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 Florida2. U.S. Fish and Wildlife Service3. U.S. Geological Survey4. Florida International University

* Liuz@ufl.eduAAG annual conferenceLas Vegas, NV, 3/2009

2

OutlineOutline Introduction

– Alligator hole– EDEN

Methodology – Data– Methods

Results– Hole morphometry– Hole morphometry and EDEN data– Hole drought

Conclusions

3

IntroductionIntroduction 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

sawgrass marsh

sloughalligator holes

tree islandswet

prairiewet prairie

4

Alligator Alligator hole by hole by

habitat typehabitat type

Tree island hole in Taylor Slough

Sawgrass hole in Shark Slough

Wet Prairie hole in Southern Marl Prairie

5

Everglades Depth Everglades Depth Estimation Network Estimation Network

(EDEN)(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

Alligator hole dataAlligator 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)

Water and sediment profile of NW1 in NW ENP (E-W transect)

An alligator hole in NW ENP (NW1)

NW1

-260.0

-210.0

-160.0

-110.0

-60.0

-10.0

40.0

90.0

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37

Sampling interval (m)

Elev

atio

n (c

m)

Water level Ground Bedrock

Cattails marshWillowhead Open water

Hole water depth

Sediment depth

Marsh water depth

7

EDEN hydrologic dataEDEN 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

MethodsMethods GIS analysis

Nonparametric statistical methods–Spearman’s rank correlation–Wilcoxon signed rank test for paired

data –Kruskall-Wallis ANOVA

9

Results: I. Hole Results: I. Hole morphometrymorphometry

Habitat Parameter All holes Sawgrass Tree Island Wet Prairie Sample size 62 41 12 9 Diameter (m) 8.0 4.0 8.0 3.8 8.4 4.2 7.5 4.9 Surface area (m2) 59.5 60.4 58.2 57.9 63.4 56.3 59.9 81.7 Circularity index 0.9 0.1 0.9 0.1 0.9 0.2 1.0 0 Basin depth (cm) 74.9 27.0 78.3 24.5 77.6 30.1 55.8 28.5 Sediment depth (cm) 152.3 50.7 148.7 49.0 156.9 54.6 163.9 58.1 Hole water depth (cm) 88.9 28.3 92.8 26.0 81.7 37.3 80.9 24.1 Marsh water depth (cm) 14.0 16.5 14.5 15.1 4.1 14.3 25.1 19.3

Note: Data shown are means ± s.d.

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

0

20

40

60

80

100

120

Shark Slough(n=20)

NW ENP(n=15)

RockyGlades(n=16)

SouthernMarl Prairie

(n=6)

Taylor Slough(n=5)

Location

Wat

er d

epth

(cm

)

Spearman’s rank correlationr = 0.41, P = 0.001

0

50

100

150

200

250

300

0 50 100 150 200 250 300

Area (sq. m)

Sedi

men

t dep

th (c

m)

Spearman’s rank correlationr = 0.76, P < 0.0001

20

40

60

80

100

120

140

160

20 50 80 110 140 170

Water depth (cm)

Bas

in d

epth

(cm

)

11

Results: II. Morphologic Results: II. Morphologic features and EDEN features and EDEN

hydrologic variableshydrologic 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

Var1 Var2 r P-value

Hydroperiod

Area 0.30 0.02

Basin depth 0.31 0.02

Hole water depth 0.01 0.92

Average hydroperiod(2000-2007)

Area 0.18 0.18

Basin depth 0.44 0.0003

Hole water depth 0.25 0.046 Note: r - Spearman’s rank correlation coefficient.

Hole morphology and hydroperiod

13

Hole bottomdrought

–2000 – 2007 –18 holes;

270 days (14 holes: < 16

days)

Results: III. Results: III. Hole droughtHole drought

N= 270 (days)

0%

5%

10%

15%

20%

25%

April May June July

Month

Perc

enta

ge F

requ

ency

NW ENPRocky GladesShark SloughSouthern Marl PrairieTaylor Slough

14

Hole top drought

–2000 -2007

–62 holes

15

Hole top Hole top droughtdrought

(2000-2007; 62 (2000-2007; 62 holes)holes)

16

Hole drought by habitatHole drought by habitatHole bottom drought

Sawgrass(N = 72)

0%

10%

20%

30%

40%

50%

60%

4 5 6 7

Month

Tree Island(N = 111)

0%

10%

20%

30%

40%

50%

60%

5 6 7

Month

Wet Prairie(N = 87)

0%10%20%30%40%50%60%70%

5 6 7

Month

Hole top droughtSawgrass

(N = 20,879)

0%

10%

20%

30%

1 2 3 4 5 6 7 8 9 10 11 12

Month

Tree Island(N = 10,077)

0%

10%

20%

30%

1 2 3 4 5 6 7 8 9 10 11 12

Month

Wet Prairie(N = 6,123)

0%

10%

20%

30%

1 2 3 4 5 6 7 8 9 10 11 12

Month

17

ConclusionsConclusions 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 StudiesFuture 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

AcknowledgementsAcknowledgements 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.