Slough Discharge and Its Effects on Crabs

Bennett Adkinson Meghan DeFord Mike Finger Chrissy Michalik

Slough Discharge and its Effects on Crabs

Abstract

The discharge of the South Slough on Ossabaw Island, Georgia changes throughout the day according to tidal flux. The experiment was to determine whether or not organism composition in the slough changed with this daily change in water volume. A negative correlation between crabs (hermit, blue, and fiddler) and discharge was found, but the small data set yielded statistically insignificant results. Determining the dynamics of biodiversity in this undisturbed barrier island habitat can serve as a reference for developed island habitat restoration.

Introduction

Ossabaw Island is one of several barrier islands that guard the coast of Georgia from the Atlantic Ocean. When examining maps of Ossabaw Island from 1992 and 1999, the changes in the orientation of the slough that runs along the southeastern edge of the island are apparent. In 1992, the slough inlet is roughly 1.5 miles northeast of the South Pavilion. In 1999, the slough inlet has migrated southwest, and is pictured approximately 1/3 of a mile southwest of the South Pavilion.

Our group wondered if the dynamics of the slough, specifically how the slough migrates over time, could be approximated based on the amount of water that enters verses the amount of water that exits each day. In addition, we were curious if these factors contributed to the presence of different species in the slough throughout the day, as these species must be tolerant of a somewhat dynamic environment on a longer time scale, as well as the daily fluctuations. The knowledge of biodiversity in an undisturbed habitat, such as Ossabaw Island, during the fluctuations of daily tidal conditions could serve as a model for the restoration of disturbed locations, such as the nearby Tybee Island.

Our experiment focused on determining whether or not the change in daily discharge of water in slough according to the tides correlated with a change in the species composition along a particular point in the slough. We believed that as the discharge increased that species composition would decrease in number and diversity.

Materials and Methods

Materials Ø Ruler Ø Rope Ø Permanent marker Ø Pole Ø Duct Tape Ø Orange Ø Stopwatch Ø Stakes Ø Camera Ø Mask/snorkel Ø Field notebook

Study sites

We observed two study sites along the slough. The first site was located at the beach access point from camp. The transect was identified by it’s adjacency to the cord grass bed. The second site was near the inlet to the ocean; the transect was performed at the apex of the last large meander bend, near the “tree graveyard” at South End Beach.

Sampling

Cross sectional area: Once the transect locations were identified, we marked their position with stakes. A 14-meter length of rope was marked with metric units using a permanent marker. The rope was used to measure the length across the slough at the transect location. Depth measurements were taken every meter across with the pole that had also been marked with metric units using a permanent marker and duct tape. Cross sectional area was then derived by multiplying the length by the average depth.

Velocity: To determine the velocity, we dropped an orange in the slough and let it float 1 to 10 meters as measured with the rope. An orange was used because it floats slightly below the surface, thereby giving a more accurate representation of the velocity vertically through the water column. The time it took for the orange to complete the float was recorded in seconds. This procedure was repeated 3 to 5 times according to the relative length of the transect. Velocity could then be calculated by dividing the distance traveled by the number of seconds required to travel that distance.

Discharge: Discharge of the slough was calculated by multiplying the cross sectional area by the average velocity. This results in an approximation of the volume of water that moves past a particular point in a certain amount of time.

Biology: Organisms were counted on a visual and tactile basis. During times of shallower depths, organisms could be easily viewed directly or with aid of a mask. The observer walked the transect line and counted all organisms that were present in the vicinity of the line (approximately a .5-meter wide window along the transect). During times of deeper water, the turbidity prevented clear observation, even with a mask. We relied on feeling for organisms along the transect, with approximately a 1-meter wide window along the line.

Times: In order to cover the variations of daily tidal change, the discharge and biology samples were taken on five occasions according to the tide chart for the day, May 29, 2004. The times for the samples were spaced with equal intervals; however, some time elapsed between the sampling of site one and the sampling of site two. · 8:51 am · 10:51 am (low tide) · 12.52 pm · 2:53 pm · 4:53 pm (high tide)

After discussing our initial experimental design with Dr. Arthur Stewart, we made minor changes to our materials (e.g., substituting the orange for a fishing bob for the velocity determination).

Results

Table A. Data collected from site 1. Time Avg Depth (cm) Depth (m) Width(m) Area (m2) Velocity (m/s) Discharge(m3/s) 1 38.5 0.39 12.1 4.66 0.16 0.73 2 36.1 0.36 12 4.33 0.11 0.49 3 36.7 0.37 11.7 4.29 0.09 0.39 4 48.3 0.48 31.2 15.07 0.53 8.05 5 64 0.64 42.7 27.33 0.21 5.7

Table B. Data collected from site 2. Time Avg Depth (cm) Depth (m) Width(m) Area (m2) Velocity (m/s) Discharge(m3/s) 1 13 0.13 13.7 1.78 0.33 0.58 2 11.1 0.11 12.7 1.41 0.28 0.39 3 10.6 0.11 11.1 1.18 0.24 0.29 4 44.9 0.45 43.3 19.44 0.51 9.88 5 49.2 0.49 61.5 30.26 0.05 1.49 The discharge results for the five trials at two sites are shown in Tables A and B. During sample times 1 through 3, the water was flowing out to the ocean, and during sample times 4 and 5 water was flowing inland. The cross sectional area decreased at both sites from time 1 to time 3, and then increased dramatically from time 3 to time 5. Velocity was the greatest at the sample time before the high and low tides (sample time 1 and sample time 4).

We observed several different organisms in the slough, including hermit crabs, blue crabs, fiddler crabs, snails, minnows, bamboo worms, and one large, unidentified fish. The crabs offer a more reliable presence that can be variable (i.e., the bamboo worm count should be the same on every sample because they are sessile). A regression analysis was run with the total number of crabs and the discharge. There seems to be a negative correlation between the two (R=-.6912), but the lack of data renders this result statistically insignificant (p=1962).

Discussion/Conclusions

Our results indicate that as more water rushes into the slough from the ocean, benthic organisms, such as hermit crabs and blue crabs, are more than likely swept downstream. The noticeable decrease in their numbers at both sites as the discharge increased seems to imply this pattern. However, without more data over a longer time period, the current trend is rendered insignificant. In addition to gathering more data for this study, future studies on the species diversity over time would be important. Regular seasonal sampling would be crucial in determining the actual number of organisms present and the changes in discharge in the slough on a yearlong basis. In order to compare these data, a similar study on a more developed island would also be important.

We would also make several modifications to our study. As mentioned above, we would need to gather many more data points over a longer time frame on more sample sites in order to yield statistically significant results. In addition, our largest obstacle was an accurate means of organism sampling, especially when the discharge is high; any repeats of the study should include a more reliable means of sampling organisms. The other modifications include more slightly more sophisticated equipment, such as a flowmeter for discharge, a field tape measure for distances, and sturdy stakes for marking the transect.

Bibliography

Schoettle, Taylor. (2003). The Natural History of Georgia's Barrier Islands. Retrieved June 10, 2004, from http://sherpaguides.com/georgia/barrier_islands/natural_history/ Crabs vs. Time (Site 1)

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3 # 0 1 2 3 4 5 Sampling Intervals

Crabs vs. Time (Site 2)

15 s l 12 a u Hermit d 9 i v i Fiddler d 6 n

I Blue f 3 o

# 0 1 2 3 4 5 Sampling Intervals Discharge vs. Crab Frequency

12 ) y c 10 n Site 1 e u Site 2 q

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0 0 2 4 6 8 10 12 Discharge (m3/s)