Hypereutrophication of the Ala Wai Canal, Oahu, Hawaii: Prospects for Cleanup

Abstract

Studies of trophic conditions in the Ala Wai Canal were carried out during a 6-week period during the summer of 1991. The canal is a partially mixed estuary whose water quality and trophic status are impacted to a large extent by run-off from residential and light industrial portions of the City of Honolulu. Gross photosynthetic rates are about 5.5 g C m-2 d-1 and increase by a factor of three from the mouth to the head of the estuary. Photosynthesis appears to be limited only by the availability of light and the concentration of phytoplankton. Allochthonous inputs of organic carbon exceed photosynthetic rates by about 60%. Of the total allochthonous plus autochthonous organic carbon input to the system, respiration consumes about 70%,18% accumulates in the sediments, and about 12% is flushed out at the mouth of the canal. Sedimentation amounts to about 7-8 x 10 3 m3 yr-1 and has greatly altered the bathymetry of the canal. Concentrations of particulate carbon, particulate nitrogen, and chlorophyll a are comparable to values reported 20 yr ago, despite dredging of the canal in 1978-1979. Surface waters are supersaturated with oxygen during the day and undersaturated at night. Shallow subsurface waters undergo even greater diel oxygen changes because of inefficient oxygen exchange with the atmosphere. Oxygen concentrations below a depth of 3 m frequently violate Environmental Protection Agency water quality criteria. Flushing the canal by pumping in seawater at its head at a rate of about 104 m3 hr-1 will probably do much to improve the aesthetic condition of the canal and increase oxygen concentrations in the bottom waters.