By Bernardo Vargas-Ángel
Analyses of quantitative data from surveys conducted last year in American Samoa corroborate anecdotal patterns of differences in coral cover and demographics between the northern areas and the southern and central areas of Faga`alu Bay. Here, we provide a summary of our analyses and the project activities conducted by scientists from the PIFSC Coral Reef Ecosystem Division (CRED) and partner organizations in this bay between March and August 2012 as part of a project funded by NOAA’s Coral Reef Conservation Program.
Entitled “Inter-disciplinary study of flow dynamics and sedimentation effects on coral colonies in Faga`alu Bay, American Samoa,” this CRED-led project provides information necessary to better understand the effects of land-based sources of pollution (runoff and sedimentation) on the coral reef community in Faga`alu Bay. This information supports the development, implementation, and effectiveness of local, reef-to-ridge watershed conservation and management action plans. Benthic percent cover and coral demographic data were collected at 37 study sites along the shallow backreef and deep forereef in Faga`alu Bay. These quantitative data support the notion of patterns that previously had been observed only casually in the field: coral development is conspicuously prominent along the central and southern portions of the reef in Faga`alu Bay (Fig. 1a, b), compared to the northern areas, where coral growth is quite limited and depauperate (Fig. 1c, d).
Results from these quantitative surveys also indicate that, on average, percent live-coral cover was nearly twice as high along the southern area of the reef compared to the northern sector (Fig. 2a). Levels of crustose coralline algae were not distinctly different between the northern and southern sectors of the reef (Fig. 2b), but percent cover of turf algae was much greater along the northern forereef and backreef than along the other sampled portions of this bay (Fig. 2c). The northern areas of the reef in Faga`alu Bay are directly affected by terrigenous siltation and runoff. Surveys corroborate this appraisal, as exemplified by the reef-builder ratio, which is the ratio of corals and crustose coralline algae to nonaccreting organisms (macroalgae and turfalgae) calculated with values of mean percent cover. The reef-builder ratio was greater along the southern backreef and forereef than along the coral-impoverished northern reefs (Fig. 2d). In actively growing coral reefs, calcifying organisms—corals, crustose coralline algae, and other calcifying plants—typically dominate coral communities. In contrast, communities dominated by noncalcifiers, such as turf algae, cyanobacteria, and other macroalgae, are common in areas with suboptimal conditions for coral growth, including areas with elevated levels of nutrient inputs, pollution, turbidity, and sedimentation.
We assembled an ordination (multidimensional scaling) plot on the basis of the benthic percent cover data. Illustrated in Figure 3, this plot indicates that overall benthic composition was distinct between reef zones (forereef vs. backreef) and cardinal position (north vs. south). A similarity profile (SIMPROFF) analysis supported a clear segregation between most forereef and backreef sites; and, within the backreef subdivision, the SIMPROFF analysis also showed the segregation between most of the northern and southern sites, corroborating the visual observations discussed above (Fig. 2). The separation between the northern and southern forereef sites was not strongly supported by the SIMPROFF analysis. These analyses provide quantitative evidence that the ecological communities of the northern and southern backreef in Faga`alu Bay are ecologically distinct from each other. This variation is driven most likely by the differences in levels of water quality, clarity, and terrigenous sedimentation.
Figure 4a illustrates estimates of coral-colony density of 4 important reef-building coral genera in Faga`alu Bay. Overall colony densities were relatively higher along the southern backreef and forereef (10.1 colonies/m2, standard error of the mean [SE] 0.90) than along the northern sector of the reef (6.01 colonies/m2, SE 0.81), and these differences were statistically significant (P=0.012, Student’s t-test). Differences in coral generic composition and density also were evident: corals of the genus Porites were heavily dominant along the shallow northern and southern backreef and corals of the genus Montipora occurred primarily along the deeper forereef. Additional notable spatial and structural differences indicated a preponderance of encrusting and foliose corals of the genera Montipora and Pavona, respectively, along the shallow northern backreef and, in contrast, the presence of branching corals of the genera Pocillopora and Acropora throughout the southern backreef. Fast-growing branching corals, such as Pocillopora and Acropora, are better adapted to the shallow, well-lit habitats of the southern backreef, compared to encrusting and foliose species that appeared to tolerate the lower levels of light and conditions of higher turbidity prevalent on the northern backreef (see Rodgers 1990; Crabbe and Smith 2005).
Differences among habitats also were observed in values of coral generic richness (Fig. 4b), with a greater mean number of genera occurring along the deeper forereef (10.95, SE 0.67) compared to the shallow backreef (6.29, SE 0.25), and these differences also were statistically significant (P=0.001, Student’s t-test). Such variation is expected given the disparate range of environmental conditions (for example, light, depth, water circulation) and of available microhabitats present on the forereef compared to the shallow, relatively homogeneous backreef.
Except for one site on the southern backreef, low levels of bleaching were commonplace across habitats and depths in Faga`alu Bay (Fig. 5a). Similarly, mean prevalence of coral disease (Fig. 5b) was low (0.56%, SE 0.16) overall; however, observed levels of disease were greater at north-facing backreef and forereef sites (0.82%, SE 0.35) than at south-facing sites (0.52%, SE 0.19). Although they were small, these differences could be associated with the elevated, chronic terrigenous runoff and sedimentation that affects these areas.
For another aspect of this project, in addition to the 37 study sites, 3 permanent coral reef monitoring stations were installed along the shallow backreef and channel of Faga`alu in March 2012 to evaluate long-term changes in community structure and composition. These stations are periodically monitored by students of the American Samoa Community College and staff of the American Samoa Department of Marine and Wildlife Resources. Also, oceanographic instrumentation, including wave-and-tide recorders, current meters, and salinity and temperature recorders, were deployed in March and April 2012 at strategic sites inside and outside of Faga`alu Bay to better profile water flow patterns and sediment residence times. Project activities will continue throughout 2013. CRED plans to conduct additional demographic surveys in 2013 to expand and complement the benthic assessments conducted thus far. CRED scientists plan to gather supplementary information on coral-colony sizes, density of juvenile corals, and coral bleaching and disease. After activities have concluded, results of the biological and oceanographic surveys will be made available to all relevant parties.
Many partners have provided instrumental support to this project. Partner organizations include the NOAA Pacific Islands Regional Office (Fatima Sauafea-Leau), American Samoa Department of Marine and Wildlife Resources (Domingo Ochivallo), San Diego State University (Trent Biggs and Alex Messina), American Samoa Community College (Kelley Anderson Tagarino), and the Faga`alu watershed community working group.
Rodgers CS. 1990. Responses of coral reefs and reef organisms to sedimentation. Mar Ecol Prog Ser 64:185–202.
Crabbe MJC, Smith DJ. 2005. Sediment impacts on growth rates of Acropora and Porites corals from fringing reefs of Sulawesi, Indonesia. Coral Reefs 24:437–441.