ATOLL RESEARCH BULLETIN 
NO. 469 
PRELIMINARY HYDROGRAPHIC SURVEYS OF SOME PONDS IN THE 
PELICAN CAYS, BELIZE 
BY 
DANIEL W. URISH 
ISSUED BY 
NATIONAL MUSEUM OF NATURAL HISTORY 
SMITHSONIAN INSTITUTION 
WASHINGTON, D.C., U.S.A. 
MARCH 2000 
.... 
. . . . . .. . 
Pelican Cays 
. . 
. . 
lkm 
............... reef 
- - - - - - - - - tidal flat 
Figure 1. Map of Pelican Cays, Belize, Central America and pond identifications 
PRELIMINARY HYDROGRAPHIC SURVEYS OF SOME PONDS 
IN THE PELICAN CAYS, BELIZE, CENTRAL AMERICA 
DANIEL W. URISH' 
ABSTRACT 
Hydrographic and hydrologic surveys of three ponds in the Pelican Cays Group were 
carried out using hand-held sonar, pressure transducer tide loggers, and YSI temperature-salinity 
probes. These ponds characteristically have a circular pattern with steeply sloping sides. The 
pond depths range from 4.6 ni in a small pond with a diameter of I00 In to 15.2 m in a large pond 
with a diameter of 840 m; in general, depth increases with increasing pond size. Depths ranged 
from 15.2 m for a distance of 100 m between cays to 28.7 m for a distance of 1,150 m. Pond 
salinities averaged 35.3 ppt with a range of 1.5 ppt, and temperatures averagcd 3 1.4" C with 
surface temperatures about 1 " C higher than bottom temperatures. 
INTRODUCTION 
The Pelican Cays lie some 13 km east of the coast of Belize on the Belize Barrier Reef 
(Fig. 1). The submerged limestone shelf on which the cays are located have the characteristics of 
submerged karst limestone topography (Purdy, 1974; Stoddart et al., 1982). These cays are 
believed to have been formed on an ancient subaerially eroded limestone plateau that was 
submerged during the Holocene Transgression some 6,000-8,000 years B.P. According to 
Macintyre et al. (this volunie). the circular pattern of the ponds in these cays is related to a 
combination of karst control and diffcrcntial reef growth. 
The cays are composcd of coral and coral rubble covered largely by mangrove forest. 
They can he characterized as low-lying land masses of coral and peat surrounding central ponds. 
The coastal margins of the cays at-e vegetated by the red mangrove Rhizophora mangle in the 
lower tidal washed sections, and some black mangrove, Avicennia gtrminans, in the slightly 
higher regions. Sand deposits with coconut trees can be found in a few higher areas. 
The ponds, which also appear as well-delineated circular bottom configurations in the 
outside lagoon, have steep walls. Many of the ponds overlap, separated only by articulate ridges. 
Some ponds are colnpletely cnclosed, but most are connected with other ponds by shallow 
channels. 
Tides along the barrier I-cef are microtidal and of a mixed semidiurnal nature, with a mean 
range of only about 15 cm. hut this, along with windset, is sufficient to cause some interpond 
currents (Kjerfve et al., 1982). l~lowever. because most of the ponds have relatively deep central 
areas, they are probably poorly flushed. except in the upper water, and are essentially individual 
'Department of Civil and 1-hvironmcntal Engineering, University of Rhode Island, Bliss Hall, 
1 Lippitt Road, Kingston, RI 0288 1 
ecosystems. 
There is no direct precipitation record for the region, but extrapolation from mainland 
records (Walker, 1973) and limited records at Carrie Bow Cay (Riitzler and Ferraris, 1982) 
suggest that annual rainfall averages about 190 cmlyr (75 inlyr). The nearest official weather 
station is Melinda Forest Station, located on the mainland coast about 45 krn to the north. A dry 
period is common from February through May, when the rainfall averages only about one-third 
that of other months. Hurricanes during the months of July to October can produce storm surges 
that completely inundate the low-lying cays. During the periods of study, pond conditions were 
little affected by precipitation or unusual sea conditions. 
During the period May 18-26, 1994, we investigated the ponds of three cays of the 
Pelican Cays Group: Pond A (Cat Cay), Pond C (Manatee Cay), and Ponds E, F and G 
(Fisherman's Cay) (Fig. 1). Additional depth measurements and investigations of the broader 
hydrographic characteristics of the regions between the cays were recorded during the period 
May 7-10, 1995. The hydrographic surveys were part of a larger scientific field study of the 
Pelican Cays undertaken by a group of scientists associated with the Smithsonian Institution. 
METHODS 
Very limited mapping information is available on the Pelican Cays. Some hydrographic 
mapping of the area can be found in U.S. naval charts based largely on British surveys between 
1830 and 1841 (U.S. Navy, 1942). In order to obtain the scale of detail necessary for this study, 
the island-pond maps (Figs. 3,6, and 10) were drawn using aerial photographs taken by Tony 
Rath (Dangriga, Belize) during May 1995. Distances and dimensions were ground-truthed in the 
field and checked against other published maps. 
Pond-bottom depth measurements were made using a Scuba-pro Model hand-held sonar 
deployed from a small boat. Survey line control across the ponds was established by making runs 
between prominent shoreline points. Locations of measurements along these lines were 
determined by "dead reckoning." That is, the measurements were made at regular timed intervals 
as the boat moved at a steady speed along the survey line. Location and depth in the interpond 
channels were determined by direct tape measurements. The latitude-longitude location shown on 
the maps was obtained with a Garmin GPS Model 50 unit. 
The tide record was obtaincd with the aid of pressure transducers and an Enviro-lab 
Model 120 data logger in the ponds. The data logger was programmed to collect level data at 
intervals of 15 min. The period of data collection was two and a half days. The vertical reference 
datum was arbitrary but was selectcd to keep all readings as positive elevations. 
Vertical profile depths werc determined with direct tape measurements. Temperature and 
salinity values for the depth profiles were obtained using a YSI Model 33 instrument. 
Measurement of temperature and salinity were taken at 0.6-m (5.0-feet) intervals. 
RESULTS 
A two-and-one-half-day record of tidal fluctuation in Pond E at Fisherman's Cay was 
obtained (Fig. 2). Pressure transduccr probes used to measure water level were located in the 
mangrove roots on the west side of Pond E of Fisherman's Cay as well as in Pond F. Although 
channel flow could be observed as the tide changed, there was very little measurable difference 
in elevation or lag time. It is believed, however, that such differences can be measured using a 
shorter data collection interval, such as one minute. 
The location of work in Pond C at Cat Cay is shown on Fig. 3. It consisted of two bottom 
hydrologic profiles (Figs. 4 and 5), and two hydrologic depth profiles (Figs. 24 and 25) in the 
deeper parts of the ponds. 
The location of work in Pond A at Manatee Cay is shown on Fig. 6. It consisted of three 
hydrographic bottom profiles (Figs. 7-9) and two hydrographic depth profiles (Figs. 26 and 27) 
in the deeper parts of the pond. 
The field work at Fisherman's Cay consisted of pond-bottom profiles and vertical 
hydrologic profiles in the deep parts of the ponds (Fig. 10). Four interpond channel cross sections 
(Figs. 11-14) were completed, two at the entrance into Pond E and two between Ponds E and F. 
Nine hydrographic lagoon bottom profiles (Figs. 15-23) and four hydrologic depth profiles (Figs. 
28-3 1) were made. 
The locations of the 16 hydrographic survey runs made in May 1995 between the various 
cays of the Pelican Cays Group are shown on Fig. 32. Table 1 summarizes the results of these 
survey runs. Depths of water between cays ranged from 15.2 m for a distance of 100 m between 
cays to 28.7 m for a distance of 1,150 m. 
Table 1. Summary of pond hydrographic and hydrologic characteristics 
Location 
Cat Cay 
Pond A 
Manatee Cay 
Pond C 
Fisherman's Cay 
Pond E 
Pond F 
Pond G 
Maximum Maximum Salinity Temperature 
Depth Length Depth Upper Lower Upper Lower 
Profile (m) (m) (PP~) (PP~) ("'4 PC) 
DAYS 
Figure 2. Plot of tidal fluctuations in Pond E, Fisherman's Cay during period of  May 21- 
23 1994. 
LEGEND 
1 - 1' BoTTOM PROFILE 
X O E P M  PROFILE 
0 GPSLOCATION 
CAT CAY 
Figure 3.  Map of Cat Cay showing locations of bottom profiles and depth profiles f o ~  
Pond A. 
DISTANCE FROM SOUTH EDGE 
Figure 4. Pond A - Cat Cay Bottom Profile 1-1'. 
DISTANCE FROM WEST EDGE 
Figure 5. Pond A - Cat Cay Bottom Profile 2-2'. 
Figure 6. Map of Manatee Cay showing locations of bottom profiles and depth profiles 
for Pond C. 
DISTANCE FROM WEST EDGE 
Figure 7. Pond C - Manatee Cay Bottom Profile 3-3'. 
DISTANCE FROM EAST EDGE 
Figure 8 Pond C - Manatee Cay Bottom Profile 4-4'. 
DISTANCE FROM SOUTH EDGE 
Figure 9. Pond C - Manatee Cay Bottom Profile 5-5'. 
LEGEND 
9- 9' BOTTOM PROFILE 
DEPTH PROFILE 
FISHERMANS CAY I' \ C . 
- - - -  .>
Figure 10. Map of Fisherman's Cay showing locations of bottom profiles and depth 
profiles for Ponds E, F, and G. 
0 4 . 8 .  m I I 1 * 
1 - f-- WNDE LAGOON ---+ 
CROSSSECTION 
t - 0.5 2 - 
-1.0 
5 I I 
ff 0 10 20 30 40 
Distance 
Figure 11. Fisherman's Cay Channel Axial Profile from Pond E to lagoon. 
m 
0.5 
-1.0 
8 
- 
5 
f? 0 10 20 30 
Distance 
Figure 12. Fisherman's Cay Cross-Section looking into Pond E. 
P 
2 3 -  
4 
5 
- 
. -- 
f-- PONDF PONDE --) 
20 30 
Distance 
Figure 13. Fisherman's Cay Channel Axial Profile from Pond F to Pond E. 
5 
ft 0 2 4 6 8 10 
Distance 
Figure 14. Fisherman's Cay Cross,Section looking from Pond E into Pond F. 
Figure 
DISTANCE FROM SOUTH EDGE 
15. Pond G - Fisherman's Cay Bottom Profile 6-6' 
DISTANCE FROM WEST EDGE 
Figure 16. Fisherman's Cay Bottom Profile 7-7'. 
DISTANCE FROM WEST EDGE 
Figure 17. Pond E - Fisherman's Cay Bottom Profile 8-8' 
20 
0 
40 m 
. 9  I I 9'l 
5 4 
- 2 
z 
b 10 - 
a 
W 
n 
- 4 
15 - 
- 
20 I 
ft 0 100 200 
DISTANCE FOM NORTH EDGE 
Figure 18. Pond E - Fisherman's Cay Bottom Profile 9-9'. 
DISTANCE FROM NORTH EDGE 
Figure 19. Pond F - Fisherman's Cay Bottom Profile 10-10'. 
DISTANCE FROM WEST EDGE 
Figure 20. Pond F - Fisherman's Cay Bottom Profile 1 1-1 1 '  
DISTANCE FROM WEST EDGE 
Figure 21. Pond F - Fisherman's Cay Bottom Profile 12-12'. 
DISTANCE FROM EAST EDGE 
Figure 22. Pond F - Fisherman's Cay Bottom Profile 13-13', 
50 I I 1 
ft 20 40 60 80 
DISTANCE FROM EAST EDGE 
Figure 23. Fisherman's Cay Bottom Profile 14-14' outside of Pond E 
Figure 24. Pond A - Cat Cay Depth Profile Al 
ft TEMPERATURE ("C) 
TEMPERATURE (OC) 
I . ' I I I B '  
TEMPERATURE 
0 SALINITY 
/7 
B 
0 
10 
20 
F 
n 30 
W 
0 
40 
50 
60 
SALINITY (ppt) 
Figure 25. Pond A - Cat Cay Depth Profile A2. 
I 
m 
0 ' I I 
0 
a ' 0 
B 
- 
* \ T m m w u w  m 0 
- 0 SALINITY idB - 5 B 
0 
- 0 m 
0 ' 10 
- B 
0 B 
- B - 15 
. . I I I 
28 29 30 31 32 33 . 34 35 36 
SALINITY (ppt) 
- 0 TEMPERATURE m 
0 SALINITY N m  
0 m 
50 I . , . , , , . ,  . , . I . , . , .  
28 29 30 31 32 33 '34 35 36 37 
SALINITY (ppt) 
Figure 26. Pond C - Manatee Cay Depth Profile C1. 
- TEMPERATURE 0 
SALINITY flm 
4 0 ~ " ' ' " ' ' ' '  
28 29 30 31 32 33 34 35 36 37 38 
SALINITY (ppt) 
Figure 27. Pond C - Manatee Cay Depth Profile C2. 
SALINITY (ppt) 
TEMPERATURE (%) 
ft rn 
Figure 28. Pond E - Fishman's Cay Depth Profile El 
0 
1 
2 
3 
0 
2 
4 
I 
I- 6 
a 
LLI 
0 
8 
10 
12 
Figure 29. Pond F - Fisherman's Cay Depth Profile F1. 
28 30 32 34 36 
I 8 
0 
- Z 
TEMPERATURE - 
- 
0 
- 
SALINITY + , 
- 
0 
- 
- 
- 0 rn 
I I 
ft TEMPERATURE (T) m 
0 .  
5 10 
Q 
a, 
n 
20 
28 30 32 34 36 38 
SALINITY (ppt) 
I I 
0 rn 
- 
0 rn 
SALINITY / - 
TEMPEPATUFE 
- 0 rn 
- 
0 rn 
- 
0 
. 
- 
0 
2 
4 
6 
TEMPERATURE (OC) 
0 
-% TEMPERATURE 
SALINITY / 
28 30 32 34 36 38 
SALINITY (ppt) 
Figure 30. Pond G - Fisherman's Cay Depth Profile G1 
ft TEMPERATURE ("C) m 
0 
1 0  
2 0  
5 
a 3 0  
0" 
4 0  
50 
6 0 
29  3 0 3 1 3 2 3 3 3 4 3 5 
SALINITY (ppt) 
Figure 3 1. Fisherman's Cay Depth Profile E2. 
I 0 I I I I a 
0 a 
- 0 a 
0 a 
- 
- 0 a 
o \  a TEMPERATURE 
- 
- 0 
SALINITY 
- 0 
/a 
a 
- a 
- 0 a 
- 
-. a 
- 
I I 
0 
4 
8 
-12 
16 
Pelican Cays 
I N 
1 
survey run 
island 
. . . . . . . . . , . . . . . . . . reef numerous coral heads 
, . . .  
. ... : .. 
- ----- . . .. 
... lidal flat 
Figure 32. Map of Pelican Cays showing locations of inter-cay hydrographic survey runs 
DISCUSSION 
Pond depths ranged from 4.6 m (15 feet) in relatively small Pond E at Fisherman's Cay to 
15.2 m (50 feet) in the large Pond A at Cat Cay. In general, there appears to be a direct 
correlation between size and depth in the ponds. Such a correlation might be better determined 
by more detailed examination of the size of the circular bottom contour pattern and the depth of 
the configuration. The large ponds seem to be a composite of smaller circular depressions. 
Results for the ponds are summarized in Table 2. 
Table 2. Inter-cay water depths of Pelican Cays group. Survey run locations are shown on 
Figure 32. 
Survey Run 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
Distance (m) Max. Depth (m) 
The variation in salinity within ponds, or between ponds is noteworthy, but not 
remarkable. The near-surface water salinity was 35.1 ppt, except in Manatee Cay, which 
averaged 35.6 ppt, while the bottom water was in most cases about 0.2-0.5 ppt or less. In two 
ponds at Fisherman's Cay, the bottom salinity was slightly higher. Near-surface salinities are 
likely to be significantly less during the rainy season. 
The variation in temperature within ponds and between ponds was not large but is 
consistent and significant. Near-surface water temperatures ranged from 3 1 .O0 C to 3 1.8 C, 
averaging 3 1.3" C, while bottom water temperatures ranged from 29.0" C for the deepest reading 
in Pond A at Cat Cay to 31.5" C for the shallowest reading in Pond E at Fisherman's Cay. This is 
not surprising since water temperature is a result of solar radiation and the volume of water 
available to absorb it. The measurements were all taken during the middle of generally sunny 
days. Significant semidiurnal, as well as seasonal, variations in the near-surface temperature are 
to be expected. 
Analysis of the hydrographic survey data for the lagoon regions between the cays (Table 
2) shows that the sides of the cays exhibit very steep slopes similar to the ponds and relatively 
flat bottoms over the major length of the runs. Analysis of 23 lagoon slopes gives an average of 
28", considerably greater than the 17" of the ponds. Again, there appears to be a direct 
correlation between water depth and distance across open water, whether in the ponds of the cays 
or in the lagoon between cays. 
CONCLUSIONS 
I-Iydrographic and hydrologic surveys of ponds in three cays-Cat Cay, Manatee Cay, and 
Fisherman's Cay--of the Pelican Cays Group show that the pond bathymetry characteristically 
shows a circular pattern with steeply sloping sides. An articulated ridge frequently occurs 
between these circular patterns. The pond depths range from 4.6 m in a small pond, such as Pond 
E in Fisherman's Cay (I00 m wide) to 15.2 m in a large pond such as Pond A in Cat Cay (250 m 
wide). The average maximum pond depth is 10.7 m. In general, depth tends to increase with the 
width of the pond. 
Measurenents of water depth between the cays showed maximum depth ranged from 
15.2 m over a survey run distance of 100 m to a maximum depth of 28.7 m over a survey run 
distance of 1,150 m. The average maximum depth of the intercay survey runs is 22.7 m, which is 
more than twice that of the interior ponds. Here, depth tends to increase with increasing distance 
between cays. Side slopes in both ponds and the lagoon are steep, averaging 17" for the ponds 
and 28" for the lagoon. 
Maximum salinities in the ponds averaged 35.3 ppt near the surface and 35.2 ppt near the 
bottom, with a range of 1.5 ppt. There was a slight tendency for salinity to be lower near the 
bottom. Temperature showed a somewhat larger variation, ranging from an average maximum of 
3 1.3" C near the surface to 30.2" C near the bottom, with a relative strong tendency for surface 
temperatures to be about 1 .O0 C higher at the surface. 
ACKNOWLEDGMENTS 
Special thanks arc given to Carrie Bow Cay station managers Tom Hawkins (1994) and 
Steve Hays (1995) for their valuable assistance in the field operations of this study, and to Tony 
Rath of the Pelican Beach Hotel for his very useful aer~al photographs of the cays. Field support 
for this project was provided by the National Museum of Natural History's Caribbean Coral Reef 
Ecosystem Program (CCRE Contribution No. 579). 
REFERENCES 
Kjertve, B., K. Riitzler, and G. H. Kierspe 
1982. Tides at Carrie Bow Cay, Belize, in the Atlantic Barrier Reef Ecosystem at Carrie Bow 
Cay, Belize. In The Atlantic Barrier Reef Ecosystenz at Carrie Bow Cay, Belize, I. 
Structure and Communities, edited by Klaus Riitzler and Ian G. Macintyre. 
Washington, DC: Smithsonian Institution Press. 
Purdy, E. G. 
1974. Karst-determined facies patterns in British Honduras: Holocene carbonate 
sedimentation model. American Association ofPefroleum Geologists 585325-855. 
Purdy, E. G., and G. T. Bertram 
1993. Carbonate Conceptsfrom the Maldives, Indian Ocean. American Association of 
Petroleum Geologists Studies in Geology, No. 34,56 p. 
Ruzler, K., and J. D. Ferrais 
1982. Terrestrial Environment and Climate, Carrie Bow Cay, Belize. In The Ailantic Barrier 
ReefEcosysfem at Chrrie Bow Cay, Belize, I .  Structure and Communities, edited by 
Klaus Rutzler and Ian G. Macintyre. Washington, DC: Smithsonian Institution Press. 
Stoddart, D. R., F. R. Fosberg, and D. L. Spellman 
1982. Cays of the Belize Barrier Reef and lagoon (western Caribbean Sea). Atoll Research 
Bulletin 256: 1-76 . 
U.S. Navy 
1942. Oceanographic Office Map of British Honduras: Ranguana Cay to Columbus Cay. 
Walker, S. H. 
1973. Summary of Climatic Records for Belize, Land Resources Division, Surbiton, Surrey, 
England, Supplement No. 3. Rainfall distribution map contained in Belize, Country 
Environmental Profile, Robert Nicolait & Associates, USAID Contract No. 505-0000- 
C-00-3001-00, April 1994.