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Coral reefs in Kaneohe Bay, Hawaii: Two centuries of western influence and two decades of data

Author(s): Hunter CL, Evans CW


Kaneohe Bay, an estuarine and coral reef ecosystem on the windward coast of Oahu, Hawaii, is often cited as an exemplary illustration of the resiliency of a natural system to environmental insult. Impacts to Kaneohe Bay coral reefs have resulted from various effects of natural processes such as freshwater flooding and erosional runoff. Additional impacts to the reef communities have resulted from anthropogenic activities concomitant with land use changes. Initially, agriculture and grazing, and subsequently urbanization, led to increased soil erosion and sedimentation, extensive reef dredging, channelization of streams, and eutrophic conditions ensuing from sewage discharges into the bay. Most of these land use changes occurred during the 1940's through 1970's, prior to and precluding comprehensive and/or quantitative studies of pristine reef conditions in the bay. One of the best documented anthropogenic changes in Kaneohe Bay focused on physical and ecological responses during a one-year period following sewage diversion. After twenty-five years of discharge, two large sewage outfalls were diverted from the bay in 1977–1978, followed by rapid and dramatic decreases in nutrient levels, turbidity, and phytoplankton abundance in the previously affected areas. There was a corresponding change in community structure from one dominated by the green bubble alga, Dictyosphaeria cavernosa, and filter or deposit feeders, to one or more closely approaching the "coral gardens" described by Kaneohe Bay visitors prior to W.W.II. By 1983, D. cavernosa had decreased to ¼ of its previous (1970) abundance while coral cover had more than doubled. The last point-source sewage discharge into the bay was diverted in 1986. Recovery of coral-dominated reef communities in Kaneohe Bay was expected to continue with a further decrease in algal cover and an increase in coral abundance. However, a 1990 survey indicated that, on a baywide basis, 1) algal cover had increased between 1983–1990 surveys, and 2) the rate of coral recovery established by surveys in 1970 and 1983 had slowed or, in some cases, reversed. Percent cover of D. cavernosa increased at 5 of 15 sites, while live coral showed slight to significant declines at nine sites compared to 1983 levels. This paper summarizes the recent history of Kaneohe Bay reefs in light of anthropogenic alterations, describes changes in reef communities in the bay over the past two decades, and discusses the potential environmental factors involved in these changes.

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