Written by Thomás Banha
Corals are threatened by temperature rises in the oceans, but some spots are proposed as refugia from this impact. Coral reefs under marginal conditions in the South Atlantic present five key features that make them possible refugia in this scenario and less susceptible to bleaching followed by death than those from the Caribbean and Indo-Pacific.
In early 2019, the South Atlantic Subtropical Anticyclone (the SASA, an important feature in atmospheric circulation) blocked cold fronts in subtropical Brazil, promoting a period of unusual high temperatures in the region. In response, NOAA’s Coral Reef Watch issued an unprecedented level 2 bleaching alert for the area. This location is under marginal conditions such as high turbidity, lower temperatures during winter and cold fronts, and massive tourist presence during weekends throughout the year.
I work in Dr. Paulo Sumida’s Lab at the University of São Paulo, Brazil. I’ve been studying the effects of climate change on coral reefs in marginal environments from the Southwestern Atlantic for more than five years. Thus, based on our previous observations, we did not expect that the heat wave could have a major impact in the colonies.
A field course was scheduled to the area (see the map below), and we were able to do a quick survey of Mussismilia hispida colonies (the only reef-building species that inhabits the area). Even knowing that these corals go through seasonal bleaching (as described by Dr. Miguel Mies and colleagues in 2018, see paper below), we were astonished when we saw the amount of bleached colonies.
We contacted some researchers to help us assess the magnitude of bleaching in the area. During the peak of the heat wave, we surveyed 1,116 colonies from coastal and insular sites. Almost 80% of the colonies bleached, the highest bleaching prevalence ever recorded for a given reef area in the South Atlantic. However, less than 2% were dead, even with degree heating weeks (DHW, a measure of accumulated heat stress) values of 18.5 and 20.5 °C-weeks (coastal and insular, respectively), the highest values recorded during a bleaching event for the entire South Atlantic. The results were then published in the scientific journal Coral Reefs (see paper by Thomás Banha and collaborators listed below).
We were aware that the marginal conditions might buffer the effects of thermal stress, but we realized that there was no scientific paper describing which features could make reef-builders from the South Atlantic less susceptible to mass bleaching than those from the Indo-Pacific and Caribbean. With this idea in mind, we started to gather and compile data and invited more researchers to contribute. We ended up with five key features that could explain this resistance (see the figure below that summarizes our findings):
(i) deeper bathymetric distribution
Since mesophotic reefs have been hypothesized as refugia to thermal stress, the depth limit seems to be a key factor to the resilience of the species.We observed that coral from the South Atlantic and Caribbean have lower depth limits than those in the Indo-Pacific. The limit for species from the Indo-Pacific, Caribbean and South Atlantic is 30.6 ± 0.7 (mean ± standard error), 59.5 ± 3.9 and 70.1 ± 9.0 m, respectively.
(ii) higher tolerance to turbidity
The adaptation to low light environments helps corals to undergo turbid conditions, which prevents high irradiance. Almost 60% of species from the South Atlantic are found in turbid conditions, against only 16% in the Indo-Pacific and 21% in the Caribbean. Also, the resistant species from the South Atlantic presented higher bathymetric limits. Thus, South Atlantic species are well adapted to turbid/low light conditions.
(iii) higher tolerance to nutrient enrichment
Nutrification works together with climate change and often triggers negative impacts on coral reefs, as shown by Jörg Wiedenmann and coauthors in 2013 (see paper below). Using nitrate as a proxy, we observed that the concentration in the South Atlantic is naturally elevated and about 2.5 times higher than both the Indo-Pacific and Caribbean, with some areas presenting concentrations higher than 5.0 µM. This means that South Atlantic reefs are more resistant and can also benefit from nutrification.
(iv) higher morphological resistance
The morphology of the coral can help to reduce the effects of temperature and irradiance stress, with massive corals being more resistant. This relationship has been explained by Kyle Zawada and collaborators in 2019 (see paper listed below). Massive growth forms are dominant and comprise two thirds of species from the South Atlantic. This proportion is higher than those from the other two areas.
(v) more flexible symbiotic associations
The flexibility of symbiotic relationships is extremely important when analyzing thermal tolerance of both coral and symbiont. This means that having more generalist coral species allows the coral-symbiont relationship to endure fluctuations in stress. South Atlantic and Caribbean reefs have a higher proportion of generalist coral species than the Indo-Pacific. From the Symbiodiniaceae perspective (those microalgae that live within the tissues of corals), South Atlantic reefs have a higher proportion (about 60%) of generalist species than the other two areas.
The figure below summarizes our findings, except from the flexibility of symbiotic relationship.
After analyzing their features, we compared the mortality of corals associated with bleaching events for the three areas. The data showed that the South Atlantic reef-builders experienced proportionally fewer bleaching episodes resulting in coral mortality, than both the Indo-Pacific and Caribbean, which presented 60 and 50% more mortality cases, respectively. It is important to note that mortality in the South Atlantic is usually associated with fire corals Millepora spp., which do not match the features we presented before.
All the data presented shows how South Atlantic reefs are prepared for climate change and may be a refugia. Also, this qualifies them as bioclimatic units, which are large coral reef areas presenting lower vulnerability to climate-driven stress and are well-connected to surrounding marine ecosystems, as proposed by Prof. Ove Hoegh-Guldberg and collaborators in 2018 (see paper below). Currently, the biggest threat to South Atlantic corals is the lack of effective management programs for the marine protected areas (MPA) where they are found. Intensified monitoring, conservation and proper MPA design may be the path to protect this unique ecosystem.
More information can be found in the article recently published in Frontiers of Marine Science by Dr. Miguel Mies and collaborators, listed below.
Banha et al. (2019). Low coral mortality during the most intense bleaching event ever recorded in subtropical Southwestern Atlantic reefs. Coral Reefs, 1-7.
Hoegh-Guldberg et al. (2018). Securing a long-term future for coral reefs. Trends in Ecology & Evolution, 33(12), 936-944.
Mies et al. (2018). In situ shifts of predominance between autotrophic and heterotrophic feeding in the reef-building coral Mussismilia hispida: an approach using fatty acid trophic markers. Coral Reefs, 37(3), 677-689.
Mies et al. (2020). South Atlantic coral reefs are major global warming refugia and less susceptible to bleaching. Frontiers in Marine Science, 7, 514.
Wiedenmann et al. (2013). Nutrient enrichment can increase the susceptibility of reef corals to bleaching. Nature Climate Change 3, 160–164. Zawada et al. (2019). Morphological traits can track coral reef responses to the Anthropocene. Functional Ecology, 33(6),