Nitrogen and phosphorus concentrations in tropical Pacific insular streams: historical data from Tutuila, American Samoa

DiDonato, Guy

Tropical Pacific insular streams are relatively unexplored compared to streams in temperate or other tropical regions, and more stream studies are necessary because human population growth threatens these unique island ecosystems. The population of Tutuila, the largest island of American Samoa, has grown exponentially in recent years. This report presents historical water quality data [total nitrogen (TN), nitrate (NO3-+NO2- ), total phosphorus (TP)] for eleven perennial streams on Tutuila sampled between 1979 and 1996. Data from 1979 show that streams impacted by human activities exhibited a higher median TN concentration (520 μg/L) than reference streams (329 µg/L). A similar difference was observed for NO3-+NO2-. There was no significant difference between impacted and reference stream TP (129 vs. 102 µg/L, respectively). Data from the entire period indicate that TN and NO3-+NO2- demonstrated more between year variability (CV=60.2% and 26.5%, respectively) than did TP (CV=19.9%). Overall, though, these data do not demonstrate any obvious trends towards increasing nutrients in these island streams. Data suggest that the baseline nutrient levels are ca. 200-300 µg TN/L and 50-100 µg TP/L for reference streams on this island. These numbers are supported by data collected in 1997 from Laufuti Stream, a stream located in an uninhabited watershed on the nearby island of Ta'u, American Samoa. Future monitoring must examine the full range of stream types to accurately assess these insular stream ecosystems.

Nitrogen and phosphorus concentrations in tropical Pacific insular streams: historical data from Tutuila, American Samoa

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