BERMUDA
Biodiversity of Coral Reefs
PAGE INDEX
PAGE INDEX
Hard Corals Soft Corals Overall Diversity Environmental Stressors
The causeway site has the lowest mean D. strigosa, and statistically insignificant lower means for the other two hard corals, following the trends recorded previously due to human disturbance, low flow, sedimentation, increased heavy metal availability, increased oxidative stress, and increased turbidity.
These study sites were conducted in shallow waters, ranging from 4 meters to less then a meter deep. Jones found that BBD (Black Band Disease) affects corals in shallow waters more vehemently. Interestingly, D. labrynthiformis appears to be immune to BBD in Bermuda, giving it competitive advantage over the dominant D. strigosa. While deeper terrace reefs are comprised of thriving D. strigosa in a 3:1 ratio with their relative, D. labrynthiformis, the shallower lagoons where disease is more prevalent (and we did our sampling) were shown to have a much more even distribution of the two species (1:1). In addition, little to no BBD has been recorded in Castle Harbor, despite the large amount of death cataloged (Jones 2008).
Do the results conform to known trends? D. strigosa is significantly less abundant in the causeway, and D. labrynthiformis, while always at a much lower density then D. strigosa, is highest in number at John Smith, where another student in the class, Kasia, found the largest incidence of BBD. Finally, while the causeway seems to be lacking one of the key hard coral species, it still has mustard coral in abundance, making up the dominant form of the coral cover. Thus, differential presence of key hard coral species may not be the biggest contributors to coral density differences seen in the data.
Plexaurella sp. (common sea rod) followed the expected trend for soft corals, it increased in mean density at the two sites with higher flow rates. The absence of Gorgonia vantalina from John Smith’s Bay could be due to disturbance such as hurricane Fabian, or lack of colonization even prior to Fabian. This was the only south shore site, if we could go collect more data I might find another south shore site nearby, perhaps one less impacted by the hurricane, and see if Gorgonia vantalina had colonized successfully. Finally, the Briareum asbestinum (Corky Sea fingers) were likely constant between all sites because their population size was relatively low, even when overall biodiversity was high.
John Smith's Bay Whalebone Bay Causeway Hard Corals D. strigosa ↑ ↑ ↓ D. Labyrinthiformis ↑ ↓ ↓ Porites astreoides ↔ ↔ ↔ Soft Corals
Gorgonia vantalina ---------------- ↑ ↑ Plexaurella sp. ↑ ↑ ↓ Briareum asbestinum ↔ ↔ ↔ Sum vector
↑↑↑ ↑↑ ↓↓
The stark difference in Shannon evenness index for the Castle Harbor site, labeled “causeway”, is due to the dominance of mustard coral. Mustard coral comprised 66.6 ± 26.2% of the total individuals found at the causeway. This standard deviation marks the differences between transects, it is particularly high because a few transects were on points that received slightly more water movement and therefore contained a bunching of fan and hairy rod corals. The mustard coral also was the only coral found in 2 out of the 11 sites. Standard deviation of causeway data is higher than for the other two sites because there were 11 transects, while John Smiths and Whalebone had 6 and 4 transects, respectively.
The causeway has very limited flow and increased sedimentation compared to the other sites, and therefore is unsuitable habitat for most coral (Flood, Pitt et al. 2005). The site was covered in an overwhelming amount of dead coral.
Wells bay is also in the harbor, so it has less water movement compared to north rock. Wells is further from pollution then the causeway and receives a bit more flow, leading to increased amounts of corals in comparison. There was little sediment in comparison to the causeway, but more than at any of the other sites. There were fewer dead corals, suggesting that this area was less impacted by the causeway construction and by the municipal pollution.
North rock had higher wave action then wells, and was deeper then wells. It was also not within the harbor, where anthropogenic forces have affected most reefs. While comparisons to the density data are merely observational, it is interesting that north rock had a higher index value then any other site, and the highest wave action.
Physical Parameters Measured By Site John Smith's Bay Whalebone Bay Castle Harbor: Causeway North Rock Castle Harbor: Wells Bay Water Depth 4 2 1 5 3 .75 - 4.5 m .8 - 2 m .25 - 1 m 2.4 - 6 m 2 - 3.5 m Wave Action 4 3 1 5 2 Pollution 0 0 1 0 1 Natural Disturbance: Hurricane 1 0 0 0 0 Disease BBD Diversity 5 7 6 6 7 SHEI 0.823 0.764 0.451 0.931 0.816
Discussion: Individual Environmental Stressors
Water Flow
The diversity increased as the site increased in relative measure of wave action. The three sites where I measured biodiversity in density had increased in their SHEI as their wave action increased. While the strength of waves was not quantified, it is well known that the south shore receives considerably stronger waves then the north, and that the causeway has created very constricted flow comparatively in Castle Harbor (Thomas 1985; Flood, Pitt et al. 2005). The two other sites also have lower diversity with lower wave action, as North Rock receives much stronger waves then Wells.
Depth
The overall diversity, measured by Shannon evenness index, increased as the depth of the site increased. The site depths peaked at 6 meters for North Rock, never reaching the peak 15 to 30 meters measured in literature.
Disturbance
Above: (L) Airport Dump (R) Causeway to the left of the Airport
Photo Sources: (L) www.bermuda-online.org/
environ.htm; (R) news.bbc.co.uk/2/
hi/americas/3087296.stm
The most devastating disturbance for biodiversity seems to be the construction of the causeway. While the south shore was subjected to a massive hurricane, John Smith’s beach has very high diversity and where there is a difference in individual coral means, it has the highest (excepting the sea fan anomaly). However, the causeway site was decimated due to the combination of physical factors associated with its almost stagnant flow and increased pollution (it did smell a little bit). The dead coral was covered in algae and there were more sponges, but very few coral individuals. While Wells bay was also in the Harbor, because of its location it received more movement and less trash (the airport causeway blocked any bulk trash flow, and it was far enough down current for diffusion to occur).
