The 2014 coral bleaching and freshwater flood events in Kāne'ohe Bay, Hawai'i

Abstract

Until recently, subtropical Hawai'i escaped the major bleaching events that have devastated many tropical regions, but the continued increases in global long-term mean temperatures and the apparent ending of the Pacific Decadal Oscillation (PDO) cool phase have increased the risk of bleaching events. Climate models and observations predict that bleaching in Hawai'i will occur with increasing frequency and increasing severity over future decades. A freshwater "kill" event occurred during July 2014 in the northern part of Kāne'ohe Bay that reduced coral cover by 22.5% in the area directly impacted by flooding. A subsequent major bleaching event during September 2014 caused extensive coral bleaching and mortality throughout the bay and further reduced coral cover in the freshwater kill area by 60.0%. The high temperature bleaching event only caused a 1.0% reduction in live coral throughout the portion of the bay not directly impacted by the freshwater event. Thus, the combined impact of the low salinity event and the thermal bleaching event appears to be more than simply additive. The temperature regime during the September 2014 bleaching event was analogous in duration and intensity to that of the large bleaching event that occurred previously during August 1996, but resulted in a much larger area of bleaching and coral mortality. Apparently seasonal timing as well as duration and magnitude of heating is important. Coral spawning in the dominant coral species occurs early in the summer, so reservoirs of stored lipid in the corals had been depleted by spawning prior to the September 2014 event. Warm months above 27 °C result in lower coral growth and presumably could further decrease lipid reserves, leading to a bleaching event that was more severe than would have happened if the high temperatures occurred earlier in the summer. Hawaiian reef corals decrease skeletal growth at temperatures above 27 °C, so perhaps the "stress period" actually started long before the bleaching threshold of 29 °C was reached. Hawai'i is directly influenced by the PDO which may become a factor influencing bleaching events in subtropical Hawai'i in much the same manner as variations in the El Niño Southern Oscillation (ENSO) influences bleaching events at low latitudes in the tropical Pacific. Records show that offshore temperatures measured by satellite will not always predict inshore bleaching because other factors (high cloud cover, high wind and wave action, tidal exchange rate) can limit inshore heating and prevent temperatures in the bay from reaching the bleaching threshold. Low light levels due to cloud cover or high turbidity can also serve to prevent bleaching.

Keywords: Climate change; Coral bleaching; Corals; Freshwater reef kill; Global warming; Hawaiʻi.

Figure 1. Bleaching events in Kāneʻohe Bay.

Extent of impact in the 1996 (cross-hatch) and 2014 (gray) bleaching events in Kāneʻohe Bay, Oʻahu Hawaiʻi. (A) Enlarged photograph of Oʻahu, Hawaiʻi. Kāneʻohe Bay is indicated by the white square. (B) Impacted area from the proceeding freshwater event (July 2014) is indicated in the area bounded by the black square. Location of temperature recorders are shown on reef flat (black star), back reef (red star) and on reef slope (blue star).

Figure 2. Variations in water temperatures in Kāneʻohe Bay at different reef locations.

Water temperatures at depth of less than 1 m on reef flat (solid black line for patch reef 37), depth of 1–2 m on reef slope (dashed blue line for HIMB weather station) and on the back reef at 1–2 m depth (solid red line for NOAA tide gauge station) during the 2014 bleaching event 20 August 2014–1 December 2014. Color of lines matches the color of the stars marking the respective location of each measurement site as plotted in Fig. 1.

Figure 3. Extent of 2014 bleaching event.

Surveyed sites during the bleaching event in Kāneʻohe Bay in October 2014 (A) and after initial recovery in December 2014 (B). Area impacted by the flooding event is indicated in the black square. Proportion of surveyed corals are shown as normal (red), pale (yellow), bleached (white), and dead (black).

Figure 4. Long term Sea surface temperatures (SST).

Combined SST record using IGOSS-NMC temperature data (1992–2014) and corrected NMFS data for Koko Head, Oʻahu (1956–1992).

Figure 5. Comparison of temperature during the 1996 and 2014 bleaching events.

Comparisons of the summer water temperatures during the 1996 (blue) and 2014 (red) bleaching events. Time of initial reports of bleaching for each event are indicated by respective arrow.