We investigated the potential of coral skeleton δ15N (CS-δ15N) records for tracking anthropogenic-N sources in coral reef ecosystems. We produced a 56yr-long CS-δ15N record (1958–2014) from a reef flat in Guam that has been exposed to varying 1) levels of sewage treatment 2) population density, and 3) land use. Increasing population density (from <30 to 300ind·km−2) and land use changes in the watershed resulted in a ~1‰ enrichment of the CS-δ15N record until a sewage treatment plant (STP) started operation in 1975. Then, CS-δ15N stabilized, despite continued population density and land use changes. Based on population and other considerations, a continued increase in the sewage footprint might have been expected over this time. The stability of CS-δ15N, either contradicts this expectation, or indicates that the impacts on the outer reef at the coring site were buffered by the mixing of reef water with the open ocean.

High sedimentation rates have well-documented, deleterious impacts on coral reefs. However, few previous studies have attempted to quantitatively describe a coral reef community across a large continuous sediment gradient. In this study distinct benthic assemblages in Fouha Bay, Guam, were identified using a Moving Window Analysis conducted along a two-order of magnitude sediment gradient, with transition boundaries that were generally consistent with sediment thresholds identified in the literature. Coral richness dropped exponentially with increasing sedimentation rate. Richness was nearly three times greater in assemblages with sedimentation rates <10 mg cm⁻² d⁻¹ compared to assemblages experiencing rates between 10 and 50 mg cm⁻² d⁻¹, and nearly 30 times greater than assemblages experiencing rates between 50 and 100 mg cm⁻² d⁻¹. No corals were found in assemblages with sedimentation rates >110 mg cm⁻² d⁻¹. Reducing sedimentation in this area could result in a shift of more diverse and abundant coral assemblages toward the head of the bay.

The goals of this study were to evaluate the contribution of sewage-derived N to reef flat communities in Guam and to assess the impact of N inputs on coral disease. We used stable isotope analysis of macroalgae and a soft coral, sampled bimonthly, as a proxy for N dynamics, and surveyed Porites spp., a dominant coral taxon on Guam’s reefs, for white syndrome disease severity. Results showed a strong influence of sewage-derived N in nearshore waters, with δ15N values varying as a function of species sampled, site, and sampling date. Increases in sewage-derived N correlated significantly with increases in the severity of disease among Porites spp., with δ15N values accounting for more than 48% of the variation in changes in disease severity. The anticipated military realignment and related population increase in Guam are expected to lead to increased white syndrome infections and other coral diseases.

A ridge-to-reef framework was developed for 26 watersheds around Guam. Dissolved inorganic nitrogen (DIN) data were collected for one year at the base of streams while coral and fish surveys were conducted on adjacent reefs. Two independent analyses revealed a similar 0.10 mg/l DIN threshold beyond which negative impacts to water quality and coral reefs existed. The influence of DIN was next partitioned with respect to a second primary stressor, fishing pressure. While coral diversity was negatively influenced by DIN, the cover of some stress-tolerant corals increased, such as Porites rus, making coral cover alone a poor indicator of watershed pollution. Less intuitive, DIN predicted increased food-fish biomass that was accounted for by generalist herbivores/detritivores, representing homogenized assemblages, while fishing pressure reduced biomass. Our DIN thresholds resonated with a similar study in American Samoa suggesting broader guidance for water quality legislation may be emerging.