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

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Title: Human Impacts on Fluxes of Nutrients and Sediment in Waimanalo Stream, O'ahu, Hawaiian Islands
Authors: Laws, Edward A.
Ferentinos, Lisa
Issue Date: Apr 2003
Publisher: University of Hawai'i Press
Citation: Laws EA, Ferentinos L. 2003. Human impacts on fluxes of nutrients and sediment in Waimanalo Stream, O'ahu, Hawaiian Islands. Pac Sci 57(2): 119-140.
Abstract: 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 reflects
the high iron content of Hawaiian soils, which effectively immobilize P in
ISSN: 0030-8870
Appears in Collections:Pacific Science Volume 57, Number 2, 2003

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