Reports

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TR-392

Lake Granbury and Lake Whitney Assessment Initiative Final Report

B.L. Harris, D. Roelke, J. Grover, B. Brooks

Research Approach

A team of Texas AgriLife Research, Baylor University and University of Texas at Arlington researchers studied the biology and ecology of Prymnesium parvum (golden algae) in Texas lakes using a three-fold approach that involved system-wide monitoring, experimentation at the microcosm and mesocosm scales, and mathematical modeling. The following are conclusions, to date, regarding this organism's ecology and potential strategies for mitigation of blooms by this organism.

Ecology of P. parvum in Texas lakes

Our in-lake monitoring revealed that golden algae are present throughout the year, even in lakes where blooms do not occur. Compilation of our field monitoring data with data collected by Texas Parks and Wildlife and Brazos River Authority (a period spanning a decade) revealed that inflow and salinity variables affect bloom formations. Thresholds for algae populations vary per lake, likely due to adaptations to local conditions, and also to variations in lake-basin morphometry, especially the presence of coves that may serve as hydraulic storage zones for P. parvum populations.

More specifically, our in-lake monitoring showed that the highly toxic bloom that occurred in Lake Granbury in the winter of 2006/2007 was eliminated by increased river inflow events. The bloom was flushed from the system. The lower salinities that resulted contributed to golden algae not blooming in the following years.

However, flushing is not an absolute requirement for bloom termination. In Lake Whitney, the highly toxic bloom that occurred that same winter was also stopped by high river inflow events. Flushing, however, did not terminate this bloom as the lake rose 10 feet but no water was released from the dam at this time. It was the influx of nutrients that stopped toxin production. This, coupled with the high rates of photodegradation for P. parvum toxins (which we determined in laboratory experiments), allowed other phytoplankton to out-compete golden algae.

Our laboratory experiments have shown that growth of golden algae can occur at salinities ~1-2 psu but only when temperatures are also low. This helps to explain why blooms are possible during winter months in Texas lakes.

Our in-lake experiments in Lake Whitney and Lake Waco, as well as our laboratory experiments, revealed that cyanobacteria, or some other bacteria capable of producing algicides, were able to prevent golden algae from blooming. Identification of this organism is a high priority as it may be a key to managing golden algae blooms.

Our numerical modeling results support the idea that cyanobacteria, through allelopathy, control the timing of golden algae blooms in Lake Granbury.

Our in-lake experiments in Lake Whitney and Lake Waco also revealed that as golden algae blooms develop, there are natural enemies (a species of rotifer, and a virus) that help slow the population growth. Again, better characterization of these organisms is a high priority as it may be key to managing golden algae blooms.

Potential management of P. parvum in Texas lakes

Manipulation of cove waters may be a key to mitigating P. parvum blooms. Should coves serve as a hot spot for bloom initiation, preventing bloom development in those areas could prevent large lake-scale blooms altogether. Should blooms develop elsewhere, manipulation of coves may still prove beneficial, as it may create refuge habitat for fish, thereby accelerating the recovery of fisheries after blooms have subsided.

Our laboratory and in-lake experiments and field monitoring have shown that nutrient additions will remove toxicity and prevent golden algae from blooming. In fact, other algae displace the golden algae after nutrient additions. Additions of ammonia are particularly effective, even at low doses (much lower than what is employed in fish hatchery ponds). Application of ammonia in limited areas of lakes, such as in coves, should be explored as a management option.

Our laboratory experiments and field monitoring also show that the potency of toxins produced by P. parvum is greatly reduced when water pH is lower, closer to neutral levels. Application of mild acid to limited areas of lakes (but not to a level where acidic conditions are created), such as in coves, should be explored as a management option.

Our field monitoring and mathematical modeling revealed that flushing/dilution at high enough levels could prevent P. parvum from forming blooms and/or terminate existing blooms. This technique could work using deeper waters within a lake to flush the surface waters of limited areas of the same lakes, such as in coves and should be explored as a management option. In this way, water releases from upstream reservoirs would not be necessary and there would be no addition of nutrients in the lake.

Biomanipulation of P. parvum, i.e., through taxon-specific algal pathogens and/or grazing by toxin-resistant zooplankton, has promise and should be explored in future research.

Finally, our laboratory and in-lake experiments have also shown that additions of barely straw extract (useful for controlling some nuisance algae) have no effect on golden algae blooms and should not be employed as a management technique.

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