Diet analysis and community characteristics of Lepomis (sunfish) species congregating near natural spring vents of the Rainbow River, Florida by Michael S. Sears A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Environmental Science, Policy, & Geography College of Arts and Sciences University of South Florida St. Petersburg Major Professor: James Gore, Ph.D. Randy Edwards, Ph.D. Melanie Riedinger-Whitmore, Ph.D. Date of Approval: November 1, 2010 Keywords: natural spring vents, Lepomis , stomach content, spring-fed river, diversity, electroshocking, stomach pump Copyright © 2010, Michael S. Sears Table of Contents List of Tables ............................................................................................................ ii List of Figures ......................................................................................................... iii Abstract ................................................................................................................... iv Introduction ............................................................................................................. 1 Materials & Methods ............................................................................................ 12 Study Site .................................................................................................... 12 Sampling Methods ....................................................................................... 17 Electroshocking Sampling Methods ................................................. 18 Hook and Line Sampling Methods ................................................... 22 Plankton Net Sampling Methods ...................................................... 24 Statistical Analysis ....................................................................................... 27 Results ................................................................................................................... 29 Fish Populations .......................................................................................... 29 Stomach Content Analysis .......................................................................... 32 Plankton Net Sampling Results ................................................................... 36 Discussion ............................................................................................................. 37 List of References ................................................................................................. 44 Appendix ................................................................................................................ 51 i List of Tables Table 1. Upstream and downstream Rainbow River pH data from 2004 and 2005 .................................................................................... 17 Table 2. Mean monthly discharge (cfs) from the Rainbow River during sampling events ..................................................................... 17 Table 3. Upstream and downstream Fish species composition percentages ........ 29 Table 4. Upstream fish species diversity measurements ....................................... 30 Table 5. Downstream fish species diversity measurements .................................. 31 Table 6. Upstream and downstream Lepomis species length results .................... 31 Table 7. Upstream Lepomis species stomach contents ......................................... 33 Table 8. Downstream Lepomis species stomach contents .................................... 34 Table 9. Upstream Lepomis species stomach content diversity ............................ 36 Table 10. Downstream Lepomis species stomach content diversity ..................... 36 ii List of Figures Figure 1. Rainbow River Watershed Land Use Map of 1944 ................................... 3 Figure 2. Rainbow River Watershed Land Use Map of 1999 ................................... 4 Figure 3. Spring vent locations within the Rainbow River ..................................... 12 Figure 4. Rainbow River water quality sampling site .............................................. 15 Figure 5. Upstream and downstream Rainbow River temperature data between 2006 and 2009 ...................................................................... 16 Figure 6. Upstream and downstream Rainbow River dissolved oxygen data between 2006 and 2009 ...................................................................... 16 Figure 7. Upstream electroshocking sampling locations ........................................ 19 Figure 8. Downstream electroshocking sampling locations ................................... 20 Figure 9. Hook and line sampling locations ............................................................ 22 Figure 10. Upstream plankton net spring vent and tow sampling locations ........... 25 Figure 11. A Lepomis species at a sample site ...................................................... 26 Figure 12. View of the plankton net setup over a spring vent ................................ 26 Figure 13. Up-close view of a spring vent/boil ........................................................ 28 iii Abstract Little is known about the ecology of natural springs and the influence spring vents have on fish populations. This study explores fish population characteristics, such as abundance, diversity, and length distributions near headwater spring vents and downstream away from the spring vents within the Rainbow River, located in central Florida. The population characteristics examined are useful to resource managers when evaluating the possible impacts on fish populations from increased groundwater withdrawals and subsequent lower discharge rates into large spring-fed rivers. Initial field observations led to the hypothesis that Lepomis species feed on organisms expelled from the spring vents. Electrofishing and hook and line sampling were utilized to collect fish specimens for analysis, and Lepomis species were frozen for later stomach analysis, or stomach contents were pumped in the field. Spotted sunfish ( Lepomis punctatus ) were dominant upstream (28.8% composition), while bluegills ( Lepomis macrochirus) were most abundant downstream (38.0% composition). Upstream populations of Lepomis species had significantly larger individuals (P < 0.05) than Lepomis species sampled downstream. Lepomis species stomach analysis revealed Hyalella azteca as the most abundant food source upstream, and chironomids as the most abundant prey taxa downstream. Upstream Lepomis species stomach analysis yielded an average of 19.05 organisms identified per individual fish, while 4.01 iv organisms per fish were observed downstream. These differences in Lepomis species populations and their diets indicate that future groundwater withdrawals could impact these stable state spring environments, and have substantial impacts on the ecology of the Rainbow River. v Introduction Natural freshwater springs are unique ecosystems that provide essential habitat for many species of flora and fauna (Odum 1957; Hubbs 1995; Walsh 2001). These habitats are vulnerable to flow alterations and habitat loss from increased demand for groundwater withdrawals by residential and agricultural land use. This demand continues to increase on aquifers that sustain natural springs, like the Rainbow River, in central Florida. Over the last several decades, much of the Rainbow River watershed and recharge area were converted from forested lands to agricultural and residential land uses (Figures 1 and 2). As of 2004, the population of the area has increased by 37.5 percent since ten years prior (SWFWMD 2004). The increasing development of the area and demand on water poses a threat to the relatively constant water flows, water temperatures, and water quality that makes the Rainbow River and other spring habitats unique natural resources (Odum 1957; Hubbs 1995; Berndt et al . 1998; Barquin and Scarsbrook 2008). Spring-fed rivers are unique for providing a annually stable habitat for many species, including resident endemic species (Hubbs 2001), and seasonal species seeking thermal refuge in extreme cold or warm conditions (Patterson 1996). Future impacts from groundwater withdrawals on spring-fed rivers would have negative effects on these unique ecosystems by decreasing flows from springs and possibly altering fish and macroinvertebrate communities (Barquin and Scarsbrook 2008). 1 Initial visual site observations from a boat, with confirmation by preliminary electrofishing, indicated that fishes of the family Centrarchidae are abundant and likely dominant in the Rainbow River system, including bluegill ( Lepomis macrochirus ), spotted sunfish ( Lepomis punctatus ), and largemouth bass (Micropterus salmoides ). During these field visits, Lepomis species were observed congregating around the spring vents at high frequencies for reasons unknown. It is expected that decreased flow from the spring vents might alter this unique habitat and its fish populations. 2 Figure 1. Rainbow River watershed land use map of 1944 (SWFWMD 2004). 3 Figure 2. Rainbow River watershed land use map of 1999 (SWFWMD 2004). 4 Lepomis species, such as bluegill are characterized as generalist feeders (Werner et al. 1981), and select prey according to size (Werner and Hall 1974; O’Brien et al. 1976). As flow is concentrated directly adjacent to and downstream from individual spring vents, Lepomis may utilize the area of flow to maximize predation on drift prey expelled directly from the spring