ABSTRACT: Seagrasses are critical to productive coastal ecosystems but are endangered and poorly studied, especially in the tropical Indo-Pacific. A key local threat is the increasing anthropogenic input of nitrogen (N). We hypothesized that such N eutrophication is occurring on Guam, the largest and most populated of the Northern Mariana Islands. We examined the seagrass Enhalus acoroides over 5 years and applied stable isotope analysis to document N dynamics and impacts. Although δ¹⁵N values indicated that sewage-derived N was the dominant source, the δ¹⁵N values were unrelated to either seagrass cover or growth rate. Strong relationships between seagrass δ¹⁵N values and both distance to the nearest source of N inputs and population density of adjacent municipalities suggest that N inputs are highly localized. Although there was no evidence of a negative impact of sewage-derived N inputs on seagrasses, this study provides a useful reference for evaluating mitigation efforts and for tracking the effect of increasing anthropogenic eutrophication on Guam.