Excess Nutrients in a Coral Reef Ecosystem: Dynamics, Sources and Tracer Techniques

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 growth of benthic algae. In Vatia Bay (American Samoa), resource managers have hypothesized that nutrient pollution from the local village may be contributing to a decline in coral ecosystem health in the Bay. In this study, water samples were collected monthly at sixteen sites selected from a stratified random design for analysis for nitrate, nitrite, ammonium, urea, total nitrogen, orthophosphorus, total phosphorus, silica and salinity. Land based contributions of nutrients come from a variety of sources, such as fertilizer, animal waste, human waste and fossil fuel combustion; in Vatia the most likely sources are piggeries and septic systems. Discriminating between mammalian (human vs pig) sources of nutrients can be problematic using traditional techniques (e.g. stable nitrogen isotopes, microbial methods). To solve this problem, two compounds (caffeine and sucralose) present only in human diets, and persistent enough to be measured in the environment after excretion, were quantified in water samples from the Bay. Data showed the presence of caffeine and/or sucralose in 82% and 51% of samples respectively, conclusively showing that human waste is reaching Vatia Bay. Additionally, caffeine and sucralose are correlated with elevated nutrient levels, confirming the importance of human waste to the nutrient budget of this system. 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.