Reefs are important economic, cultural, and ecological resources; however, on Guam poor land management, anthropogenic fire regimes, and soil chemistry contribute to soil erosion and subsequent sedimentation of rivers and reefs. Sediment plumes can be seen at river inputs all along the coast of Guam. Although there are significant interspecific differences, heavy sedimentation rates (greater or equal to 95% coverage of substrate) have been shown to inhibit coral settlement completely, while more moderate rates (between 50-90%) severely inhibit settlement (Hodgson 1990). Sediments can smother and kill adult coral colonies by decreasing light penetration and disrupting polyp gas exchange. Some adult species can be effectively smothered at relatively low sedimentation rates, while others are able to secrete mucous and remove these sediments and are thus more tolerant of elevated sedimentation levels. Even if sedimentation is not visibly impacting adult coral colonies, sedimentation rates may be high enough to inhibit all or some of the juvenile recruitment.
Juvenile corals are more susceptible to environmental disturbances and can be used as an indicator of reef health. Reefs that appear “healthy” may not be receiving sufficient recruits to replace adults, and over time the coral community will deteriorate. This phenomenon has been observed already on some Guam reefs (Richmond 1993, Richmond 1994), concerning some resource managers about the future, long-term health, and stability of the reefs and marine resources on Guam
Soil erosion on Guam and at WAPA is occurring at significant levels. The sedimentation load may be acting as a barrier to coral recruitment, especially at certain locations along the reef at War in the Pacific, NHP (WAPA), which in recent years has received considerable (and most likely elevated levels of) terrestrial runoff associated with poor land management, wildfire, arson, and development. Investigating the relationship between sediment load and recruitment is vital to mitigating disturbance and understanding one of the mechanisms that regulate the park’s benthic populations and mediate species coexistence on the reef (Underwood and Fairweather 1989). The objectives of this study are to assess spatial and temporal patterns of coral recruitment at WAPA; assess the relationship between the sediment deposition and coral recruitment rates at WAPA; and to provide baseline data on coral recruit taxonomy at WAPA. With this information natural resource managers will be able to identify recruitment limited/inhibited areas due to sedimentation, and develop best management practices for erosion mitigation in the adjacent watershed.
A pilot study to assess the feasibility of settling corals onto experimental plates, determining the level of taxonomic resolution for recruit identification, and to determine appropriate study sites for the project has been conducted within the park since March 2004. Park scientists have determined that four sites of similar benthic composition and morphology will be used in the study. The careful selection of study sites enabled the paired sites to retain essential environmental features (coral cover, species richness, wave action, temperature, salinity), while applying elevated sedimentation levels to only one of each paired sites. This design should isolate sedimentation as an inhibiting variable to coral recruitment.
Although not many corals have been settled on experimental settling plates (see photo), Park scientist believe that the results will be significant. Coral recruits from the Pocillopridae family have been observed to settle within the park (see photo). Additionally, in conjunction with the University of Guam Marine Lab and Dr. Peter Schupp’s lab, live coral recruits are being settled in lab aquaria. These recruits (see photo) will be placed at the experimental sites to assess the survivorship of corals once they settle on the benthos.
This project was funded by NRPP-Small Parks, and is intended to run for FY2005. It is expected that a correlation between coral recruitment (settlement and survivorship) will be realized.
References
Hodgson, G. 1990. Sediment and the settlement of larvae of the reef coral Pocillopora damicornis. Coral reefs 9[1], 41-43.
Richmond, R. H. 1993. Coral reefs: Present problems and future concerns resulting from anthropogenic disturbance. American Zoologist 33[6], 524-536.
Richmond, R.H. 1994. Effects of coastal runoff on coral reproduction. pp. 360-364 In Proc. Colloq. On Global Aspects of Coral Reefs, R.N. Ginsburg (ed.) University of Miami.
Underwood, A. J., and P. G. Fairweather. 1989. Supply-side ecology and benthic marine ecology. Trends in Ecology and Evolution 4:16-20.
