Brown water: natural tannins or sign of looming red tide?

Tannin-stained waters are blasting out of some Southwest Florida passes as rain water continues to wash off the watershed and into the Gulf of Mexico.

Water quality scientists and others worry nutrients in that water could eventually feed a red tide bloom that's already on the horizon.

"The volume of water and the amount of nitrogen that’s being delivered to the nearshore Gulf is an issue," said Calusa Waterkeeper John Cassani. "We’re starting to see background levels of Karenia (red tide), so the timing is really bad. Enriching nutrients for the nearshore water, it couldn’t happen at a worse time as we're heading into the red tide season."

Florida Fish and Wildlife Conservation Commission reports from Tuesday show low to medium concentrations of red tide in Sarasota Bay.

Karenia brevis is the organism that causes red tides in the Gulf of Mexico. It's naturally occurring at background levels but can devastate the entire coastline during major blooms.

Last year, a massive red tide bloom killed millions of pounds of marine life, including hundreds of dolphins and sea turtles, manatees and even a whale shark.