Introduction
Coral reefs are diverse communities that consist of many marine organisms interacting together. Coral reef ecosystems serve many important purposes, such as creating natural barriers that serve to protect shorelines from storms and erosion, support fisheries as a source of food, as well as a source of income in tourism (Pendleton et al. 2016). Unfortunately, in recent years, coral reefs have reduced in abundance. The reduction could be linked to the intermediate disturbance hypothesis or other factors such as direct and indirect impacts from human activity. Direct impacts include anchor strikes from boats breaking coral or, physical interaction of people with coral reefs (fin kicks, standing on the reef, etc.). Indirect impacts, such as ocean acidification, runoff, and sedimentation, could also be a major factor in the coral reef decline. The intermediate disturbance hypothesis states that coral reef communities will experience maximal diversity when disturbances are at intermediate levels (Aronson and Precht, 1995, Rogers, 1993)
In Hawaii, the most significant cause of coral reef disturbances is physical disturbances from waves, which have been long recognized as one of the most influencing factors that can control coral growth and reef developments (Dollar, 1982). If the time between disturbances increases, diversity would increase because there would be more time available for the invasion of more species (Connell, 1978). The purpose of this study is to compare coral abundance at two sites on Oahu, Hawaii.
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Methods
This study compared the coral abundance at two different sites on Oahu, Hawaii. The first site was at Pokole Beach Access at Lanikai Beach and the second site was Makai Pier in Waimanalo. Pokole is characterized by relatively low wave activity, while Makai Pier is characterized by relatively high wave activity. Although the two sites differ in their wave activity, they share some similar characteristics: they are both exposed to disturbances, patchy distribution of coral reefs, and comparable coral species.
The coral abundance at both sites was surveyed using a 15 meter lead transect line, which was selectively placed on reef patches that were approximately 35 meters offshore. For each site, this was repeated 8 times. The data collected were benthic substrate observed every half meter along the transact line and all coral colonies under the transect line were recorded. T-tests were conducted to compare the abundance of corals, crustose coralline algae (CCA), and turf.
Results
Coral reef diversity and abundance of three different species of corals were compared between two reefs on Oahu, along with other benthic substrates such as sand, rubble, CCA, turf, and algae. The two study sites had morphological similar coral distribution pattern and both experienced disturbances. Pokole experienced relatively lower wave activity than the second study site, Makai pier.
Figure 1 shows that at Pokole, corals and algae were more abundant than Makai Pier. Meanwhile, Figure 2 shows Montipora had a significantly higher abundance at Pokole than Makai Pier, while the other two species of corals ( Porites and Pocilipora ) were not significantly different. A one-tailed t-test showed that corals located in Pokole beach were significantly more abundant than corals located at Makai pier (P<0.05).
Discussion
Coral reefs can be affected by many factors since they are fragile communities. Those factors can range from direct and indirect human disturbances to natural disturbances such as wave activity, hurricanes, etc. Wave activity is considered to be the main factor affecting coral reef communities (Dana, 1979). According to the intermediate disturbance hypothesis, we can expect coral reef communities will experience maximal diversity when they experience intermediate levels of disturbances (Aronson and Precht, 1995, Dial and Roughgarden, 1998, Rogers, 1993). Other environmental factors such as storms and hurricanes may affect coral reef cover and abundance but not its diversity (Porter et al.1981, Rogers et al. 1983, Rogers, 1993).
Coral reef abundance and diversity were surveyed at two different sites in Oahu, Hawaii. Each site differed in the wave activity that it experienced. Pokole beach at Lanikai experienced a much lower wave activity than Makai Pier because it is protected while Makai Pier is exposed to the ocean. The patchiness nature of coral reefs may be caused by disturbances (Grigg1983). The results indicated that Pokole beach had significantly more corals than Makai Pier, this could be due to the lower wave energy it experiences thus lower disturbance. The time between disturbances also affects the coral abundance by allowing more recovery time for corals to re-settle in the community (Connell et al. 1997, Lewis, 1997). The results also indicated that Montipora abundance in the two sites differed, this could be due to low tolerance of Montipora to wave activity, thus in areas where the disturbance by waves is high such as at Makai Pier, the abundance of Montipora decreases. More studies are required to access the tolerance of Montipora to high wave activity.
References
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