Summary
The Edwards Aquifer in south-central Texas is one of the most productive karst aquifers in the world. Covering an area of approximately 3,600 square miles, it is the primary source of drinking water for over 2.3 million people in San Antonio and its surrounding communities. In addition, it supplies irrigation water to thousands of farmers and livestock operators in the region, which can account for as much as 30 percent of the total water pumped from the system each year. The Edwards has extremely high-yield wells and springs that respond quickly both to rainfall events and to water withdrawals. The region experiences periodic droughts, some of which have been severe enough to drastically reduce flow in the Edwards Aquifer and its major springs (Comal Springs in New Braunfels and San Marcos Springs in San Marcos).
Comal Springs and San Marcos Springs and their river systems house several plants and animals found nowhere else in the world. Eight of these species are listed as threatened or endangered under the federal Endangered Species Act (ESA): the fountain darter, the San Marcos gambusia (which is presumed extinct), the Texas blind salamander, the San Marcos salamander, the Comal Springs dryopid beetle, the Comal Springs riffle beetle (CSRB), the Peck’s Cave amphipod, and Texas wild rice. To protect these species, the Edwards Aquifer Authority (EAA) and four other local entities created a 15-year Habitat Conservation Plan (HCP). After the HCP was approved in 2013, the EAA requested the input of the National Academies of Sciences, Engineering, and Medicine (the National Academies) to review the plan and its implementation (the statement of task is found in Chapter 1). This report is the third and final product of a three-phase National Academies
NOTE: LTBG = long-term biological goal; SAV = submerged aquatic vegetation.
study to provide advice to the EAA on various scientific aspects of the HCP that will ultimately lead to improved management of the Edwards Aquifer.
The third and final phase of the National Academies’ study focuses on the biological goals and objectives found in the HCP for each of the listed species. The first part of the Committee’s statement of task asks whether the biological objectives, which have flow, water quality, and habitat components, can meet the biological goals, which are often stated as population measures for the listed species. The second task asks whether the conservation measures in the HCP, also called minimization and mitigation (M&M) measures, can meet the biological objectives.
The biological goals, biological objectives, and M&M measures are shown in Figure S-1 for four of the listed species: fountain darter (shown separately for each system), CSRB, Texas wild rice, and San Marcos salamander. These four species have been identified as sentinel or indicator species that can serve as proxies for the other listed and petitioned species (as discussed in Chapter 2). The gold arrows in Figure S-1 link biological objectives to the biological goals for each species. In particular, these arrows indicate that the spring flow, water quality, and habitat components of the biological objective will work in concert to reach a biological goal (as discussed in greater detail in Chapter 3). The colored arrows in Figure S-1 link M&M measures to certain components of the biological objectives, as indicated by their particular color. For example, the recreation management measures (purple) are intended to help achieve the habitat component of the biological objectives for Texas wild rice and San Marcos salamander. The spring flow protection measures (red) are intended to help achieve the flow component of the biological objectives for all species. Chapter 4 describes the extent to which the five classes of M&M measures can meet their corresponding biological objectives. Chapter 5 considers several overarching issues, including new analyses for the fountain darter and macroinvertebrates and planning for catastrophic events, such as invasive species and floods. Each chapter has conclusions and recommendations that synthesize more technical and more specific statements found within the chapters; the most important conclusions and recommendations are repeated in this summary. This report is intended to be useful to the EAA and other stakeholders of the HCP, other water suppliers dealing with ESA issues, state and federal regulators, and academic and consulting communities.
THE LISTED SPECIES
For each of the four listed species, Chapter 2 includes a description of the organism’s life history and habitat; the biological goals and objectives found in the HCP; and the monitoring, modeling, and applied research done for each organism by the EAA and its contractors. The biological
goals for each species tend to reflect the desired population of an organism in the system, such as the number of organisms per unit area, as well as a habitat goal, such as acreage of submerged aquatic vegetation (SAV) or the maintenance of silt-free substrates. The following conclusions and recommendations are made regarding the four sentinel species and their biological goals.
The habitat-based approach for the biological goals for fountain darter (fountain darter density times submerged aquatic vegetation acreage), rather than an actual measure of fish abundance, is reasonable. However, the use of the cumulative median density in determining whether the biological goals are being met is problematic because this metric is very insensitive to year-to-year changes in fountain darter densities. It is imperative that the EAA consider a metric that reflects fountain darter density in recent years (e.g., a running mean or median over the most recent four years, or similar) for each vegetation type, and they should monitor it relative to an unchanging baseline (e.g., the cumulative median from the first ten years in the Variable Flow Study dataset, or other appropriate baseline data). The development of the fountain darter population model was very effective in integrating the available information and should be leveraged in the future.
The long-term biological goals for Texas wild rice (desired acreage in various reaches of the San Marcos River) are appropriate, and this species has benefited from extensive monitoring and decades of study. The EAA and its partners have taken particular care in mapping of Texas wild rice in the San Marcos River and documenting its restoration since implementation of the HCP. Considerable work done over the last century has revealed the life history and physiology of Texas wild rice, which has expanded our knowledge of the genetic framework of this species and its relatives. There are still some questions about relative competition of Texas wild rice versus other native and nonnative SAV, which could be addressed with mesocosm studies.
The long-term biological goals for Comal Springs riffle beetle density (number of beetles per cotton lure) should be updated during Phase 2 of the Habitat Conservation Plan to reflect more quantitative and standardized monitoring methods. The density goals were based on data derived from the Variable Flow Study, which used an unstandardized sampling methodology with no standard operating procedure. It would also be useful to conduct new CSRB studies under the Applied Research Program to better substantiate the biological goal of maintaining silt-free substrate. Beyond the uncertainties that went into deriving these biological goals, uncertainties associated with continued population monitoring and a lack of monitoring of the effects of riparian restoration on maintaining silt-free substrate make it difficult to understand compliance. A reevaluation of how annual
median values of beetle abundance are calculated for compliance purposes is needed.
Both long-term biological goals for the San Marcos salamander—target densities in three reaches and maintenance of silt-free gravel—are reasonable and biologically justifiable. To meet the abundance goals, the EAA should discontinue calculating cumulative median densities, and instead adopt a metric that reflects salamander density in recent years. Furthermore, the EAA needs to begin monitoring adherence to the goal of maintaining silt-free substrates. Given the considerable spatial variation in salamander abundance data and the inability to accurately estimate salamander numbers, the current sampling method could be supplemented with an additional protocol that uses occupancy estimation. It is also important to eliminate any sampler biases during salamander monitoring. Finally, the San Marcos salamander would benefit from additional studies on its life history, particularly using refugia populations, similar to what has been done for the fountain darter, CSRB, and Texas wild rice and other SAV.
WILL THE BIOLOGICAL OBJECTIVES MEET THE BIOLOGICAL GOALS?
Chapter 3 addresses whether the biological objectives can meet the biological goals for the listed species. The biological objectives are different for each species, although they have three similar components: flow, water quality, and habitat. For all species in the Comal system the total spring flow discharge can go no lower than 225 cubic feet per second (cfs) as a long-term average, while the minimum flow can go no lower than 30 cfs. For the San Marcos system, the long-term average can go no lower than 140 cfs and the minimum no lower than 45 cfs. The water quality objective is that water quality cannot exceed a 10 percent deviation from historically recorded water quality conditions. The conditions are measured at the spring openings for species that dwell near or in the springs (such as CSRB) and in the river systems for the other species (fountain darter). The habitat objective varies by species and generally refers to restoring the physical aspects of a species’ habitat.
The likelihood that the combined effects of the flow objective, the water quality objective, and the habitat objective can achieve the biological goals for each species is given one of four possible ratings: highly likely, likely, somewhat likely, and unlikely. The rating highly likely corresponds to no concerns about achieving biological goals, likely implies the objective is expected to achieve biological goals, somewhat likely implies the objective may reach the goals but there are significant concerns, and an unlikely rating is given where the objective is not expected to reach biological goals. For each species, the evidence and reasoning that led to the determination
are given in Chapter 3, and actions that could be taken to move the determination for a species up to a higher rating are described. The following conclusions and recommendations about the biological objectives meeting the biological goals are made.
It is likely that the biological objectives will meet the biological goals for fountain darter. Fountain darters are clearly associated with SAV and there have been no recent downward trends in fountain darter densities by habitat type or systemwide changes in SAV coverage, despite the drought and flood years of 2013–2014 and 2015, respectively. The flow objectives are consistent with the habitat suitability modeling for the fountain darter, and adaptive management was used successfully to adjust the long-term biological goals. The rating could be improved by repeating the habitat suitability modeling using more recent data; by further examining fountain darter median densities over time, by vegetative habitat type, and abundance indices both within reaches and systemwide; and by performing a power analysis on fountain darter data to guide the interpretation of false negatives.
It is likely that the biological objectives will meet the biological goals for Texas wild rice. This conclusion is based on empirical observations of gains in the coverage of Texas wild rice, even in the face of recent floods and droughts; on the compatibility of the flow objective with the habitat suitability modeling for Texas wild rice; and on the adaptive management changes that now include Texas wild rice as fountain darter habitat. As with the fountain darter, monitoring and successful restoration of Texas wild rice to date have led to conclusions offered with relatively high confidence. The rating could be improved by repeating the habitat suitability modeling using more recent data, by creating a defined water quality objective for Texas wild rice, and by adding a habitat objective to continue to remove nonnative SAV.
It is somewhat likely that the biological objectives will meet the biological goals for Comal Springs riffle beetle. This conclusion is based on the limitations associated with (1) the lack of quantitative monitoring of CSRB populations, (2) determining whether riparian restoration can actually eliminate or significantly reduce siltation at spring openings, and (3) the lack of habitat suitability modeling for CSRB in the monitored reaches. To improve the rating, the following actions should be undertaken. First, it is important to continue to standardize and move toward quantitative sampling of CSRB in order to better understand what the true beetle populations are in the monitored reaches. Second, it is highly recommended that a plan be developed to quantitatively monitor CSRB habitat sedimentation associated with continuing riparian restoration efforts. Finally, if the habitat suitability modeling were repeated in the long-term biological goal reaches,
it would increase confidence in the ability of the flow objectives to meet the biological goals.
It is somewhat likely that the biological objectives will meet the biological goals for San Marcos salamander. A robust monitoring program that could provide evidence of upward trends in abundance is lacking for this species. Much of the current scientific information on the species is based on observations and experiments with captive individuals. There is no water quality objective for the salamander or information on the effects of aquatic gardening. The rating could be improved by creating a water quality objective for San Marcos salamanders, better regulating recreational access to the 50-m reach of the San Marcos River just below Spring Lake Dam, quantifying the outcomes of aquatic gardening and maintenance of silt-free gravel at the salamander study reaches, and augmenting the current sampling protocol with a new method to estimate proportion of area occupied and detection probability of San Marcos salamanders. Controlling access just below Spring Lake Dam and quantifying the maintenance of silt-free gravel should be made high priorities since they could be implemented soon and will help ensure that the stated salamander goals are met.
WILL THE MINIMIZATION AND MITIGATION MEASURES MEET THE BIOLOGICAL OBJECTIVES?
Chapter 4 addresses whether the M&M measures will meet the biological objectives. Rather than consider the dozens of M&M measures individually, the chapter is organized by category of M&M measure, with five major categories being identified: (1) flow protection measures, (2) measures to protect water quality, (3) planting of SAV (including Texas wild rice) and removal of nonnative vegetation, (4) recreation management, and (5) riparian restoration. For each category, the section describes the relevant M&M measures and the extent of their implementation, it shows monitoring data when available, and it summarizes what is known about the effectiveness of M&M measures. Each section concludes with a determination that the suite of measures in that category are highly effective, effective, somewhat effective, ineffective, or it cannot be determined with available information. Each section also suggests what might be done in the near future to increase the rating for that category. A description of the M&M categories is presented below, followed by the major conclusions and recommendations.
Flow Protection Measures
The four flow protection measures of the HCP are Critical Period Management Stage V, the San Antonio Water Supply Aquifer Storage and
Recovery, the Voluntary Irrigation Suspension Program Option (VISPO), and the Regional Water Conservation Program. These four flow protection measures are the most expensive elements of the entire HCP and make up 71 percent of the HCP expenses, totaling $12.2 million through 2017. These four measures have been designed to maintain the minimum flows required by the HCP in the Comal and San Marcos systems during the Drought of Record. Given their central importance, a determination of whether these flow protection measures are effective is crucial to evaluating the overall success of the HCP.
The flow protection measures will be effective in meeting the flow component of the biological objectives for all listed species. Throughout the 2014 drought, during which both VISPO and Critical Period Management Stage IV and V restrictions were triggered, spring flows remained above threshold levels. Recent validation of the MODFLOW model during a drought period suggests that the model conservatively estimates both indicator-well levels and minimum spring flows, particularly at low flows. The model predicted that triggering of the four spring flow protection measures would prevent simulated flows from going below the minimum HCP flow requirements during the Drought of Record. The rating for flow protection measures will move toward highly effective if results of the uncertainty analysis show that the errors are low or if model improvements continue to demonstrate that the model is biased low (i.e., conservatively underestimates well levels and spring flows).
Water Quality Protection Measures
The M&M measures designed to protect water quality in the Comal and San Marcos systems include stormwater control measures, golf course management, and the management and removal of litter and floating vegetation. These measures are appropriately directed toward watershed activities and not direct action in the rivers, with the exception of removing litter and floating vegetation. Most of the stormwater control measures have not yet been implemented, whereas golf course management and the removal of floating litter and vegetation are ongoing.
The water quality protection measures are meant to achieve the biological objective of maintaining water quality within 10 percent of historical conditions. However, for the CSRB, this objective applies to spring water quality, which is not a target of the M&M measures evaluated in Chapter 4. Hence, this section pertains most directly to the water quality component of the biological objective for the fountain darter.
The water quality protection measures, focusing primarily on stormwater control, will be somewhat effective in meeting the water quality component of the biological objective for the fountain darter in the Comal
Submerged Aquatic Vegetation Restoration
Restoration and maintenance of SAV is a key component of reaching the biological goals for fountain darters because these fish are strongly dependent on a vegetated habitat. There are four M&M measures related to aquatic plants. The first is planting of Texas wild rice, one of the listed species and also recognized as habitat for fountain darters. The other three M&M measures all deal with some aspect of SAV management, including removal of exotic or invasive SAV species and either active planting or maintenance of desired native plants that have been documented as fountain darter habitat. The guiding principles for these three measures are that SAV species known to be able to support the fountain darter at abundances on the order of 5 individuals/m2 or greater are targeted for planting, explicit areal coverages for each SAV type and reach are based on historical records of plant abundance, and nonnative SAV species (even if known to be fountain darter habitat) are actively removed.
The SAV restoration measures, including the replanting of Texas wild rice, will be effective in meeting the habitat component of the biological objective for Texas wild rice and the fountain darter. These measures have been in place since 2013 and have seen incremental and positive progress in moving the systems from being dominated by nonnative SAV, such as Hydrilla and Hygrophila, to housing a variety of native SAV species. Removal of nonnative SAV has reduced fountain darter habitat, but this was a known consequence, and future plantings of native SAV combined with expanded areas should compensate. The planting program for Texas wild rice has been particularly successful. The ratings could improve if there were less reliance on intensive planting efforts and less dependence on bryophytes as fountain darter habitat in the Comal system.
Recreation Management
Human recreational use of the San Marcos and Comal systems has occurred for decades, and continued recreational use of these natural re-
sources is identified as one of the activities covered by the HCP. Some of the recreation-associated M&M measures target habitat protection and water quality issues, such as siltation and turbidity, whereas others are meant to mitigate recreation-associated damage to covered species and are often most important during periods of low flow. The recreational M&M measures include management of recreation in both the Comal and San Marcos systems, the designation of permanent access points and bank stabilization in the San Marcos system, regulation of diving and boating in Spring Lake and Sewell Park, and the creation of State Scientific Areas in the San Marcos system.
The recreation management measures will be effective in meeting the habitat component of the biological objectives for the San Marcos salamander and Texas wild rice. Establishment of permanent river access points is complete, including terraces and walls to stabilize the riverbank and facilitate river access by the public. Native vegetation has been planted between permanent access points to eliminate public access in these areas. Exclusion areas within the San Marcos River have been actively implemented and maintained when low-flow conditions occur, and substantial outreach efforts have been undertaken to ensure compliance by recreational users. Actions to improve the rating include enrollment of all outfitters in the Certificate of Inclusion program; better control of recreational access to the 50-m stream reach immediately below the Spring Lake spillway; and sustaining, enforcing, and monitoring the suite of actions currently in place.
Riparian Management
Riparian management measures include restoring native riparian vegetation, stabilizing riparian banks, and preventing shoreline erosion and sedimentation. They are considered critical to the CSRB, but also have relevance for the San Marcos salamander, which shares a goal with the CSRB of maintaining silt-free gravel. Well-executed and monitored riparian management activities may also have positive effects on other listed species, for example, by mediating sediment loading and transport in the San Marcos system and thereby affecting the survival of Texas wild rice or by controlling shading and sedimentation, which can affect growth of native SAV in both systems. Riparian management also supports recreation management by blocking access to portions of the rivers and funneling people to specific access points.
The Committee is unable to determine whether riparian management measures will contribute to achieving the biological objectives of the Comal Springs riffle beetle. This is due to a lack of quantitative monitoring of the riparian measures to show that they are preventing siltation of adjacent springs, as well as to the substantial maintenance requirements of erosion-
control structures. There is the potential for negative effects of nonnative riparian plant removal and replanting activities, such as increased sedimentation of spring substrates. For the other riparian restoration activities (e.g., bank stabilization) in both systems that do not directly affect CSRB habitat, site visits and observations suggest that the riparian restoration measures are effective for reducing erosion and sedimentation that might inhibit the growth of SAV and for supporting recreation management by funneling people to permanent access points.
OVERARCHING ISSUES
As the EAA plans for implementation of Phase 2 of the HCP and ultimately a renewal of the incidental take permit, it should begin to consider several overarching issues and concerns that may ultimately suggest improvements to the biological goals and objectives and the HCP to better protect the listed species.
Fountain Darter
Although the habitat-based biological goals for the fountain darter are reasonable because they are easy to measure and quantify (see Chapter 2), the ultimate goal is to ensure that the fountain darter population is sufficiently large to provide a buffer against environmental variation and other possible factors that can affect population abundance. This requires estimates of the total number of fountain darter individuals in each system. Further exploration of the population abundance of the fountain darter, especially with the monitoring data available, could help determine the viable population size. An approach to examining how well population abundances offer a buffer to variation and can lead to recovery of the species is population viability analysis (PVA) modeling. Much of the needed information to construct a PVA model is available because of projects from the Applied Research Program, continued monitoring including responses to extreme events and restoration, and the development of the fountain darter ecological model. As the EAA fine-tunes the biological goals (as was done recently via the nonroutine adaptive management action), there should be some confirmation of the numbers of fountain darters that are dictated by the goals.
Submerged Aquatic Vegetation
While the existing M&M measures for SAV were found to be effective (see Chapter 4), there are some issues worth considering as work proceeds and certainly in planning for the next phase of the HCP. First, specific areal
targets for SAV species may not be necessary. There is a relatively small difference in the number of fountain darters across the species of SAV subject to active management (most commonly managed Ludwigia, Sagittarria, Cabomba, and Potamogeton), bringing into question the fine-scale, precise management of areal targets that is currently being implemented. Second, it is important to better understand controls on SAV in general, and relative species contributions in particular. If mechanistic understanding is improved, an opportunity may exist to evaluate SAV species targets with the benefit of species-specific habitat requirements that would better inform restoration efforts. Taken together, a relaxation of the targets for species-specific areal SAV coverages and a stronger attempt to identify what factors control SAV success could lead to a lower overall effort without sacrificing the ultimate goal for fountain darters.
Macroinvertebrate Data Analysis
Macroinvertebrate monitoring is not formally part of the HCP, although a long-term monitoring program has been in place since 2003. Multiple aspects of the macroinvertebrate monitoring program provide great potential to the HCP. First, macroinvertebrate monitoring could serve as a general proxy for the overall ecosystem health of the two spring systems, like that routinely done throughout the United States for wadeable streams. Second, the general monitoring of aquatic invertebrates can provide substantial understanding of, and a powerful database on, the complex natural history of the aquifer. Third, comparisons of the general invertebrate community composition and dynamics could be paired statistically with CSRB population estimates to provide an evaluation of the cotton lure sampling approach. Finally, standard ecological community analyses for macroinvertebrates could ultimately serve as a useful surrogate metric for evaluating the overall HCP, and specifically, the efficacy of the M&M measures related to protecting all troglobitic invertebrates in the Edwards Aquifer.
Invasive Species, Exotics, and Disease
The HCP addresses control of some nonnative species, particularly those already present in the Comal and San Marcos systems. However, other as yet unknown threats may pose even greater risks than those already present in these systems, including species that introduce diseases or parasites. The introduction and establishment of a high-impact nonnative species could make these systems permanently uninhabitable for one or more covered species, even if all these suitable habitat conditions are maintained. The opportunity to eradicate an introduced species is often limited to a short period before it becomes abundant or widely distributed. Once a
population is well established, it can be difficult or impossible to eliminate, rendering reintroduction of covered species from refugia populations infeasible or ineffective. There is an urgent need to develop and implement a plan for early detection of nonnative species and for rapid response to eradicate them before they become established. Given the intensive sampling and monitoring that occur in both spring systems, formalizing an early detection strategy should not be difficult.
Catastrophic Events
The HCP represents a detailed and comprehensive planning process that is focused on meeting the recognized challenges to the listed species and the Comal and San Marcos spring and river systems. However, there is the potential for catastrophic events that are far outside the historical record and could pose unrecognized challenges to listed species and the systems, and their frequency may increase due to climate change. For example, an event the size of Hurricane Harvey could completely destroy much of the restored SAV in the Comal and San Marcos rivers, directly affecting Texas wild rice and fountain darter habitat, and lead to substantial erosion and sedimentation in areas of the rivers, affecting silt-sensitive species. The MODFLOW model is a potential tool to partially address some of the scenarios that could occur in the future, by evaluating how flow protection measures operate in extreme scenarios. Other models would need to be deployed to evaluate the impacts of extreme events on overland flow, surface water hydrology, sediment transport, and habitat loss. Although not part of the HCP, catastrophic events should begin to receive evaluation for possible inclusion in future take permits and HCP planning.
