By: Mary Rollins and Skyler Paoli
Figure 1. Coral Reefs of the World. By: Isabela Rios In recent decades, coral reefs are under an increased threat of dying out due to many factors like ocean acidification, ocean warming, and eutrophication (excessive nutrient content in waters leading to a decrease in dissolved oxygen). Corals are adapted to live in a very specific set of conditions and when their environmental conditions are changed either through anthropogenic impacts, or natural disturbances in the ecosystem, the ecosystem may suffer the negative consequences. With this in mind, coral refugia is an extremely important concept for researchers wishing to understand how coral reefs can continue to live in rapidly changing oceans.
“...coral refugia is an extremely important concept for researchers wishing to understand how coral reefs can continue to live in rapidly changing oceans.”
Coral refugia is defined as an area where coral species can survive during a period of adverse conditions where elsewhere corals would die. Many factors contribute to coral refugia, such as depth of reefs, upwelling, coral associated bacteria, and others. In an article titled Thermal refugia against coral bleaching throughout the northern Red Sea, researchers identified the northern Red Sea as a site of coral refugia where corals resist mass bleaching events despite experiencing extreme warming events (Osman et al. 2017). This is an example of thermal refugia, where corals located in a certain area are protected against warming events. Additionally, in the article, Assessing the ‘deep reef refugia’ hypothesis: focus on Caribbean reefs, the researchers discuss the Deep Reef Refugia Hypothesis (DRRH), the idea that deeper reefs are more protected from anthropogenic and natural disturbances than shallower reefs, and can act as a place for shallow reef species to reproduce and recover after a disturbance (Bongaerts et al. 2010). These two articles provide insight on how coral refugia is an increasingly important topic for the preservation of coral reefs throughout our oceans.
"The degradation of coral reefs negatively impacts ocean health, and in turn, humans that rely on the ocean for their livelihood."
As coral reefs are under threat in today’s oceans, coral research is important to better understand corals, their importance, how they survive and thrive in different environments, and alternately what factors lead to their declining health and death. Coral reefs are essential to the world’s oceans as they are indicators of overall ocean health, they provide habitat and a basis for food chains in many areas, and they are heavily relied upon in the fishing and tourism industries. The degradation of coral reefs negatively impacts ocean health, and in turn, humans that rely on the ocean for their livelihood.
Figure 2. Thermal refugia against coral bleaching throughout the northern Red Sea. Map and figures depicting sea surface temperature (SST) and degree heating weeks (DHWs) across different areas of the Red Sea from years 1982–2012. Identifying refugia is a direct way to ensure the survival of coral reefs. Refugia are areas where corals can survive despite adverse conditions, and their importance to worldwide coral reef survival is only beginning to be studied and understood. However, for many reasons, scientists face obstacles when identifying potential areas of refugia. For example, deep reefs are not easily accessible, making the necessary equipment to assess whether they will make good refugia expensive. Additionally, there are many factors that lead to coral death, so pinpointing why a specific area is a good refugia candidate can be time consuming and difficult. Identifying locations such as the Red Sea and deep coral reefs is the first step in discovering what factors help corals survive the adverse conditions they are facing in recent times. In order to preserve the world’s corals, refugia needs to be researched further and applied to broader areas.
"In order to preserve the world’s corals, refugia needs to be researched further and applied to broader areas."
The primary objective of the article, Thermal refugia against coral bleaching throughout the northern Red Sea, was to examine different sections of the Red Sea coral reefs and determine the thermal sensitivity of the corals based on sea surface temperature (SST) and bleaching event records from three geographical sections (north, central, south). This was done in order to determine if Red Sea coral reefs have an increased tolerance for high heat/stress environments, and can thus be considered a site of coral refugia. The article identifies the northern Red Sea as a site for coral refugia based on the data suggesting that even though corals live at lower annual ambient temperatures in the northern Red Sea compared to the central/southern Red Sea, the northern Red Sea experienced drastic seasonal periods of warming, called degree heating weeks (DHWs) (Figure 1). Despite the DHWs experienced by the northern Red Sea, coral reefs in that area resisted mass bleaching events more than their central/southern reef relatives.
Corals in the northern Red Sea experienced maximum temperatures of 34° C for ten days and still did not experience mass bleaching. To compare, their typical ambient SST experienced is 26-29° C on average, which suggests an extreme thermo-tolerating ability, as many corals experience bleaching after an increase of just 1° C beyond the annual SST. Researchers suggest that circulation patterns, density driven currents, and coral bacterial associates may have a link to the differential thermotolerance and resistance to bleaching events experienced throughout the coral reefs in the Red Sea, but further research is needed to identify the exact cause of their thermotolerance, and also how this information can be utilized in areas outside of the Red Sea.
The article Assessing the ‘deep reef refugia’ hypothesis: focus on Caribbean reefs evaluates the strengths and weaknesses of the Deep Reef Refugia Hypothesis. The researchers assert that while there is evidence to support the hypothesis (Table 1), further research is needed to affirm it. The article focuses on reefs in the Carribean, and maintains that this data can be extrapolated to reefs elsewhere. The depth range that is considered suitable for refugia is from 30 meters depth to a depth where there is not enough light to sustain corals that rely on phototrophic obligate symbiotes, generally around 60 meters depth. These mesotrophic reefs are protected from natural disturbances such as hurricanes and disease, which are increasing in frequency as global temperatures continue to rise, as well as anthropogenic disturbances, such as overfishing and physical injury to coral due to divers. However, it is not known for sure that mesotrophic reefs are protected from temperature bleaching, and more long term studies are needed. Most importantly, it needs to be assessed whether the depth-generalist species (those that can thrive at any depth, independent of other variables) are good candidates for restoring shallow reefs. It is believed that the same species of coral living at lower depths could provide propagules for restoring shallower reefs, but genetic assessments of the individual populations is needed for this to be confirmed. The DRRH is a promising hypothesis, and it is crucial that it is explored further.
Table 1. Assessing the ‘deep reef refugia’ hypothesis: focus on Caribbean reefs. Different documented disturbances such as hurricanes and coral diseases, with their location and when they occurred listed. Researchers documented the depth range of the reef affected by the disturbance, and the total depth range of the coral reef that they examined in order to determine if disturbances have less of an impact on the deeper parts of the reef (supporting the deep reef refugia hypothesis).
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