A Magnificent Frigate chick and "thrown-up" squid in Grahams Harbor, San Salvador, Bahamas. See other beautiful phenomena from the Bahamas.
Is Eutrophication Present in Graham’s Harbor?
Eutrophication can be simply defined as a process that increases the rate of supply of organic material to a particular ecosystem. Eutrophication usually leads to high nutrient concentrations, which stimulate blooms of algae (phytoplankton). Eutrophication is also a natural process by which aquatic systems (like lakes) age and gradually become more and more productive. This “natural” process usually takes thousands of years. Unfortunately humans (anthropogenic influences) have significantly accelerated this process. This human-caused acceleration is sometimes referred to as “cultural eutrophication”.
Humans add plant nutrients primarily in the form of phosphorus, nitrogen and carbon to aquatic ecosystem (streams, lakes, oceans) in an assortment of ways. In our part of the world one of the largest sources of these nutrients comes from runoff from agricultural fields, golf courses, and urban lawns. Another large contributor is untreated or partially treated sewage. In Sal Salvador the contributions from sewage to eutrophication seemed to be more important that the runoff from agricultural fields and other sources. For this reason our study focused on the sewage pipe that ran from the Gerace field station to the ocean. The excess nutrients interfere with the health and diversity of native fish, plant and animal populations. Consequently, biodiversity in these aquatic ecosystems is often disturbed and depleted. Sewage usually contains large amounts of phosphates stemming from detergents. The phosphates are used to soften water and improve cleaning action, but negative side effects include stimulating algal growth.
The algal blooms primary affect the marine ecosystem in two ways. Initially the concentration of algae blocks live giving sunlight. This causes underwater grasses to die as well as the zooxanthellae (algae) living within the corals. As a result a domino effect commences. The grasses provide food, shelter and/or a spawning and nursery habitat for many marine species. When the grasses are negatively affected so are the many marine organisms that depend on them. Secondly the algae eventually die and decompose using up precious oxygen. The lowered oxygen levels make it difficult for other aquatic organisms to survive.
Among the organisms affected by the eutrophication are corals. By nature corals require the cleanest water of any coastal ecosystem. Over time corals have evolved to depend on the lowest nutrient environments in the world. In this type of environment plants usually consume all available nitrogen and phosphorus. A slight increase in nutrients could be beneficial, however only a slight increase above this could cause the beneficial effects to turn negative. The surplus of nutrients stimulates algal blooms which eventually smother and kill coral.
Our predictions for our research project are as follows: the areas nearest to the sewage pipe will have a higher concentration of algae and grasses, the areas nearest to the pipe will have lower species diversity, and algae and grasses nearest the flow from the pipe will be taller.
The first part of this experiment was to delineate the number of transects and the distance between transects. We decided that five transects would be enough with the pipe being the central transect. Therefore, we had two transects on either side of the pipe. The transects were at 10m intervals on either side of the pipe. Once the transect intervals were established then we worked on creating intervals for the outward (away from shore) measurements. We decided that we would go away from the shore 20m and based on the distance we divided the transect into 4 meter increments, meaning that we took 5 measurements along each transect. Then using a 0.5m2 metal quadrant we took two sets of data measurements at each point. At each spot we measured three things; the approximate height of the vegetation, the percent coverage of the vegetation, and the number of species. In order to measure the percent coverage we divided it into three categories. The lowest category was for vegetation that covered 0-33% of the total area of the quadrant. Medium coverage was defined as 34-66% coverage, and high coverage was 67%-100% coverage of the quadrant. In addition salinity testing was conducted in order to delineate any differences at the different distances from the pipe.
In order to evaluate the data we used the Analysis of Variance (ANOVA) to analyze the data. For our research we decided that a p-value of 0.05 would be significant enough to examine our data. This means that we can be 95% confident in the results of our analysis. In addition to this analysis we preformed a post hoc to identify which groups were significant different from each other. This post hoc was called a Scheffe’s test. The test was performed because the ANOVA will only tell us if there is a significant difference somewhere in all our transects, but the post hoc actually tells us where the significant differences are.
For species diversity we can accept our hypothesis because at the 95% confidence level we can say that there is a significant difference (p-value 0.0014<0.05) among the transects (Figure 1). Visually Figure 1 further displays the difference among transects with the graph. As you can see the transect at the pipe had the smallest species diversity, and diversity increased as the distance from the pipe further increased. The transects that were 20 m up or down stream of the pipe were the most diverse.
Figure 1: ANOVA Species Diversity
When the post hoc analysis (Scheffe’s) was conducted it supported the results that could be interpreted from the graph. The post hoc confirmed that the transect 20m down stream from the pipe was significantly different from the pipe, and that the transect 20m up stream from the pipe was also significantly different from the pipe (Figure 2).
Figure 2: Post Hoc of Species Diversity
For percent coverage we have to reject our hypothesis because at the 95% confidence level we can say that there is a significant difference (p-value 0.0001<0.05) among the transects (Figure 3). Visually Figure 3 further displays the difference among transects though the graph. As you can see the transect at the pipe had the lowest percent coverage, and percent coverage increased as the distance from the pipe increased. The transect that was 20m down stream had the highest percent coverage out of all the transects.
Figure 3: ANOVA Percent Coverage
When the post hoc analysis (Scheffe’s) was conducted it supported the results that could be interpreted from the graph. The post hoc confirmed that the transect 20m down stream from the pipe was significantly different from the pipe (Figure 4). The post hoc also went further by showing us that the transect up stream 10m was significantly different from the 20m down stream (Figure 4). In addition the transect up stream 20m is significantly different from the transect down stream 20m (Figure 4). Finally the transect down stream 10m is significantly different from the transect down stream 20m (Figure 4). As you can see even transects that are right next to each other (Down 10 & Down 20m) can be significantly different from one another in terms of percent coverage of vegetation.
Figure 4: Post Hoc Percent Coverage
For vegetation height we have to reject our hypothesis because at the 95% confidence level we can say that there is a significant difference (p-value 0.0006<0.05) among the transects (Figure 5). Visually Figure 5 further displays the difference among transects though the graph. As you can see the transect at the pipe had the lowest vegetative height and vegetative height increased as the distance from the pipe increased. The transect that was 20m down stream had the tallest vegetative growth out of all the transects.
Figure 5: ANOVA Vegetation Height
When the post hoc analysis (Scheffe’s) was conducted it supported the results that could be interpreted from the graph. The post hoc confirmed that the transect 20m up stream from the pipe was significantly different from the pipe (Figure 6). The post hoc also went further by showing us that the transect down stream 20m was significantly different from the pipe (Figure 6).
Figure 6: Post Hoc Vegetation Height
The only hypothesis that was correct, was our prediction that the number of species would decrease closer to the pipe. We had to reject our predictions that the percent coverage and the height of the vegetation would increase the closer you got to the pipe. Overall we found that the algae and grasses were taller and more abundant coverage downstream from the pipe.
One of the conclusions that we came to from this study is the nutrient overloading near the pipe favored certain species, and led to a reduced number of species right at the pipe. When nutrients are high they are going to favor the species that can out-compete the rest. A possible explanation for the decrease in the coverage and the height of the plants near the pipe could be the increased freshwater flow from the sewage pipe.
Another finding in this study helped to confirm that there might be a slight influence from the freshwater. Salinity measurements were made along the shoreline at each transect. The salinity at the pipe was found to be 3.6, which was slightly lower than the 4.2 measurement that we got from the transects 20m from the pipe. Based on the difference in salinity we can say that the inflow of freshwater may affect the success of the algae and grasses that normally favor saline conditions. In the spots closest to the pipe where salinity was lower there was less diversity, while the vegetation cover, and the average height of the vegetation was lower. The freshwater could have made the environment harsh for the algae and grasses. In the future further testing could confirm our findings.
If we had more time we could increase the number of transects in order to see where the patterns (more diversity, less coverage, shorter at 20m away from pipe) stop. Additional water quality test should be done in order to determine the nutrient content, pH, and other chemical compounds. Measurement of biomass from the algae and grasses at each spot would give us a better idea of how the vegetation responses to inflow of nutrients and freshwater. In addition a universal index of species diversity could be used in the future to measure species richness and abundance.
Pinckney, James L., Paerl, Hans W., Tester, Patricia, and Richardson, Tammi L. (Oct. 2001). “The Role of Nutrient Loading and Eutrophication in Estuarine Ecology.” Environmental Health Perspectives 109(5)
2001. “The Impact of Phosphorus on Aquatic Life: Eutrophication.” http://www.agnr.umd.edu/users/agron/nutrient/Factshee/Phosphorus/Eutrop.html
“Eutrophication: Trend, Pressure, State, Response.” http://www.grida.no/soeno97/eutro/
“Eutrophication and Water Quality.” Global Coral Reef Alliance. http://globalcoral.org/Eutrophication%20and%20Water%20quality.html
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