Coral reef bleaching is a growing concern for coral reefs around the world. Climate change is affecting the intensity and frequency of coral reef bleaching which puts the reefs at risk of increased mortality, directly affecting the communities that rely on the species biodiversity supported by the coral reefs (Baker, Glynn et al. 2008). One of the main causes of coral reef bleaching is rising ocean temperatures (Baker, Glynn et al. 2008). The intensity of solar radiation and warming events such as the El Niño Southern Oscillation (ENSO) are two important influences on ocean temperature. Higher aerosol levels, such as cloud cover or dust, and water turbidity are two factors that mitigate damaging effects of solar radiation (Baker, Glynn et al. 2008). Recent research has found that coral reef bleaching events were less severe with higher atmospheric aerosol levels during ENSO events (Mumby, Chisholm et al. 2001; Gill, Watkinson et al. 2006). These events and storms lead to increases in water turbidity which lowers water temperatures and reduces coral bleaching. The intricate interaction among cloud cover, water turbidity, solar radiation, and ocean temperature is important to understand for the future of coral reefs. We propose a controlled experiment that will carefully measure the impact of cloud cover and water turbidity on solar radiation and the resulting effects on water temperature and subsequently coral reef bleaching. Once we understand these interactions with confidence, we can apply this knowledge to improve models and forecasts of future bleaching events to minimize the negative impact of bleaching.
Coral reef bleaching
Reef-building corals have a crucial symbiotic relationship with plankton that provide necessary energy for growth and are responsible for making coral colorful (Baker, Glynn et al. 2008). Corals depend on the plankton for survival and any stress on their relationship puts corals at risk.
“Coral reef bleaching” refers to the expulsion of the plankton and the altered color of the corals. Bleaching does not necessarily equate to coral death because sometimes corals recover, but the added stress of bleaching decreases the chance for their survival (Baker, Glynn et al. 2008).
Coral reef bleaching events have increased in frequency and intensity over the past few decades mostly due to rising sea temperatures (McWilliams, Cote et al. 2005; Baker, Glynn et al. 2008). An increase in sea surface temperatures is related to the increase in bleaching events (McWilliams, Cote et al. 2005). Therefore, the ability of cloud cover and water turbidity to mitigate the effects of solar radiation on rising sea temperatures is important to understand.
The negative bleaching effects experienced by coral reefs are directly related to the amount of solar radiation that is absorbed by seawater, thus raising the sea temperature. As increased carbon emissions lead to increased climate change and ozone depletion, solar radiation will become more and more intense and heat oceans to a greater extent. Before the 1980s, it seemed that only sporadic bleaching occurred. Since the 1980s, larger-scale bleaching is occurring more often and some scientists believe that this consistent damage to coral reefs worldwide can be attributed to solar radiation, especially ultraviolet (UV) radiation (Glynn, 1992).
Even seemingly small sea temperature increases of a fraction of a degree Celsius have the potential to disrupt coral reproduction and lead to coral reef death and bleaching (Glynn and D’Croz 1990; Jokiel and Coles 1990). Solar UV radiation can also be affected by many factors, which we will control for in the proposed experiment. These factors include cloud cover, which “dilutes” the strength of oncoming rays of radiation, and water turbidity which also acts to dissolve radiation before it reaches the water surrounding coral reefs.
Some corals have mechanisms to fight off damaging UV radiation. However, these mechanisms are ineffective when the radiation is emitted at a high dose rate. Coral reefs require gradual exposure to solar radiation in order to build up the compounds that absorb UV radiation, mycosporine-like amino acids. This is the reason why recent large-scale ENSO events have triggered more regular, instead of sporadic, coral reef bleaching. These events are occurring at greater intensities, catching coral reefs off guard without the built up immunity of mycosporine-like amino acids.
When studying coral bleaching on a large scale, one of the many factors affecting the severity of the bleaching is cloud cover. Cloud cover is defined simply by the fraction of the sky that is obscured by clouds when observed from a particular location. It is suggested that the amount of cloud cover in a given year is directly related to the amount of coral bleaching (Baker, Glynn et al. 2008). With increased cloud cover, there exist fewer sun hours and ultimately a greater reduction of radiative stress, resulting in the lessening of large-scale coral bleaching.