Master Naturalists: How brown tide works

By Paul and Mary Meredith
April 19, 2012 at midnight
Updated April 18, 2012 at 11:19 p.m.

Brown tide blooming was confirmed in Laguna Madre recently by a sample taken from the western shoreline along the King Ranch property.

What's brown tide

Brown tide, Aureoumbra lagunensis, is a microscopic alga only 4 to 7 microns (millionths of an inch) in diameter. Several hundred brown tide cells sitting side-by-side would stretch across this sentence's period. When 1 to 2 million cells are found in each milliliter of water, the water turns brown, the same color as the alga.

What's brown tide do

Research has shown brown tide can be deadly to fish larvae under certain conditions. But it's unclear if their death results from a direct toxic effect of the alga. Their death may be caused by changes in water quality because of high numbers of brown tide cells in the water.

There's no evidence brown tide harms people. And research has found no apparent effect on juvenile or adult fish.

Where, how brown tide lives

Brown tide can tolerate a wide range of salinities and temperatures. It does best in warm temperatures. And it has very few enemies. It's unique to the Gulf of Mexico, first discovered in Laguna Madre and named Aureoumbra lagunensis. Laguna is a high-salinity bay stretching 120 miles from Corpus Christi to Port Isabel. Brown tide has also been found in Florida and Mexico.

Some researchers think it's part of a natural cycle in Laguna Madre. Wind can move its patchy blooms. So an area made murky by a bloom in the morning may be clear in the afternoon because wind blew the brown tide "away."

Brown tide's effect on fish larvae

Laguna Madre produces the most fish in Texas, even more than Galveston Bay. Adult fish go to Laguna to spawn; juveniles use its seagrasses for their nursery.

Almost all of Texas' seagrass meadows are in Laguna's waters. Brown tide that's highly-concentrated enough blocks the sunlight the seagrasses need to survive, and over several months the grasses die.

Zooplankton (small floating animals, such as amoeba), food for other organisms, decline during brown tides. Laguna Madre's animals and plants suffer, along with all animals that need Laguna's fish production.

Previous famous bloom

Laguna Madre had what's believed to be the longest continual algal bloom in history (1990-1997), which originated in Baffin Bay. Researchers with the University of Texas Marine Sciences Institute in Port Aransas taking routine water samples along Laguna's entire length found an scientific unknown: "brown tide."

Possible reasons identified for the 1990-1997 bloom include a late-1989 hard freeze, killing both brown-tide-eating fish and other invertebrates. Moreover the area's two- to three-year drought had left Baffin Bay's salinities unusually high.

High salinities killed some organisms, lowering the bay's diversity. Then, brown tide had fewer predators. Decomposing organisms killed in the freeze raised the bay's level of nitrogen, possibly fertilizing the brown tide and promoting brown tide's uncontrolled bloom.

Paul and Mary Meredith are master naturalists. Contact them at



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