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Master Naturalists: Fall is phytoplankton bloom season on Texas coast

Oct. 13, 2011 at 5:13 a.m.

Phytoplankton samples from Corpus Christi show large numbers of Pseudo-nitzschia, a potentially poisonous variety, in this scanning electron microscope image taken by scientists at the Phytoplankton Monitoring Network facility in Charleston, S.C.  The long thin cells are a Pseudo-nitzschia species, but their exact species is not clear at this magnification (1400X).  Further analysis at magnification of 20,000 times actual size was not definitive either, so scientists are doing genetic testing to figure out species and toxicity.

By Paul and Mary Meredith

Late summer/early fall along the coastal bend is a transitional period. Days shorten, the air cools, school starts, and phytoplankton begin to bloom in bays and the Gulf. Although this year is drier and the bays are saltier because of lack of river water entering their estuaries, various species of potentially hazardous phytoplankton have reached very high counts in our waters.


Sept. 9 was the first reported bloom, near Brownsville in the channel, bays and Laguna Madre. And it spread northward on the ensuing days. Two species - one of them Karenia brevis (K. brevis), the classic red tide - were found in large numbers. Classic red tide bloom symptoms were found: kills of bay fish like red drum, southern flounder, striped mullet, croaker and spotted seatrout. By Sept. 19, the bloom had moved out to the Gulf, and beach-goers were experiencing symptoms, respiratory distress, congested eyes, etc.

By Sept. 22, red tide had been confirmed from San Luis Pass to the Brazos River, with fish kills and reports of distressed beach-goers breathing the neurotoxin brevetoxin produced by K. brevis. Samples collected from Surfside's beach, near the Brazos River locks, and the old Brazos River/Dow barge canal confirmed the bloom.


Reports of a bloom in Corpus Christi Bay occurred Sept. 22. Pseudo-nitzschia, which can produce domoic acid, which causes amnesic shellfish poisoning, was found in concentrations so dense they caused discolored water inside Corpus Christi Bay at a number of locations. Our samples at Port O'Connor about the same time showed few of the poisonous diatoms.


By month-end, the red tide and fish kills had declined in South Texas, with only limited numbers of newly killed fish in limited locations. But things began to change farther north. K.Brevis were found in Galveston Bay (low concentrations) and at Sargent Beach (moderate concentrations). Galveston Beach Patrol lifeguards reported respiratory irritation. Surfside and Freeport areas, and Sea Center Texas all had bloom conditions.

On Oct. 5, Texas Department of State Health Services, closed three major commercial oyster reefs in Galveston Bay due to concentrations of K.Brevis. And while the bloom is declining farther south, birds that probably ate killed fish are showing erratic flight patterns, symptoms of neurotoxic poisoning - like the West Coast birds that inspired Alfred Hitchcock's movie "The Birds." NOAA scientists in South Carolina, testing Corpus Christi's pseudo-nitzschia to determine species and toxicity, said genetic analysis is not yet conclusive as to species.

On Oct. 6, we were advised by TPWD's Meridith Byrd that a fish kill is under way at Port O'Connor, and K.Brevis was found in samples there. Re-sampling was done by TPWD Oct. 6. And after consultation with NOAA in South Carolina, we are going to take samples to determine whether other hazardous species are occurring along with K.Brevis.


When the weather cools more, and we get some clouds and rain to help interrupt the reproduction of these hazardous species, the blooms generally taper off.

Until then, we wait.

Paul and Mary Meredith are master naturalists. Contact them at paulmary0211@sbcglobal.net.



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