Watershed staff report: Austin Lakes Index, hydrilla, shoreline plantings and microplastics in reservoirs

Dr. Brent Bellinger, Conservation Program Supervisor for the City of Austin Watershed Protection Department, updated the Environmental Commission on the Austin Lakes Index, aquatic vegetation trends in Lake Austin and Lady Bird Lake, shoreline planting survivorship on Lake Austin, and initial sediment sampling for microplastics.

Bellinger summarized the Austin Lakes Index (ALI), an annual index compiled by Watershed Protection that aggregates multiple measures of reservoir ecological condition. He said ALI scores for Austin reservoirs typically range from the mid-40s to mid-60s and that the index helps guide where the city invests engineering and restoration resources. He presented three main topics: aquatic vegetation, shoreline/riparian protections and a microplastics sediment study conducted in partnership with the University of Texas.

On vegetation, Bellinger described differing management aims across reservoirs. Lake Austin has seen cycles of hydrilla (an introduced aquatic plant) and stocking of sterile (triploid) grass carp to control vegetation; recent grass-carp stockings and population changes have been followed along with periodic mechanical harvesting in high-use areas. Bellinger said Texas Parks and Wildlife had advised targets of roughly 15–25% vegetated area for some reservoirs but that Lake Austin’s narrow, sunlit morphology (and zebra mussel-driven water clarity) complicates management.

Lady Bird Lake, Bellinger said, lacks hydrilla but supports native aquatic plants such as fanwort (Cabomba caroliniana) and bladderwort; the city’s approach there has been a light touch with mechanical removals for high-use recreational channels. Lake Walter E. Long (Decker Lake) is managed differently; it has a narrower shoreline vegetation zone and is drawn down for dam work as of the presentation.

Bellinger reported a systematic review of closed Lake Austin shoreline site plans (2010–2020). He said watershed staff visited 84 closed site plans that filed mitigation or shoreline plantings under contemporary codes and graded compliance on a six-point scale. He said most sites did not fully retain the number and diversity of required plants over time and that demonstration projects such as the established Emmaline (Emmalong) Park shoreline restoration performed strongly. He noted that clustering plantings (denser groupings) can improve survivorship compared with widely spaced plants.

On microplastics, Bellinger described grab samples of surface sediment collected at reservoir and tributary points and processed by University of Texas researchers. He said counts of microplastic particles per 100 grams of sediment increased approaching the urban core, and Lady Bird Lake samples at mouths of major urban tributaries showed particularly high counts, with road- and tire-related particles prominent in analysis. He and commissioners discussed limitations of sediment-only sampling and recommended additional water-column sampling and flow-standardized loading estimates for future work.

Commissioners asked about biological controls for hydrilla, tree/shoreline plant survivorship, enforcement of shoreline plantings, outreach to landscaping contractors, and whether the city should limit recreational use if ecological stressors intensify. Bellinger said some biocontrol work had been attempted historically (insects, fungal approaches) but results were limited; he described grass carp, mechanical removal and targeted management as current tools. He noted Watershed Protection and partners are pursuing more frequent vegetation surveys with TPWD and continuing collaborations with the University of Texas on contaminants of emerging concern.

The presentation concluded with commissioners encouraging continued monitoring, stronger outreach and follow-up on enforcement for shoreline mitigation plantings that were not persisting.