Group Experiment
Ossabaw Island
2006
A Survey of Three Bodies of Water on Ossabaw Island:
Ecosystem Inhabitants and Salinity Values
Experiment Group
Lisa Taylor
Amy Baker
Susan Smathers
James Robertson
Abstract
Ossabaw Island and other barrier islands along the coast of the Southeastern
United States support a variety of plant and animal life. Some of the species represented
are similar to other well-known ecosystems on the mainland and other coastal regions.
Some of the flora and fauna are unique to these kinds of areas. This study surveyed three
selected areas on the island that served as habitats to three very different living
ecosystems. In addition to inventorying the living communities in these areas, this study
focused on the bodies of water that characterized each site. Salinity values at each site
were measured using a hydrometer.
Introduction
“Ossabaw Island is the third largest of Georgia’s Sea Islands and lies along the Atlantic
Ocean about 20 miles south of Savannah. The island encompasses over 16,000 acres of
tidal marshes and 9,000 acres of high ground. Ponds, salt water creeks, one river and
many sloughs, inlets, and beaches make the high ground of Ossabaw a complex of small
islands.” (Georgia Department of Natural Resources, 2002)
There are three unique ecosystems on the island, which can generally be classified
as beach/dune, saltmarsh/estuary, and forest communities. Each of these areas supports
distinctly different categories of flora and fauna. From a physiological perspective, it
might be assumed that these unique collections of plant and animal species have
developed characteristics that have allowed them to adapt to certain conditions that are
present in these particular areas. One specific condition that is remarkable is the variety
of types of bodies of water found in such a limited area.
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Since the aquatic areas are in such close proximity to one another, and seem to be
so interconnected, one may presume that they are all very similar. However, this may
contradict the idea that the species have any special ability to survive in a habitat that has
differentiation or fluctuations in water salinity.
Is there a division or separation between salt/brackish water and fresh water, and
where are those areas? Is there a definite distinction between what plants and animals
were found in those areas?
In order to try to determine the answers to these inquiries, a survey of three
specific sites on the island was taken. Observations were recorded of what types of
organisms were found at each site. Water samples were also taken at each site to
determine if there were any differences in salinity.
Salinity values were measured in terms of specific gravity. Also known as relative
density, specific gravity is a dimensionless measurement in which the density of a
substance is compared to a reference density. Generally, the density of water is used as a
common reference. In this case, water’s specific gravity is identified as “1.” The densities
of other substances are described in relation to water. For example, a substance with a
specific gravity <1 has a lower density relative to the density of water. Similarly, a
specific gravity >1 indicates that the substance is more dense than water (Wikipedia, June
1, 2006).
Specific gravity of liquids can be measured using a hydrometer. According to
Wikipedia (June 21, 2006), “the function of the hydrometer is based on Archimedes’
principle that a solid suspended in a liquid will be buoyed up by a force equal to the
weight of the liquid displaced.” Although, laboratory hydrometers are usually made of
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glass, the field hydrometer used for this study was a plastic model, consisting of a
demarcated containment section. A weighted marker was situated in this section so that
when liquid filled the container, the weight would sink or float based on the relative
density of the substance.
Experiment
Objective: To determine, if any, the difference in salinity of the inland marshes and the
slough of the south end of Ossabaw Island.
Materials: Bucket
Rope
Hydrometer
Thermometer
GPS locator
Map of Ossabaw Island
Collection sites: (see Figure 1)
• Causeway
Northside
Southside
•
Upper Slough
•
Inlet Slough
•
Beach at Tidal Pool
GPS Coordinates
N 31° 44 min. 55.4 sec
W 081° 07 min. 20 sec
GPS Coordinates
N 31degrees 44 min. 41.76 sec
W 81 degrees 06 min 24.80 sec
GPS Coordinates
Data Not Available
GPS Coordinates
N 31 degrees 43 min 38.25 sec
W 81 degrees 07 min 32.90 secs
Procedure:
Collections of water samples were each taken at low tide at 5:55 a.m. on May 29 as well
as at high tide at 5:50 p.m. at each of the designated collection sites. Each site had three
samples taken at each time. The salinity values were calculated through the use of a
hydrometer and recorded. As a standard of measure a sample was also taken from the
well water.
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Results:
Well Water
Sample 1 – Well
Sample 2 – Well
Sample 3 - Well
1.001
1.001
1.001
16 ppt
16 ppt
16 ppt
Low Tide Readings:
Causeway (Figure 2)
Sample #1 < 1.012 or 16 ppt
Sample #2 < 1.012 or 16 ppt
Sample #3 < 1.012 or 16 ppt
Sample 1- Causeway
Sample 2 –Causeway
Sample 3 - Causeway
1.012
1.012
1.012
16 ppt
16 ppt
16 ppt
Upper Slough (Figure 3)
Sample #1 1.0215 or 29 ppt
Sample #2 1.022 or 30 ppt
Sample #3 1.022 or 30 ppt
Sample 1 Upper Slough
Sample 2 Upper Slough
Sample 3 Upper Slough
1.0215
1.022
1.022
29 ppt
30 ppt
30 ppt
Beach (Figure 4)
Sample #1 1.0205 or 28 ppt
Sample #2 1.0205 or 28 ppt
Sample #3 1.0205 or 28 ppt
Sample 1 Beach
Sample 2 Beach
Sample 3 Beach
1.0205
1.0205
1.0205
28 ppt
28 ppt
28 ppt
Tidal Pool (Figure 5)
Sample #1 1.0205 or 28 ppt
Sample #2 1.02 or 28 ppt
Sample #3 1.0205 or 28 ppt
Sample 1 – Tidal Pool
Sample 2 – Tidal Pool
Sample 3 – Tidal Pool
1.0205
1.02
1.0205
28 ppt
28 ppt
28 ppt
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High Tide Readings:
Causeway ( South)
Sample #1 1.020 or 27 ppt
Sample #2 1.019 or 25 ppt
Sample #3 1.0195 or 26 ppt
Sample 1 S. Causeway (H)
Sample 2 S. Causeway (H)
Sample 3 S. Causeway (H)
1.020
1.019
1.0195
27 ppt
25 ppt
26 ppt
Causeway (North)
Sample #1 < 1.012 or 16 ppt
Sample #2 < 1.012 or 16 ppt
Sample #3 < 1.012 or 16 ppt
Sample 1 N. Causeway (H)
Sample 2 N. Causeway (H)
Sample 3 N. Causeway (H)
1.012
1.012
1.012
16 ppt
16 ppt
16 ppt
1.022
1.022
1.022
27 ppt
27 ppt
27 ppt
Upper Slough
Sample #1 1..022
Sample #2 1.022
Sample #3 1.022
Sample 1 Upper Slough (H)
Sample 2 Upper Slough (H)
Sample 3 Upper Slough (H)
Beach
Sample #1 1.0205 or 28 ppt
Sample #2 1.0205 or 28 ppt
Sample #3 1.0205 or 28 ppt
Sample 1 Beach
Sample 2 Beach
Sample 3 Beach
1.0205
1.0205
1.0205
28 ppt
28 ppt
28 ppt
Slough Inlet
Sample #1 1.021 or 28.5 ppt
Sample #2 1.0215 or 28.5 ppt
Sample #3 1.021 or 28.5 ppt
Sample 1 Slough Inlet (H)
Sample 2 Slough Inlet (H)
Sample 3 Slough Inlet (H)
1.021
1.0215
1.021
28.5 ppt
28.5 ppt
28.5 ppt
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Tidal Pool
Sample #1 1.0225 or 30 ppt
Sample #2 1.0225 or 30 ppt
Sample #3 1.0225 or 30 ppt
Sample 1-Tidal Pool
Sample 2 –Tidal Pool
Sample 3 – Tidal Pool
1.0225
1.0225
1.0225
30 ppt
30 ppt
30 ppt
Discussion and Findings:
Fresh water has a salt concentration of less than 0.10%, slightly saline or salty
water is 0.10% to 0.30% salt, moderately saline water is 0.30% to 1.0% salts, and highly
saline water is 1.0% to 3.5% salts. The hydrometer that was utilized in our water
measurements has a scale with its “low-end” value of 1.6%. This falls within the “highly
saline” category. The “fresh water” samples that were taken from the well water at the
camp produced a reading well below the 1.6%, but the scale did not extend to that level.
In fact, the needle rested on the floor of the instrument well. Therefore, we established
this as an indicator for whether or not a sample was “fresh water” or not.
Based on our established scale interpretation, the data show we had sampling sites
that were salt water and a site that was fresh water. We sampled at low tide and at high
tide. Due to low water levels, we were unable to obtain a sample from the south side of
the causeway. The beach, south side of causeway, upper slough, and slough inlet all
produced readings as highly saline/salty. The north side of the causeway produced
readings that were regarded as fresh.
Additional observations help to corroborate our findings. In all the areas that
tested as high in salinity, we observed plants and animals that would be typically found in
those areas. Animals such as sand pipers and other wading birds and a variety of crabs
were observed. Cord grass, rush, and other salt water adapted plants were observed. At
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the north side of the causeway, the only site that tested as fresh water, we observed
alligators, a large variety of wading birds, frogs, and no crabs. Along with typical marsh
grasses, we observed a large amount of duck weed. Duck weed inhabits fresh water
ponds and impoundments.
Based on our findings, there exists on Ossabaw Island areas that are definitely salt
water and definitely fresh water. An interesting item to note is in regards to the causeway
sampling site. At the causeway, salt water is separated from fresh water by the earthen
dam that supports the road that leads to the south end camp area.
For Further Consideration
Future studies that may result from the findings of this survey include:
1. What plant or animal species can survive in more than one or all of these
ecosystems? What adaptations make this possible?
2. Would a survey of more sites reveal how and where the salt water/fresh water
“boundary” is formed?
3. What is the source of the fresh water on the island?
4. How has the presence of humans impacted the distribution of salt water/fresh
water? How does that impact the living communities on the island?
5. Can soil samples on the island indicate that the wetland areas act as a filter to
remove minerals from ocean water, producing fresh bodies of water?
6. With a more sophisticated measuring instrument, can the specific mineral
contents of the water samples be analyzed more closely to obtain more
accurate hypotheses?
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References
Georgia Department of Natural Resources (November 19, 2002). General information
about Ossabaw Island. Retrieved June 24, 2006, from
http://georgiawildlife.dnr.state.ga.us/content/displaycontent.asp?txtDocument=22
5.
Wikipedia (June 21, 2006). Hydrometer. Retrieved June 24, 2006 from
http://en.wikipedia.org/wiki/Hydrometer.
Wikipedia (June 1, 2006). Relative density. Retrieved June 24, 2006 from
http://en.wikipedia.org/wiki/Specific_gravity.
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Figure 1: James marking GPS
Figure 2: Causeway
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Figure 3: Upper Slough
Figure 4: Lisa taking water sample at the beach location
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Figure 5: Tidal Pool
Figure 6: Amy and James calculating salinity values
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