CRAMP Study Sites: Kahe Point, Island of O‘ahu
Geographic Name: Kahe Point
CRAMP Site Code: OaKpo
Geographic Coordinates: 21° 21.396’ N 158° 07.974’ W
Management Status: Open access
Coral assemblages in the Kahe nearshore area have been under intensive investigation since 1970 and 1971, initially in relation to thermal discharges by the Kahe Power Station. Sand beach shoreline. Limestone reef surrounded by sand. Fairly low coral cover. Southern exposed site. Evidence of previous hurricane damage.
Kahe Coral Monitoring Study
By: Steve L. Coles, Bishop Museum
The Kahe Coral Monitoring Study began in 1980 as part of the monitoring required for obtaining and maintaining the National Pollution Discharge Elimination System (NPDES) permit for operation of the thermal outfall associated with the Kahe Generating Station (Figure 1A) Ten quadrats were established at three stations in the Kahe area and at a control station off Nanakuli Park (Figure 1B), approximately 2.4 km from the Kahe outfall. Each quadrat has been photographed annually, and the percent coverage of dominant species and total live coral has been determined for each quadrat by a point intercept method. Statistical analysis of paired comparisons of quadrat coverage enable determination of significant changes in cover through any time period within the study’s duration, and the photographic images provide a permanent record of conditions. Surveys are conducted annually in August-September and a report is submitted to the Hawai‘i Department of Health the following March. These annual reports and numerous earlier marine environmental studies completed during construction and operation of the Kahe station are listed in the References.
The coral assemblages in the Kahe nearshore area have been under intensive investigation since 1970 and 1971, when the first surveys of the effects of the Kahe Power Station thermal outfall were conducted by Jokiel and Coles (1974). This study determined that the thermal discharge, which at that time was being released from a shoreline outline, was causing coral bleaching and mortality in the path of the discharge plume at depths shallow enough to be in contact with thermal effluent during low tides. These lethal and sub-lethal effects occurred only during annual ambient high water temperatures when discharge limits exceeded tolerance limits for Hawaiian corals (Jokiel and Coles 1977; Coles and Jokiel 1978), but an increase in the Kahe Station’s generating capacity by about 25% from 1970 to 1971 more than doubled the area of coral damage. Further surveys in 1973 indicated that coral coverage had stabilized under the then existing discharge conditions (Coles 1975). However, the increased thermal discharge resulting from planned expansion in the generating capacity of the power station were considered likely to cause unacceptable negative impacts on the coral reef community if the point of discharge remained at the shoreline. (Stearns Roger Inc. 1973, URS Research Co. 1973). Construction of an offshore outfall was therefore required as a condition of the NPDES permit for operation of additional generating units.
Construction of the offshore outfall involved the addition of a transition basin (Figure 2B) which links the former shoreline outfall to two pipelines which conduct effluent to 4 m diameter discharge nozzles (Figure 2C) approximately 250 m offshore at a depth of 8 m. Thermal effluent of 5-6 deg. C above ambient temperature (Figure 3) entrains receiving water by turbulent mixing, and the mixed effluent forms a thermal plume at the water surface which does not come into direct contact with subtidal benthic organisms. One year after the offshore outfall began operation in 1976, coral settlements were noted on the surface of the outfall nozzle, within centimeters of the discharged effluent. By 1983 a prolific coral assemblage had become established on the outfall nozzles and headwall, and this high coral coverage continued to exist through 1997. A seven year study of the colonization of surfaces in the vicinity of the outfall (Coles 1984) indicated that coral settlement and growth was highest adjacent to the outfall and decreased exponentially with distance from out fall in all directions, possibly resulting from stimulation of coral planulae to settlement by brief exposure to thermal effluent (Coles 1985).
The Kahe Coral Monitoring Study began in 1980 when 10 quadrats at each of three stations (2B, 6B, 8A) were established in the Kahe area and at one control station off of Nanakuli Beach Park (Figure 4A). The corners of each quadrat 1 m X 0.66 m dimension were marked by galvanized iron bolts, which serve as a platform for a camera frame (Figure 4B). Images taken with a Nikonos camera with a 28 mm wide angle lens at a focal distance of four feet correspond to the field of view of 0.66sq. m for each quadrat. Percent coverage of each species on each quadrat are determined by the point intercept method using 485 points superimposed on the projected image. Because the quadrats are permanently marked, paired comparisons determine significant changes in cover on each quadrat through any time period within the study’s duration, and the photographic images at the four stations (Figures 4C, 4D, 4E and 4F) provide a permanent record of conditions at any quadrat. Ambient water temperatures have been continuously monitored since 1976, at a remote offshore site from 1976 to 1986, and at the shoreline intake from 1986 to present.
The quadrats have been re-photographed annually since 1980, and 33 of 40 quadrats established in 1980 still remain. This time period has included two major natural events that have impacted the Kahe area: Hurricane Iwa, which occurred in November 1982, and Hurricane Iniki in August 1992. Surveys conducted soon after the hurricanes (Coles et al 1983; Brock 1992) indicated that the short term direct impacts of Iwa were strongest about 1 km offshore of the present study’s Station 2B and for Iniki, these impacts were most apparent directly offshore of Station 8A. The present results enable an analysis of the long term effects of the two hurricanes in these impacted areas against the background of normal fluctuations in reef coral live coverage.
Six species of hard corals and one soft coral were sufficiently abundant to be recorded in the quadrats, and percent coverage of each of these species, sand cover and total coral coverage are shown in Table 1. Plots of annual average coral coverage at the four stations (Figure 5 A thru D) suggest that coral coverage undergoes periodic natural fluctuations on which have been superimposed the impacts of the two hurricanes that have occurred in the last 20 years. All four stations show patterns of decreasing total coral coverage in the early 1980s and again in the early 1990s, both periods followed by recovery in the late 1980s and again in the late 1990s. Although these patterns correspond with the occurrences of both hurricanes, hurricane impacts do not fully explain the decreases in coverage. For example, substantial decreases in cover occurred at the control station in 1981 to 1982 and in 1991 to 1992 and at Station 8A in 1991 to 1992, prior to the arrival of either hurricane.
The patterns shown for total coral (Figure 6 A thru D) closely reflect the dominant species Porites lobata, which comprises most of the coverage at each station and showed coverage peaks in the early 1980s and early 1990s. The second most abundant species, Pocillopora meandrina, was stable at around 5% cover at Station 2B , but showed larger fluctuations and a peak in coverage in 1987 at Stations 6B and 8A and the control station. This pattern has resulted from this species’ rapid colonization of new surfaces after the 1982 hurricane, growth to terminal growth size, normal mortality and negative effects of the second hurricane in 1992. This fluctuating pattern is most pronounced at Station 8A, which was in the area most heavily impacted by Hurricane Iniki.
Although substantial changes in mean coral coverage from year to year were noted in many cases, not all were significant, due to substantial variability among quadrats at each station. Average differences in percent coverage for yearly and decade changes and significant changes determined by paired comparisons indicate those periods for which significant changes in total coral coverage occurred at each station in the last decade. Significant annual decreases in total coral occurred at Station 2B in 1992-93 and again in 1993-1994, one and two years following Hurricane Iniki. Coral coverage at Station 8A decreased annually every year from 1990 to 1995, and these decreases were highly significant in 1990-91 and 1992-93 and significant in 1994-95, at which time total coral cover averaged only 4.6% for the ten quadrats at this station. Significant increases occurred at this station during the following two years of recovery, bringing total % cover to nearly 9% in 1997. Significant increases also occurred at Station 6B and the control station in 1996-97, during a year when water temperatures were among the highest that have occurred at Kahe in a decade, and moderate bleaching of Pocillopora meandrina was observed throughout the Kahe area (Environmental Department 1997). Changes in mean total coverage were negative at all four stations in 1997-98 and the decrease was significant at Station 2B.
A longer-term perspective of changes in coverage is provided by the comparing the decadal differences (Figure 7 A thru D) with the average the coverage in given years. At Stations 2B and 6B all ten year changes were positive up through 1987, and these increases were significant for 1981-91 and 1982-92 at Station 2B, and for 1982-92 at Station 6B. However, large and significant ten year declines were shown by 1998 and 1999 at these stations from the high coverage that occurred in the late 1980s. The control station had positive ten year changes for three decadal comparisons, with one significant increase for 1983-93, but ten year changes have been negative since 1996, with a highly significant decrease shown for 1988-98. By comparison, all decadal changes at Station 8A were negative from 1981 to 1999, and all of these decreases were significant or highly significant. This is due to the series of significant annual decreases that occurred from 1990 through 1995 following Hurricane Iniki. Mean total coverage at Station 8A in 1998 was still only about 50% of the 1987 maximum value, despite significant annual increases that occurred in 1995-96 and 1996-97.
The Kahe Monitoring Study provides the world’s longest continuous records of coral coverage using a consistent sampling method and annual measurements. Use of permanently placed photoquadrats repeatedly photographed over nearly 20 years of annual monitoring enables statistical analysis of perceived changes in coral cover and a photographic record that can be reviewed when questions arise about conditions at any time during the period monitored. Results indicate that coral coverage along the Kahe coastline has fluctuated substantially since 1980, and that the long-term effects of major events, such as the two hurricanes that struck the area in 1982 and 1992, can extend beyond the initial storm impacts. This was shown at the station most heavily impacted by Hurricane Iniki in 1992, where significant annual decreases in coral cover continued three years after the hurricane, reducing average coral cover at Station 8A to about 25% of its maximum value of 1987. Although significant increases occurred during the next two years, coral coverage in this area was still significantly lower than ten years earlier. Most decadal comparisons at the other sites indicate a trend of general decrease in the last ten years from relatively high coverage that occurred in the late 1980s and early 1990s.
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Last Update: 02/24/2011
By: Dan Lager
Hawai‘i Coral Reef Assessment & Monitoring Program
Hawai‘i Institute of Marine Biology
P.O. Box 1346
Kāne‘ohe, HI 96744