Over the past ten years field observations have noted a decrease in healthy coral cover in Vatia Bay, on the island of Tutuila, American Samoa. The cause for this is unknown, but one hypothesis is that nutrient pollution from the local village may be driving the decline. Excess nutrients (nitrogen and phosphorus) can impact corals directly by lowering fertilization success, and reducing both photosynthesis and calcification rates, or indirectly such as through stimulation of the grown of benthic algae. Declining coral health adversely affects the biodiversity of the Bay and likely decreases ecosystem services. In this study, a comprehensive benthic demographic assessment of Vatia Bay was completed. This utilized a survey of replicate transects at preselected stratified random sites. At each site, the belt-transect method was employed by divers who quantitatively assessed generic richness, colony density, and size class of coral colonies. In addition, still photographs were collected to record the benthic community composition at predetermined points and later analyzed as the basis to estimate the benthic cover and composition at each site. Revisiting these photo quadrats quarterly provided a digital-image time series throughout the duration of the project. Water samples were collected monthly at sites selected from a stratified random design for analysis for nitrate, nitrite, ammonium, urea, total nitrogen, orthophosphorus, total phosphorus, silica and salinity.
Biological surveys found that reef habitat was more degraded in the inner portion of the Bay, which coincides with elevated levels of nitrogen and phosphorus. However, other stressors (sedimentation and increased turbidity) may also be driving this pattern. Land based contributions of phosphorus and reactive nitrogen can enter the environment from a variety of sources, but in Vatia the most likely sources are piggeries and septic systems. Analysis of water samples for tracers of human waste (caffeine and sucralose) confirmed that human derived nutrients are contributing to the nutrient budget of the Bay. These data are useful not only to enhance our understanding of the role that anthropogenic nutrients play in the biodiversity and ecosystem health of the Bay, but also serve as an important “baseline” against which to measure future change. Current ongoing research will attempt to model the nutrient budget of the watershed that drains the Bay, as well as focus on event sampling to better capture precipitation events.