Human Impacts on Fluxes of Nutrients and Sediment in Waimanalo Stream, O'ahu, Hawaiian Islands

Waimanalo Stream, on the windward side of the island of O'ahu in the Hawaiian Islands, has been greatly altered by human activities. Native riparian vegetation has been removed along much of the course of the stream, and significant sections of the stream have been hardened to control flooding. Absence of shade from riparian vegetation has allowed California grass (Brachia mutica), wild sugarcane (Saccharum officinarum), and other vegetation to proliferate in the stream channel. Some reaches of the stream more closely resemble a wetland than a natural watercourse. During fair weather and moderate storms, this vegetation effectively traps sediment. During a year when rainfall was ~40% below average, dissolved N and P accounted for most of the N and P transported by the stream. N and P content of the suspended solids was comparable with that of terrestrial organic matter, but with a slightly lower N/P ratio, probably due to the high iron content of Hawaiian soils. Concentration of suspended solids in the stream was only about 4% of the average concentration in fluvial systems that discharge to the ocean. Base flow accounted for about 32% of the P, 58% of the suspended solids, and 96% of the N transported by the stream. The very high contribution of base flow to the N flux was apparently related to contamination of shallow groundwater in the lower reach of one tributary, in which nitrate N concentrations during base flow were about 7 mg liter-1. Flux of N in the stream was comparable with the amount of N produced by livestock waste in this predominantly agricultural watershed. Cesspool seepage and/or leaching of N from animal waste into shallow groundwater and seepage of that groundwater into the stream may account for the anomalously high N loading to the stream. Absence of a similarly high P flux probably re- flects the high iron content of Hawaiian soils, which effectively immobilize P in groundwater.