On Oct. 22, I spent a full day on the waters of Belton and Stillhouse Hollow lakes with Jason Locklin, a professor at Temple College, and three 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 chair of the biology department, is 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 takes a close look at the zebra mussel populations in Belton Lake, which was first infested in 2013, and in Stillhouse Hollow, which was infested in 2016.
Locklin indicated that the purpose of the study is “... 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 feel the data may lead. The study will confirm or refute these predictions.
Lockin wrote, “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, stating, “If the proposed research results confirm this prediction, it will help to explain the causes of the declines observed in older Texas mussel populations.
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.”
During our time on the water, the four-person crew checked five locations on each lake. A variety of tasks were performed, depending on the location. These tasks included: 1) checking on 20 centimeter by 20 centimeter, flat, square plates which had been previously emplaced at a range of depths at each location to see if mussels were colonizing on them, 2) the attachment of electronic logging devices to measure temperature and dissolved oxygen over time, 3) pulling data off of logging devices previously installed, and 4) attaching small pouches of live mussels to the equipment at each site to check for growth over time.
As the study concludes in October of 2022, the participants hope to deliver to TPWD four distinct determinations, stated by Locklin in his grant application to that organization:
First, determination of the change in the zebra mussel population dynamics in two closely adjacent lakes of similar hydrological status after respective invasion times of eight years (Belton) and five years (Stillhouse Hollow) with emphasis on understanding the causes of observed mussel population declines after initial establishment in Texas water bodies.
Second, a comparative determination of the degree that spatial effects impact zebra mussel population dynamics and physiological condition by periodic sampling of mussel populations different sites throughout each lake.
Third, determinations of the extent that high water temperatures and primary productivity impact summer declines in mussel physiological condition measured as shell length-specific dry tissue weights leading to observed declines and die-offs of the previous year’s spring and fall mussel cohorts. The results will inform the degree to which summer declines in mussel physiological condition are associated with the long-term declines in mussel densities that have been observed in Texas water bodies.
And finally, determination of the causes of long-term decline of zebra mussel densities in Texas water bodies will assist in the development of cost-effective mussel management plans in both public access facilities and water using facilities (i.e., potable water treatment plants, power stations and water using industries). If Texas mussel populations decline through time relatively quickly after initial invasion as observed in lakes Texoma, Ray Roberts and Belton, it may not be cost-effective to develop and install costly control equipment or implement costly management programs immediately after invasion.
Once this study is concluded in the fall of 2022, I will share the findings in this column.