Climate change is happening at a rate alarmingly faster than at any other time in Earth’s past. Genetic adaptations in corals haven’t kept up with the rapid pace.
Coral thermal thresholds (the temperature they can withstand) have not changed over the past 20 yrs but bleaching and mortality have increased. This means that corals are not changing rapidly enough to prevent bleaching. As corals lose their symbionts which provide their, color the outer tissues become transparent and the white skeleton beneath becomes visible. This is referred to as bleaching.
Coral thermal thresholds (the temperature they can withstand) have not changed over the past 20 yrs
Since the 1980s, regional bleaching events have occurred on coral reefs throughout the world with increasing frequency and increasing geographic extent. As a result, almost every coral reef region in the world has now suffered extensive stress or mortality.
Until recently, the Hawaiian Islands had escaped widespread bleaching events where high mortality occurred. However, in 2014 and 2015, Hawai‘i experienced unprecedented widespread bleaching and mortality. Most islands reported well over half of the corals exhibiting signs of paling and bleaching and subsequent death. The continued increase in global temperature due to anthropogenic production of CO2 from fossil fuels and other human induced emissions means that future bleaching events will be increasingly more severe and frequent.
These coral bleaching events highlighted several science and management needs. It will be critical to establish a baseline to determine what is there before the bleaching then we can monitor the extent of the bleaching and mortality. With this information, we can then identify places where corals are more resilient so we can determine why these species and sites are more resistant to temperature effects.
These coral bleaching events highlighted several science and management needs.
A simple and easy field method of determining the severity of bleaching is the comparison of different shades of color over time and between sites and corals. The only existing color card to make this assessment was developed in 2002 for Australian corals. It has been used extensively throughout the world including for Hawaiian corals, but the shades are not fully representative of corals in regions beyond Australia. Our Hawaiian Ko‘a (coral) card will represent the colors of the major species of Hawaiian corals as they lose their symbionts. It allows more precise measurements through a gradient of shades in the commonly recognized system of degrees. This allows transfer of information about the color without referencing the card. For example, if a corals color starts at 10o and is assessed at 50o a month later we know precisely how much color it has lost.
There is a growing need in Hawai‘i for such a tool as more scientists, students, community members, and resource managers become involved in monitoring of the changes taking place on our reefs.
* The Coral Watch coral health chart developed in 2006 by the University of Queensland, Australia to determine extent of coral bleaching.
** A preliminary representation of the Hawaiian Ko‘a Wheel a color reference card currently under development to assess coral health and bleaching.
The Hawaiians understood the importance of corals to the preservation of the people and its ecological connectivity to the ecosystem. The Kumulipo, the Hawaiian creation chant, recants the creation of Po’ele and Kumulipo. The first living organism to be created was the coral. “Hanau ka uku ko’ako’a, Hanau kana, he ‘ako’akoa puka”. Born was the coral polyp, born was the coral. From coral all other marine organisms were brought forth signifying the vital importance of corals.
To develop the card is not as simple as it appears. We are using professional underwater photographers to assure natural colors and uniform consistency. They will establish the initial range of healthy corals and photograph coral fragments at each stage of bleaching.
We are conducting laboratory trials with different species of corals. We increase temperatures slowly and take samples of each colony as the colors change.
To understand what is happening to the corals physiologically as they bleach, we take pulse-amplitude modulation (PAM) measurements.
From each fragment we extract and measure the chlorophyll a content using a spectrophotometer which measures the reflectance or transmittance at a certain wavelength. This will tell us the concentration chlorophyll as it relates to color.
We are counting the hundreds of zooxanthellae (the tiny single celled algae that have a symbiotic relationship with the corals) that provide color in corals and are reduced as bleaching progresses. Originally developed to count blood cells we use a hemocytometer to determine the symbiont densities in coral. This will give us a link from cell counts to color.