by Lynde L. Dodd
University of North Texas, Institute of Applied Sciences
and
Gary Owen Dick and Richard M. Smart
US Army Engineer Research and Development Center
Lewisville Aquatic Ecosystem Research Facility
Background
Lake Conroe has a volatile history of aquatic vegetation management. Hydrilla (Hydrilla verticillata) infested the reservoir soon after the reservoir filled, resulting in coverage of approximately 10,000 acres by the early 1980’s. To control hydrilla, large numbers (270,000 or about 30 per hydrilla-acre) of diploid grass carp (Ctenopharyngodon idella) were stocked. Grass carp not only controlled hydrilla, but also eliminated most other aquatic vegetation in the lake and continued to inhibit plant growth into the late 1990’s. Identified as poor habitat for fish and other aquatic wildlife following loss of aquatic vegetation, efforts to re-establish native vegetation in Lake Conroe were initiated by Texas Parks and Wildlife Department (TPWD), the US Army Corps of Engineers Research and Development Center’s Lewisville Aquatic Ecosystem Research Facility (LAERF), and local stakeholders. In those efforts, founder colonies of native plants were established in the upper end of the lake. These colonies produced propagules (seeds, fragments, etc.) that eventually spread to other parts of the lake, resulting in significant habitat improvements: native vegetation increased from no appreciable coverage in the mid 1990’s to over 1,200 acres in the early to mid 2000’s. As grass carp declined and hydrilla (and other nuisance species such as water hyacinth, Eichhornia crassipes, and giant salvinia, Salvinia molesta), began to re-infest the lake, management efforts, primarily herbicides, were implemented to control them. Despite these efforts, the scale of infestation continued to increase, and by 2006 hydrilla had spread to cover more than 800 acres. Large numbers of grass carp were again stocked, this time using triploid fish. By spring of 2008, hydrilla covered more than 2,000 acres, resulting in additional grass carp stocking, reaching 130,000 fish (65 per hydrilla-acre) having been stocked over a three-year period. Hydrilla declined dramatically beginning in the summer of 2008 to cover only several acres; unfortunately, native vegetation had also greatly declined, leaving the lake without vegetation once again.
Probable long-term persistence of grass carp in the lake coupled with other herbivores (e.g., semi-aquatic turtles, common carp, etc.) may suppress recovery of native plants from seed and tuber banks for the foreseeable future. During this time, seed and tuber banks may become depleted past the point that will enable recovery. To counter this problem, this project proposed to re-introduce native aquatic plants into Lake Conroe to provide seeds and other propagules needed for recovery and to serve as immediate moderate-scale habitat for fish and other aquatic wildlife. Methodologies used in Lake Conroe during the late 1990’s and early 2000’s to establish founder colonies have been refined and re-employed, coupled with an adaptive management approach to make modifications to the general plan as needed to accommodate changes in the system during the project’s progress.
Approaches
Aquatic vegetation does not establish easily in man-made reservoirs. Many species that may be suitable for a lake are often not found in the immediate vicinity of newly impounded reservoirs, and therefore no propagules are readily available for their establishment, as was the case in Lake Conroe in the 1990’s. The few propagules that may be brought in (by waterfowl, boaters, etc.) or remaining propagules from plants lost to management strategies are often consumed by grazers such as common carp (Cyprinus carpio) and semi-aquatic turtles (mostly Emydids), and in the case of Lake Conroe, large populations of grass carp. Poor water quality (e.g., high turbidities) and fluctuating water levels also inhibit plant establishment, especially from seeds, the most likely natural dispersal and recovery mechanism in Texas lakes. Natural re-establishment of native plants, therefore, may take years to occur. In the meantime, absence of vegetation causes other ecosystem components to suffer (e.g., poor fishery development), or provides open niches for re-invasion by exotic species, such as hydrilla, which results in degraded water quality, poor wildlife habitat, and interference with human activities. Shortening the process of natural establishment of native aquatic plants would therefore greatly benefit Lake Conroe.
Because the large size of most reservoirs precludes full-scale establishment, in recent years the goal of native aquatic vegetation restoration efforts has been the establishment of founder colonies. Founder colonies provide immediate small-scale habitat improvement and serve as sources of propagules for spread to other parts of a lake. By having seeds, fragments, and other reproductive structures present in a reservoir at all times, biotic and abiotic limitations can be overcome to allow natural colonization throughout the system.
While more typically applied to aquatic ecosystems that lack or support a paucity of vegetation, the founder colony approach can be used for replacing nuisance aquatic vegetation with beneficial vegetation during and following application of control strategies. Removal of nuisance plants with selective herbicides, biocontrol agents, etc., will result in freed resources (nutrients, space, light, etc.) that provide an opportunity for enhancing native vegetation growth. Establishing native plants at this time not only mitigates lost habitat, but serves to fill niches left open by the control of nuisance species, reducing the likelihood or intensity of re-infestation of the weedy species. Timing can be critical: waiting until the control measure(s) is no longer present can result in rapid re-infestation of the nuisance species. We elected to begin re-establishing native vegetation, while grass carp were still present, to take advantage of their declining numbers as they age. At that time, we anticipate spread of native plants, particularly those less preferred by grass carp (e.g., wild celery, Vallisneria americana, has proven to grow well in Lake Conroe in the presence of grass carp). We (LAERF) followed this approach for re-establishing a number of native aquatic plant species in Lake Conroe, beginning in summer 2011 in a partnership with the San Jacinto River Authority (SJRA) and TPWD.
Founder Colony Installation
High quality fishery habitat is represented by diverse aquatic plant communities that generally include a variety of growth forms, including submersed, floating-leaved, and emergent vegetation. Previous revegetation work in Lake Conroe identified a number of native aquatic plant species that readily establish in the lake. Of those, a group of plants was selected based upon their suitability and their (mostly) relative non-palatability to grass carp (Table 1).
Table 1. Native aquatic plant species recommended for Lake Conroe, Texas. Many of these species have shown tolerance to herbivory in that lake.
Establishment of submersed aquatic plant founder colonies in Lake Conroe required: 1) a source of plants and 2) protection from grazing by grass carp and other herbivores for some species. We used the following as a general outline for founder colony establishment.
- Plants used in this project were nursery-grown, containerized species native to southeast Texas. Plants were produced at LAERF facilities in Lewisville, Texas or the SJRA nursery located in Conroe, TX. Containerized plants have been shown to establish more successfully than plugs, tubers, and seeds; additionally, fewer numbers of containerized plants are needed to establish founder colonies.
- Previous restoration work in Lake Conroe and elsewhere indicated that some species of plants exhibit a degree of tolerance to grass carp feeding. While high densities of grass carp currently in the lake will most likely preclude selective feeding, emphasis on plant species selection was placed on those that are known to grow and spread in the presence of grass carp. Several species, including wild celery, water stargrass (Heteranthera dubia), American water lily (Nymphaea odorata), American lotus (Nelumbo lutea), spikerushes (Eleocharis spp.), bulrushes (Schoenoplectus spp.), and water willow (Justicia americana) appear to be unpalatable or capable of withstanding grazing by grass carp. Regardless of their ability to grow in the presence of grass carp, many aquatic plant species are known to require protection from other herbivores during initial establishment in the lake. We therefore used exclosures to protect some plantings, particularly those more susceptible to grazing from herbivores other than grass carp (e.g., turtles feed readily on wild celery). Others, such as emergent spikerushes, bulrushes and water willow, none of which are susceptible to most herbivores, were planted without protection. For durability, materials for exclosures consisted of 2-in x 4-in PVC-coated welded wire, supported by T-posts.
In August 2011, a large, single founder colony was installed along a five-mile stretch of the Caney Creek Arm in Lake Conroe (see Appendix for site map). At the time of installation, Lake Conroe was 3.8-ft below normal pool, with additional water loss anticipated due to high use demands from the City of Houston and an ongoing drought. Because successful establishment of aquatic plants can be hampered by low (or high) water levels, additional plantings were planned for 2012 in Lake Conroe to compensate for losses due to desiccation following the August planting.
The founder colony consists of thirty-nine 10-ft x 20-ft x 5-ft tall pens installed on approximate 650-ft centers. Pens were installed along the then-current shoreline, or in about 3 ½ to 4-ft of water at normal pool (201ft ASL). Pens were anchored in place by six T-posts, each topped with safety caps. After construction, pens were planted with one each mature containerized American pondweed, wild celery, and water stargrass. Approximately one-half of the pens (those located closest to the mouth of the creek) were additionally planted with a single, containerized American water lily, and the other half of the pens were planted with a single American lotus plant and American lotus seeds by broadcast. Additional seeds were broadcast outside of each American lotus pen. In addition to planting in pens, unprotected, containerized water willow was planted along the shoreline between pens on approximate 100-ft centers. Because the lake was unusually low, and water willow was not expected to establish at depths greater than three-feet, plants were installed in moist sand approximately 2-ft below normal pool. Additional planting of water willow (and other emergent species) was initiated in 2012 to achieve project goals of 25-ft on-center shoreline plantings.
Founder Colony Evaluation
Lake Conroe’s founder colony was evaluated April 2012 by LAERF researchers. There were aquatic plants in all pens and water willow was establishing between most pens in about at depths of 12” to 18” of water. Most water willow plants had spread and many were flowering. American water lily, water stargrass, and American pondweed were the most successful within the pens, evidence that these species can handle extended periods of desiccation (the pens were out of the water for a portion of the summer 2011). American lotus was found in only a few of the pens, indicating that seeds and/or plants used were not able easily established under the conditions encountered following planting.
Founder Colony Replanting
A good portion of the mature potted aquatic plants that had been grown at the SJRA nursery were field ready and a large-scale re-planting of the founder colony was coordinated with SJRA, TPWD and LAERF on May 22, 2012. Lake elevation at the time of planting was 198.5 ASL, approximately 2.5-ft below conservation pool (201 ASL). Unprotected plantings were made on 40-ft centers along the shoreline (0 to 6-in deep) within the founder colony with approximately 650 individual of emergent species, including: water willow, water smartweed (Polygonum amphibium), giant bulrush (Schoenoplectus californicus), delta arrowhead (Sagittaria platyphylla), pickerelweed (Pontederia cordata), slender spikerush (Eleocharis acicularis), flatstem spikerush (Eleocharis macrostachya), and squarestem spikerush (Eleocharis quadrangulata). This planting was made to fill in between existing water willow patches planted August of 2011, which at the time of this planting were found at an average depth of 18 inches.
Approximately 300 plants, including wild celery, Illinois pondweed, American pondweed, water stargrass, American lotus and American water lily, were added to the pens, which were now in 2-ft of water. Pens already supporting American pondweed, American water lily and American lotus were not additionally planted with those species, resulting in all pens supporting the same suite of plants. Replanting these species in pens will be made as determined by periodic monitoring; additional species may be included to increase diversity and improve founder colony success.
Conclusions
Low water conditions in Lake Conroe have already tested the use of adaptive management in this project, resulting in modifications of plantings in 2011 at depths or elevations that we do not normally target. Submersed and floating-leaved plants were planted in less than 1-ft of water, with the risk of further fluctuation that would leave the plants high and dry. Emergent species plantings, initially slated for 25-ft on center plantings, were restricted to a single species (water willow) on 100-ft centers during the first year in order to minimize potential loss due to the extended drought. Although some plants were lost following this adaptation to our plan, many survived, broadening our knowledge of founder colony limitation and providing information value to future plantings, such as those conducted in 2012.
Additionally, water willow exhibited resistance to herbivory, including grass carp tolerance, as indicated by not only its presence, but by its spread and flowering. LAERF will continue monitoring and make additional plantings as needed to ensure long-term sustainability of the founder colony. Long-term monitoring and maintenance will contribute to, not only successful establishment of the founder colony, but is necessary to realize spread to unprotected areas in the lake. Therefore, as the grass carp population declines, native plants will have an opportunity to colonize areas previously infested with hydrilla, thereby improving the fishery and other ecosystem components of Lake Conroe.
