As human population densities grow around the boundaries of protected areas in lower-income economies, there are frequently concomitant increases in environmental degradation. The purpose of this study was to examine water chemistry and the isotopic signatures of primary consumers in streams in and around a national park in the tropics to document whether park watersheds were affected by untreated wastewater. Three of the six study sites had concentrations of soluble reactive phosphorus >39 μg/L. Additionally, there was a strong, positive correlation between δN¹⁵ values of grazing snails and phosphorus concentrations of the water, suggesting that sewage effluent is influencing trophic ecology in the protected aquatic habitats. The results from this study lend support to recent calls for management beyond riparian buffers for conservation of freshwater ecosystem integrity in protected areas.