Investigators from the Hillsborough County Health Department will test bacteria levels in the water at Picnic Island Park beach today.
The testing comes four days after officials closed the beach to swimming. Samples taken July 23 and Wednesday showed high levels of bacteria there.
Officials have not pinpointed the cause of the increase. Polluted stormwater runoff, sewage and waste from boats, pets and wildlife are likely contributors, said the Health Department.
Officials last closed the beach to swimming after detecting high bacteria levels in November 2004.
If today's tests indicate high bacteria levels, the beach will remain closed and officials will test the waters again Wednesday.
People can still sunbathe or picnic on the beach.