Climate‐change refugia: shading reef corals by turbidity

Abstract

Coral reefs have recently experienced an unprecedented decline as the world's oceans continue to warm. Yet global climate models reveal a heterogeneously warming ocean, which has initiated a search for refuges, where corals may survive in the near future. We hypothesized that some turbid nearshore environments may act as climate-change refuges, shading corals from the harmful interaction between high sea-surface temperatures and high irradiance. We took a hierarchical Bayesian approach to determine the expected distribution of 12 coral species in the Indian and Pacific Oceans, between the latitudes 37°N and 37°S, under representative concentration pathway 8.5 (W m(-2) ) by 2100. The turbid nearshore refuges identified in this study were located between latitudes 20-30°N and 15-25°S, where there was a strong coupling between turbidity and tidal fluctuations. Our model predicts that turbidity will mitigate high temperature bleaching for 9% of shallow reef habitat (to 30 m depth) - habitat that was previously considered inhospitable under ocean warming. Our model also predicted that turbidity will protect some coral species more than others from climate-change-associated thermal stress. We also identified locations where consistently high turbidity will likely reduce irradiance to <250 μmol m(-2) s(-1) , and predict that 16% of reef-coral habitat ≤30 m will preclude coral growth and reef development. Thus, protecting the turbid nearshore refuges identified in this study, particularly in the northwestern Hawaiian Islands, the northern Philippines, the Ryukyu Islands (Japan), eastern Vietnam, western and eastern Australia, New Caledonia, the northern Red Sea, and the Arabian Gulf, should become part of a judicious global strategy for reef-coral persistence under climate change.

Repeated Thermal Stress, Shading, and Directional Selection in the Florida Reef Tract

“…Shading reduces stress on the symbiont's photosystem, which in turn reduces the likelihood of coral bleaching (Warner et al, 1999;Takahashi et al, 2004). Notwithstanding the obviously adverse effects that poor-water quality, with high levels of pollutants and high nutrient concentrations, has on corals (Wooldridge and Done, 2009;Wagner et al, 2010;Wiedenmann et al, 2013), some shading provided by high-primary productivity and high turbidity can benefit corals during thermal-stress events (Cacciapaglia and van Woesik, 2016). Indeed, since coral bleaching is essentially extreme photoinhibition, and high temperatures make that photoinhibition worse (Warner et al, 1999;Takahashi et al, 2004), high productivity and high turbidity during high temperature events should effectively reduce the probability of photoinhibition and coral bleaching.…”

Environmental Drivers of Variation in Bleaching Severity of Acropora Species during an Extreme Thermal Anomaly

“…Variability in bleaching among different reef habitats is also associated with site-specific turbidity levels (e.g., Williams et al, 2010). However, observed responses range from a negative effect of turbidity whereby suspended particulates are thought to act as an additional stressor that lowers temperature tolerance (Williams et al, 2010;Hongo and Yamano, 2013), to predictions that turbidity may lessen the severity of bleaching in some shallow habitats by reducing light penetration (West and Salm, 2003;Cacciapaglia and van Woesik, 2016).…”

27 years of benthic and coral community dynamics on turbid, highly urbanised reefs off Singapore

“…For example, certain chronic disturbances, such as high turbidity, may offer a degree of protection during periods of thermal stress, by reducing light stress22. Furthermore, there is no simple relationship between proximity to high human population densities and reef condition23.…”