Ridge-to-reef linkages in Apra Harbor, Guam describe spatial and temporal variation in water quality and coral reef assemblages

An integrated ridge-to-reefs (R2R) approach was applied to assess the impacts of land-based pollutants on freshwater to marine ecosystems in Apra Harbor, Guam. Multi-year monitoring at five riverine, six seawater quality monitoring sites, and ecological surveys distinct spatial and temporal patterns in water quality driven by both natural processes and anthropogenic inputs. Elevated dissolved inorganic nitrogen and total suspended solids were from domestic wastewater, stormwater runoff, and soil erosion, especially during wet seasons and extreme events. Multivariate statistical analysis addressed site-specific water quality signatures indicative of eutrophication risk. In the marine environment, inner harbor and nearshore sites exhibited high turbidity and chlorophyll-a levels, suggesting nutrient inputs from rivers, while offshore or submarine groundwater discharge (SGD)-influenced sites showed different patterns shaped by local hydrology. These findings support the "sweet spot" hypothesis in coral reef ecology, where intermediate distances from watershed discharge correlate with enhanced coral health, potentially due to phytoplankton-fueled heterotrophic feeding. The study underscores the value of the R2R framework in linking upland watershed conditions to coastal ecosystem resilience and highlights the need for targeted watershed best management practices-such as improved wastewater treatment and erosion control-to mitigate pollution impacts. Continued long-term monitoring and integration of land-sea models are essential for adaptive management, particularly in the context of urbanization and climate change.