Continuous water quality monitoring to determine the cause of coral reef ecosystem degradation for coastal windward Oahu streams during 2002 (NCEI Accession 0001070)

Kaneohe and Waimanalo streams on the windward side of the island of Oahu in the Hawaiian Islands have been hardened to prevent flooding. The hardening process has involved elimination of the natural riparian habitat and replacement of the natural stream channel with a concrete-lined conduit having vertical walls and a broad, flat bottom. The shallow depth of the water column and the absence of shade have resulted in temperatures that average as much as 4-5ºC above ambient and rise as high as 32ºC during daylight hours. Unlike most low-order streams, the hardened sections of both streams are autotrophic, as evidenced by elevated pH values and O2 concentrations as high as 150% of saturation. Several allochthonous inputs, one from a storm sewer and the other from a natural spring, introduced water with anomalously low O2 concentrations and very high nitrate concentrations. The absence of sediments in the hardened sections of the streams precludes natural sedimentary microbial processes, including denitrification. Nitrate concentrations in a section of Waimanalo Stream with a natural streambed drop dramatically from values in excess of 400 µM to concentrations less than 10 µM at the head of the estuary. Although some of this decline is due to dilution with seawater, the concentration of nitrate at the head of the estuary is only 10% of the value that could be explained by dilution effects. Biological processes associated with a natural streambed thus appear very important to the functionality of the streams and in particular to their ability to process allochthonous nutrient inputs in a way that minimizes impacts on the nearshore environment. Prevention of flooding can be accomplished by mechanisms that do not involve elimination of riparian buffer zones and destruction of channel habitat. To maintain water quality and stream functionality, it will be important that these alternative methods of flood control be utilized. Converting natural streams to storm sewers is an unenlightened way to address flooding problems.

Water samples were taken at Waimanalo Stream along the Kahawai tributary and below the confluence of Kahawai and Waimanalo Streams near the mouth of the stream where it discharges into Waimanalo Bay. Most of the Waimanalo Stream stations were sampled a total of 10-12 times at roughly 3-4 week time intervals during the period February-October, 2002. Kaneohe Stream sampling was carried out at roughly three-week intervals during the period June-November, 2002. Most Kaneohe Stream stations were sampled a total of nine times.