Evaluation of the Long-Term Elevated Temperature and Low Dissolved
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EVALUATION OF THE LONG-TERM, ELEVATED TEMPERATURE AND LOW DISSOLVED OXYGEN TOLERANCES OF THE COMAL SPRINGS RIFFLE BEETLE FINAL REPORT EAHCP Project No. 146-15-HCP Prepared by Dr. Weston H. Nowlin 601 University Drive Department of Biology Aquatic Station Texas State University San Marcos, TX 78666 (512) 245-8794 [email protected] Parvathi Nair, PhD Candidate 601 University Drive Department of Biology Aquatic Station Texas State University San Marcos, TX 78666 [email protected] and Dr. Benjamin Schwartz 601 University Drive Edwards Aquifer Research and Data Center Department of Biology Aquatic Station Texas State University San Marcos, TX 78666 (512) 245-7608 [email protected] TABLE OF CONTENTS TITLE PAGE………………………………………………………….……………….…….….……1 TABLE OF CONTENTS ........................................................................................................................2 LIST OF FIGURES AND TABLES……………………………………….………………….…….……3 EXECUTIVE SUMMARY……………………………………………………………….…….…….…4 INTRODUCTION AND LITERATURE REVIEW…….…………………………………….…….…….…5 METHODS…………………………………………………………………….………………..…….9 RESULTS……………………………………………………………………..………………..……18 DISCUSSION…………………………………………………………………………….….…..…...26 LITERATURE CITED…………………………………………..………………………..….………..30 2 LIST OF FIGURES AND TABLES Fig 1 – Pictures of riffle beetle species used in this study…………………………..…….………..….9 Fig 2 – Conceptual diagram for determining critical thresholds for temperature and DO…...……….10 Fig 3 – Schematic of limits of acclimation experimental system and set up…….……………..……..13 Fig 4 – Schematic of set up of persistent temperature or DO experiments..……………………....….15 Fig. 5 – Schematic of set up of O2 consumption rate estimates ..……………………………………..17 Fig 6 – Comparison of H. comalensis and H. glabra temperature and DO thresholds ....…….…..…..19 Fig 7 – Results of long-term persistent temperature experiments on riffle beetle survivorship.......….20 Fig 8 – Metabolic responses of riffle beetles to persistent temperature conditions……………..…….22 Fig 9 – Survival responses of H. comalensis and H. glabra to persistent DO condition experiments…………………………………………………………………………..23 Fig. 10 – Time series of temperature in Comal Spring Runs 1, 2, and 7 (January 2005 – December 2016) ……………………………………………………………….…….24 Fig. 11 – Time series of DO concentration in Comal Spring Runs 1, 2, and 7 (January 2005 – December 2016) ……………………………………………………………….…….26 Table 1 – Summary table of the species examined, the basic design and the length of the experiments conducted as a part of this study……………………………………...11 3 EXECUTIVE SUMMARY In this Final Report, we present the results of a series of experiments conducted as a part of the Edwards Aquifer Habitat Conservation Plan (EAHCP) (EAHCP Project No. 146-15-HCP) Applied Research Program. This study examined temperature and dissolved oxygen (DO) limitations of several related riffle beetle species, including the Comal Springs riffle beetle (Heterelmis comalensis). Temperature and DO limitations of riffle beetles were examined because low flow conditions at Comal Springs is presumed to affect the daily average water temperature and DO concentrations. Thus, understanding how low flow conditions affect plastron function and beetle performance and mortality is important for setting flow targets at Comal Springs that may help to maintain H. comalensis populations. We experimentally examined responses of four species of riffle beetles (Heterelmis comalensis, Heterelmis glabra, Heterelmis vulnerata, and Microcylloepus pusillus) in a series of experiments in which we manipulated temperature and DO separately. In the first set of experiments, adult H. comalensis were separately exposed to progressive 24-h changes in temperature (increasing water temperatures) and DO concentrations (declining DO) in the lab. We hypothesized that beetles would exhibit critical “threshold” temperatures and DO concentrations beyond which individuals will exhibit a loss or reduction in performance. We found that H. comalensis exhibited a pronounced change in behavior (rapid movement around the experimental chamber at (mean ± 1 SE) 32.84 ± 0.41oC and a loss of response (LOR) to an external stimulus (gentle agitation of the chamber) at 35.97 ± 0.71oC. The onset of movement threshold was significantly lower (~1oC) than the rapid movement onset temperature observed in previous experiments with H. glabra, but the LOR onset temperature did not significantly differ from those reported for H. glabra. In the Critical DO Threshold experiments, H. comalensis exhibited an LOR onset at 1.14 ± 0.20 mg DO/L and this threshold was significantly higher than the DO threshold observed for H. glabra (H. glabra threshold occurred at 0.5 mg/L). In the second set of experiments, we first acclimated and then exposed several species of elimds to persistent and unchanging suite of temperatures (23, 26, 29, and 31oC) or DO concentrations (5, 3, 2, and 1 mg/L). Mortality and metabolic rate (O2 consumption rate) of adult beetles were assessed at various time intervals throughout the 60-d experiment. Overall, H. comalensis showed significantly increased mortality at temperatures >23oC and metabolic rate markedly increased at 31oC. These results contrasted to the other non-spring associated beetle species (H. vulnerata and M. pusillus), both of which had higher survivorship and smaller increases in O2 consumption rates at higher temperatures. In the persistent DO experiments, both H. comalensis and H. glabra showed marked declines in survivorship at lower DO concentrations at the end of the 15-day experimental period, with substantial mortality (>50% mortality) in H. comalensis occurring at DO concentrations of 2 mg/L. Overall, this study suggests that H. comalensis is comparatively sensitive to changes in temperature and DO, but field data from Spring Runs 1, 3, and 7 indicate that conditions in the Comal system rarely cross the experimentally-derived thresholds identified by this study. It is recommended that these thresholds and field data be used for a future risk assessment analysis for H. comalensis in the Comal Springs system. 4 INTRODUCTION AND LITERATURE REVIEW The Edwards Aquifer Recovery and Implementation Plan (EARIP) currently sets the long-term mean and minimum daily discharge objective for Comal Springs at 225 cfs (cubic feet/second) and 30 cfs, respectively. However, modeling results from Phase 1 of the EAHCP predict that the mean and minimum daily discharge will be 197 cfs and 27 cfs, respectively (EARIP 2012). Thus, there is currently concern about the impacts of lower spring flows on Comal Springs riffle beetle (Heterelmis comalensis) populations. Historical data and modeling results indicate some of the potential loss of habitat and habitat degradation associated with the reduction in spring flows. It has been observed that Spring Runs 1 and 2 generally cease to flow when total Comal Springs flow is ~130 cfs and Spring Run 3 generally ceases to flow when Comal Springs total flow is about 50 cfs (LBG Guyton 2004). Modeling results suggest that discharge will be less than 120 cfs for a total of 127 months and less than 45 cfs for a total of 7 months during a repeat of the drought of record (in the 1950s) with Phase 1 of the HCP implemented (EARIP 2012). Modeling efforts also indicate that a repeat of the drought of record (with Phase 1 of the HCP fully implemented) will lead to the total flows in the Comal Springs system to be < 30 cfs for a two-month period (EARIP 2012). If flows drop below 30 cfs, it is expected the main spring runs in the system (Spring Runs 1 through 6) will be dry for a considerable time period and the remaining aquatic habitat within the Comal Springs system will be limited to portions of Landa Lake and the Spring Island area. Cumulatively, this information indicates that it is possible for several if not most of the spring runs in the Comal system to cease flowing for extended periods of time (from months to years) and for a significant reduction of aquatic habitat to occur if there is a recurrence of the drought of record. The Comal Springs riffle beetle and spring flows The Comal Springs system exhibits consistent temperatures (annual mean approximately 23 °C), high water transparency, and low nutrient and bacteria levels (USFWS 1997). Monitoring by the EAA at ~80 groundwater wells, eight surface water sites, and major springs groups across the region indicates little contamination in the aquifer. However, as total spring flow in the system declines, water quality in the remaining habitat will likely be a primary concern. Two of the most relevant water quality changes associated with reduction in flows that would potentially have an impact on the Comal Springs riffle are temperature and DO. It is currently thought that the occurrence of H. comalensis within the Comal system is largely limited to habitats immediately adjacent to spring outflows (USFWS 2007). Therefore, a reduction in spring flow that leads to loss of habitat (via desiccation) or reduces water quality of their occupied habitat will likely impact the fitness and survival of beetles. Obviously, water quality will not be the primary issue in the Spring Runs or along the western shoreline during substantial low-flow events because these habitats will cease to flow and the habitat associated with the presence of the Comal Springs riffle beetle (i.e., areas around spring orifices) will be dry. However, in the summer period as flows decline at Comal Springs and the remaining aquatic habitat is reduced to portions