range of all estuaries sampled nationwide. While the bay is far less chemical-laden than highly industrialized areas such as New York and Boston harbors, it contains more toxics than many Gulf of Mexico estuaries. In fact, concentrations of copper and zinc in Tampa Bay oysters were 10 times higher on average than in any other coastal areas of the Gulf.

An ongoing study financed by the TBEP's will more precisely pinpoint the sources of toxic pollution in the bay. TBEP'S also is helping to develop sediment quality guidelines that will assist in preventing further contamination. Results of these investigations will allow bay managers to tailor pollution reduction and management strategies to specific industries and segments of the bay.