BOB MAINDELLE: Stillhouse-Belton zebra mussel study concluded

Back in October 2021, I spent a full day on the waters of Belton and Stillhouse Hollow lakes with Dr. Jason Locklin, a professor at Temple College, and several of his students, providing my boat as a safe, stable work platform as they went about their duties associated with the study of zebra mussels on the two bodies of water.

Zebra mussels are an invasive species of mussel. They populate rapidly and compete with native species for waterborne nutrients.

Locklin, the Director of Natural Sciences for Temple College, was the principal investigator in a study funded in part by the Texas Parks and Wildlife Department, named “Assessing the Population Dynamics and Body Condition of Zebra Mussels Within and Between Two Texas Water Bodies with Different Population Trajectories.”

This study took a close look at the zebra mussel populations in Belton Lake, which was first infested in 2013, and in Stillhouse Hollow Lake, which was infested in 2016.

Locklin indicated that the purpose of the study was “... to determine the impacts of elevated temperatures on mussel shell growth rates and body condition measured as dry tissue mass relative to shell length in two zebra mussel populations inhabiting two closely adjacent and similar water bodies in central Texas.”

In addressing the need for the study in his grant application to TPWD, Locklin said, “Zebra mussel populations in southwestern U.S. water bodies have been shown to have elevated growth rates, shortened life spans and large-scale summer die-off events relative to populations at higher latitudes.”

Locklin added that, “A primary indirect effect of exposure to increased temperatures is increased metabolic demand. With temperature increase from 20 to 32C (68 to 90F), zebra mussel oxygen consumption can quadruple and metabolic rates increase by as much as 265%. Feeding efficiency also

declines with increasing temperatures.”

Going into the study, Locklin and his students made some predictions on where they felt the data may lead. The study, which concluded in the autumn of 2022, would confirm or refute these predictions.

Lockin wrote prior to the study’s start, “It is predicted that the observed decline in shell growth rates and densities in the older Belton Lake zebra mussel population relative to the younger Stillhouse Hollow Lake population will be related to a reduction in food availability measured by chlorophyll-a concentrations leading to extensive summer starvation (marked by loss of physiological condition) and adult mussel die-offs.”

Locklin pointed out potential benefits of the study.

“If the proposed research results confirm this prediction, it will help to explain the causes of the declines observed in older Texas mussel populations.”

Locklin continued, “If Texas zebra mussels are generally subject to long-term population declines, it will have important implications for estimating their future ecological and economic damage as well as future mussel macrofouling impacts on water-using facilities drawing from infested source waters, informing their future development of anti-fouling control strategies and infrastructure costs.”

Following a full year of work, and after logging many hours both on the lakes and in the lab, Locklin and his students finalized the study in October of 2022 and recently issued their report to the Texas Parks and Wildlife Department.

In the study’s Conclusions segment, the researchers reported both similarities and differences between the populations of zebra mussels in these two bodies of water located only miles apart.

The first similarity had to do with population growth. The report states, “Little difference was found between the population dynamics of Belton Lake and Stillhouse Hollow Lake zebra mussel populations.

The lakes were infested by mussels in 2013 and 2016, respect(ively). In both lakes, juvenile settlement was initiated in June and the shell growth rates were similar being greater at 2 meters than at 8 meters depth.”

The team also observed that, “(Zebra mussel) annual density fluctuations were similar with densities of the previous year’s Spring and Fall Cohorts declining and being lost from the population during July-September periods of maximum water temperature.”

Locklin observed, “… the mussels in both lakes gain mass through the spring but then enter a period of starvation through the summer and fall when metabolic demand can’t be met with the reduced feeding efficiencies associated with extremely warm waters.  This is likely what causes the mass summer die-offs that we observe in the fall months in Texas.”

As for zebra mussel population differences, Locklin and crew stated, “The only real difference recorded among the two populations was that larger Belton Lake mussels had higher dry tissue weights than Stillhouse Hollow Lake mussels potentially due to Belton Lake having higher chlorophyll-a concentrations than Stillhouse Hollow Lake. Increased chlorophyll-a concentrations indicating higher phytoplankton food densities at Belton Lake appeared to allow Belton Lake mussels to maintain elevated dry tissue weights relative to those in Stillhouse Hollow Lake whose lower chlorophyll-a concentrations indicated potentially lower phytoplankton concentrations.”

Even though the researchers found that the zebra mussels’ shells grew at about the same rate in the two bodies of water, this was not the case concerning the creature within those protective shells. The study stated, “What was surprising was that in spite of having similar shell growth rates, adult zebra mussels from Belton Lake had elevated dry tissue weights compared individuals from Stillhouse Hollow Lake. This result suggests that zebra mussel physiological condition may be uncoupled from their shell growth rates.”

This study, and others like it, help to form a comprehensive picture of how zebra mussels function in the unique climate of Texas waters. Understanding how these invasive organisms function can, in the long run, help shape zebra mussel management plans for infested Texas water bodies and aid in developing effective prevention and control procedures, especially for mussel fouling of raw water using utilities and infrastructure.

Locklin welcomes input from the public and encourages those interested in local research efforts to stay up-to-date with those efforts via his Facebook page at https://www.facebook.com/DrLocklin/.