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Description
The concentrations and loads of nutrients found in tropical streams are often related to anthropogenic sources in the surrounding watershed and can have an effect on the health of the coral communities living just off shore. This study quantified water quality on the island of Tutuila in American Samoa through islandwide sampling and determined which anthropogenic factors add the greatest loads of nutrients to the sea. Nitrate (NO3) and Phosphate (PO4) were analyzed in samples from 45 streams around the perimeter of the island in 2014-2015. Only 2 streams were found to be above the EPA surface quality standard of 0.3 mg/L for nitrate while 10 streams were above the standard of 0.15 mg/L for PO4. A previous survey was used to document that 95% of sampled streams had a decrease in nitrate concentrations while 69% had a decrease in phosphate between 2003-2005 and 2014-2015. A disturbance ratio (DR) [disturbed:pristine] was calculated for each watershed on the island based on samples taken upstream of the villages. The eastern part of the island had the highest mean NO3 DR with an average of 6 in the wet season and 4.3 in the dry season and the central part of the island had the highest mean PO4 DR with an average of 1.5 in the wet season and 1.7 in the dry season. Watershed variables contributing to nutrient loads were analyzed using linear regression modeling. Population density was the strongest predictor of nutrient loads in the wet and dry season while pig density and percent-sewered land showed a moderate correlation. Contrary to expectation, nutrient loads were not lower in watersheds with sewage connections, suggesting that leaky sewage systems may be an important source of nutrients, though collinearity between the percent of the population connected to the sewer and population density complicates the interpretation. The results were comparable to those found in other tropical ecosystems where pigs and leaky municipal sewer systems contributed high nutrient concentrations, which indicates that future mitigation strategies could help reduce the island-wide nutrient concentrations even further.
