The effect of distance from the ocean, volume of water and salinity on the number of gastropod individuals and gastropod species in tidal pools
Introduction
The intertidal area of the Bahamas provides a rocky habitat for many types of organisms. These organisms can be broken into groups according to what zone they are found in. The zones form according to how different organisms deal with variations in environmental factors. One factor, water, helps creates zones based on how much water an area receives (Coulombe, 1992). Some areas are constantly wet; others only during medium to high tide and still others receive spray only from large waves during a high spring tide (black lichen zone) (Course Reader, 2001).
Within the intertidal zone are numerous tidal pools that are home to various types of gastropods such as limpets, nerites, and periwinkles. These pools can be stressful to organisms because of fluctuations in environmental factors such as temperature, salinity, pH, and type of water/availability (Dando and Burchett, 1996). The animals, which live in these areas, have developed ways to deal with the stresses. For example, to prevent desiccation during low tide, the Nerite spp. will seal off by closing their operculum (Barnes, 1980).
The zone looked at in this study was the area that is submerged during high tide, but exposed during low tide, the littorine zone (pink zone) (Course Reader, 2001). The area of inquiry was how some of the environmental factors effected gastropods. The study focused on how distance from the ocean, volume of water and salinity of tidal pools affects the number of gastropod individuals and species. The three major hypothesis were as follows:
Hypothesis 1
As water volume increases, tidal pools will show an increase in the number of gastropod individuals and gastropod species.
Alternative hypothesis: As water volume increases there will not be an increase in the number of gastropod individuals and gastropod species.
Hypothesis 2
As salinity increases the tidal pools will show a decrease in the number of gastropod individuals and gastropod species.
Alternative hypothesis: As salinity increases, tidal pools will not show a decrease in the number of gastropod individuals and gastropod species
Hypothesis 3
As distance from the ocean increases, tidal pools will show a decrease in the number of individuals and gastropod species.
Alternative hypothesis: As distance from the ocean increases, tidal pools will not show a decrease in the number of individuals and gastropod species.
Methods
The study area was the North Point of San Salvador Island, Bahamas (see appendix). Three study areas were selected to represent high, medium and low energy regimes. The high-energy area was the North Point, the medium energy area was the bay in which Man Head Key was located and the low energy area was GrahamÕs Harbor. Within each study area three tidal pools were selected in a non-random manner and during non-high tide times, for a total of nine tidal pools. In order for a tidal pool to be selected it needed to be in the littorine/pink zone, easily accessible and of a reasonability measurable size (> 10 cm and < 60 cm across). Each tidal pool was marked so over the course of the study the same pools would be measured each time.
The measurements were carried out three afternoons over the course of four days in June. For each tidal pool the following parameters were measured: horizontal distance from the ocean, salinity, approximate volume of water in the pool based on the depth and diameter, and number of each species of living gastropods in the pool. It should be noted the area considered to be the pool extended to 10 cm above the water level.
The data was analyzed using Excel graphs and Chi-square tests. Three Chi-square tests were performed. In order to test the volume hypothesis tidal pools were broken down into small (< 0.2L), medium (0.4-1.8L) and large (6.7 Ð 7.5 L)) groups and then compared to number of individuals and number of species. The test for salinity was done by breaking the data groups into normal (33-30ppt), hyper (40-50ppt), and super hyper (55-100ppt) saline levels and comparing then against number of individuals and number of species. The distance from the ocean groups were 0 - 2.6m, 3 Ð 5.9m, and 7.45 Ð 11.2m which were compared with number of individuals and number of species. The graphs were scatterplots with best-fit lines. The six graphs are as follows: salinity vs. #of species, salinity vs. # of individuals, volume vs. # of species, volume vs. # of individuals, distance vs. # of species, and distance vs. # of individuals.
Results
In order to test if there was a relationship between previously mentioned factors and # of species and individuals, Chi-square tests were preformed with an alpha at 0.05 as well as scatterplots with best fit lines. The water volume results for the Chi-square test gave a P-value of 0.0011. Therefore there is a relationship between volume, number of species and number of individuals. The salinity Chi-square test gave a P-value of 0.0038; there is a relationship between the salinity, number of species, and number of individuals. For distance the value of the Chi-square test was a P-value of <0.0001 showing a relationship between distance from ocean, number of species and number of individuals. The scatterplots all had a best-fit line, which corresponded with the findings of the Chi-Square tests and gave a direction to the correlation (graphs 1-6)). The charts of the Chi-square set up and results can be found in the appendix or at TCP 134.53.98.237. The raw data can also be found in the appendix.
Graph 1: Volume of water versus Number of Individuals.
As volume of water increases there is a small increase in the number of individuals
Graph 2. Volume vs. Number of Species
As the volume increases the number of species shows a small increase.
Graph 3. Salinity versus Number of Individuals
As the salinity increases there is a marked decrease in number of individuals
Graph 4. Salinity Versus Number of Species
As salinity increases there is a decrease in the number of species.
Graph 5. Distance versus Number of Individuals
As the distance increases the number of individuals decreases.
Graph 6. Distance versus Number of Species
As the distance increases the number of species decreases.
Note: Due to lack of a statical package such as Minitab, it was not possible to conduct a Regression test. The Chi-square test shows a correlation between the different variables and factors and the graphs shows the direction of the trend. A regression test would be needed to determine in which direction the increase or decrease was and if it was significant. The hypothesizes cannot not be rejected or accepted until further statical analysis can be done.
Discussion
There is a significant relationship between all of the parameters tested as shown by the Chi-square tests and scatter plot graphs. There is a relationship between (1) volume of water, number of individuals, and number of species, with the graphs showing a trend towards as the volume of water increases the number of individuals and species increases (graph 1 and 2), (2) level of salinity, number of individuals and number of species, with the graphs showing a trend towards that as salinity increases the number of individuals and species decreases (graph 3 and 4), (3) horizontal distance from the ocean, number of individuals and number of species, with the graphs showing a trend towards as the distance increases the number of individuals and species decrease (graph 5 and 6). Further statical analysis will help give the numbers to prove or disprove the hypothesizes.
Data was collected on Nerita peloronta shell size as well. This data could be used for further study into the effects of water volume, salinity, and distance from ocean on shell size. Diversity indices could also be looked into. In a past study the relationship between the salinity decreasing the diversity increasing was looked at and this idea could be tested in this Bahamas situation. (Ysebaert, Meire, Coosen, and Essink, 1998). In future studies tidal pool temperature could be used a variable as well as the impact of low/medium/high energy sites on number of individuals and species. There could also be further studies done to look into anthropogenic stress (Ysebaert, Meire, Coosen, and Essink, 1998) and factors other then environmental such as biological stresses (food, space, etc) (Dando and Burchett, 1996). An over all improvement would be a study with more sites and a longer study period.
There were sources of error and limitations to be considered in this study. The vertical distance from the ocean was not considered, sampling did not occur at the same point in the daily tidal cycle, pool selection was not random, questionable accuracy in counting small gastropods, a mystery snail which was found in large numbers, one site was lost (#7), site disturbance when taking measurements, volume was based on a cylinder, and there was not differentiation between limpet species.
Overall the study provided preliminary data and results to the questions posed by the hypothesizes, many more doors have been opened for further research into intertidal factors and the effects on the gastropods which live there.
Works Citied
Barnes, R.D. Invertebrate Zoology. Philadelphia: Saunders College/Holt, Reinhart, and Winston. 1980.
Coulombe, D.A. The Seaside Naturalist. New York: Simon & Schuster. 1992.
Course Reader. Chapter 15: The intertidal zone adaptations and tropic relations. Pp. 205-225. Oxford Copy Shop. 2001. Note: I ripped out the pages before leaving, so I donÕt know the real citation.
Dando, M. and Burchett, M. SeaLife: A Complete Guide to the Marine Environment. Washington D.C.: Smithsonian Institution Press. 1996.
Ysebaert, T., Meire, P., Coosen, J., and Essink, K. ÒZonation of intertidal macrobenthos in the estuaries of Schilde and EmsÓ. Aquatic Ecology 32: 53-71. 1998.
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