Relationship between alligator holes and EDEN hydrologic data in Everglades National Park, Florida

1. Relationship between alligator holes and EDEN hydrologic data in Everglades National Park, Florida Zhongwei Liu, Ph.D.1 * Frank J. Mazzotti, Ph.D.1 Laura A. Brandt, Ph.D.2 Stephanie S. Romañach, Ph.D.3 Danielle E. Ogurcak 4 Aaron L. Higer1 1. Ft. Lauderdale Research & Education Center, University of Florida 2. U.S. Fish and Wildlife Service 3. U.S. Geological Survey 4. Florida International University * Liuz@ufl.edu AAG annual conference Las Vegas, NV, 3/2009

2. Outline • Introduction • Alligator hole • EDEN • Methodology • Data • Methods • Results • Hole morphometry • Hole morphometry and EDEN data • Hole drought • Conclusions

3. tree islands wet prairie sawgrass marsh wet prairie slough alligator holes Introduction • American Alligator • Top predator, keystone species, ecosystem engineer in Florida Everglades • Alligator Hole • Small but persistent ponds excavated and maintained by alligators • Dry-season refugia • Nest, colonization, and foraging sites

4. Alligator hole by habitat type Tree island hole in Taylor Slough Sawgrass hole in Shark Slough Wet Prairie hole in Southern Marl Prairie

5. Everglades Depth Estimation Network (EDEN) • Integrated network of real-time water level monitoring, ground elevation modeling, and water-surface modeling • EDEN Water-Surface Model • Developed by Pearlstine et al. (2007) with spatial interpolation of water level data from 240 gage stations in ArcGIS • Validated by Liu et al. (2009, in press, Ecohydrology) • 2000 – present • Resolution: 400 m

6. An alligator hole in NW ENP (NW1) Alligator hole data • 62 holes (1/2006 – 8/2007) • Accessed by helicopter/airboat or on foot • Transect length (major and minor axes), water and sediment depths at every 1-meter interval • Hole diameter, area, marsh water depth, basin depth, circularity index (Miller,1953) NW1 Water and sediment profile of NW1 in NW ENP (E-W transect)

7. EDEN hydrologic data • Water level • Direct EDEN model output (2000-2007) • Water depth • depth = water level – DEM (EDEN water depth) • depth = water level – estimated site ground elevation • estimated site ground elevation = water level – measured water depth • Hydroperiod • Hydroperiod: the number of days per year an area is covered by water (hole water depth >0). • Average hydroperiod: 8 years (2000-2007) • Drought • Hole bottom drought (hole water depth ≤ 0) • Hole top drought (0 < hole water depth ≤ basin depth)

8. Methods • GIS analysis • Nonparametric statistical methods • Spearman’s rank correlation • Wilcoxon signed rank test for paired data • Kruskall-Wallis ANOVA

9. Results: I. Hole morphometry

10. Morphologic variations by habitat and location • Average marsh water depths varied with habitat type (Kruskall-Wallis, H = 12.44, 2 d.f., P = 0.002). • Average hole water depths varied with hole location (Kruskall-Wallis, H = 14.52, 4 d.f., P = 0.006). • Inter-relationships • Hole area and sediment depth • Hole water depth and basin depth

11. Results: II. Morphologic features and EDEN hydrologic variables • Hole area/basin depth and EDEN water level • EDEN water level: daily value, and seasonal average (2000-2007) • Spearman’s rank correlation, all P > 0.05. • Marsh water depth and EDEN water depth • Wilcoxon’s signed rank test, W = -213.5, P = 0.14 • However, differences ranged from -60 to 30 cm (60% absolute differences >10 cm).

12. Hole morphology and hydroperiod Note: r - Spearman’s rank correlation coefficient.

13. Results: III. Hole drought Hole bottom drought • 2000 – 2007 • 18 holes; 270 days (14 holes: < 16 days)

14. Hole top drought • 2000 -2007 • 62 holes

15. Hole top drought(2000-2007; 62 holes)

16. Hole drought by habitat Hole top drought Hole bottom drought

17. Conclusions • The hole areas were from 5 - 269 m2 (60 ± 60 m2). The hole basin depths were from 30 - 148 cm (75 ± 27 cm). Most holes were circular in shape. • Hole area and basin depth were not correlated with EDEN water level, but basin depth was associated with short and long-term hydroperiod. • Hole bottom drought: southern marl prairie and Taylor Slough (May - July) • Hole top drought: Taylor Slough and Rocky Glades (March – May) • The findings would help identify ecologically and biologically important habitats/sites (particularly during dry seasons) to conduct long-term monitoring and study of alligators, fish, wading birds, and vegetation dynamics.

18. Future Studies • Field surveys within both dry and wet seasons for a single alligator hole • Drought duration • Hole volume and biomass estimation • Comparison and linkage with alligator ecological model simulations • Examination of the relationships with rainfall and evapotranspiration (ET)

19. Acknowledgements • This research was funded by U.S. Army Corps of Engineers, University of Florida, and U.S. Geological Survey Priority Ecosystems Science. • Everglades National Park and South Florida Water Management District provided logistical support including airboats, helicopters, and vehicles. • We thank other members of the field-data collection team at University of Florida: Ryan Lynch, Jemeema Carrigan, Debbie Kramp, Mike Rochford, Wellington Guzman, Joy Vinci, Mark Peyton, Brian Bahder, Clayton McKee, Matt Brian, Chris Bugbee, and Michelle Casler.