Growth of coral reef in southern Atlantic was controlled by changes in last 5,000 years | AGÊNCIA FAPESP

Growth of coral reef in southern Atlantic was controlled by changes in last 5,000 years Described in 2019, the Queimada Grande coral reef off the coast of São Paulo state arose when the ocean was warmer and stopped growing when cooler sea surface temperatures influenced the climate in the region, according to a study led by researchers at the Federal University of São Paulo (photo: Rodrigo Melo/LABECMar)

Growth of coral reef in southern Atlantic was controlled by changes in last 5,000 years

January 12, 2022

By André Julião  |  Agência FAPESP – A temperature drop occurring 3,000 years ago was probably what interrupted the growth of the southernmost Atlantic coral reef after it had flourished for some 2,000 years in the warm tropical waters that had bathed the southern coast of São Paulo state in Brazil. This is a key finding of a study published in the journal Global and Planetary Change by a group led by researchers at the Federal University of São Paulo’s Institute of Marine Sciences (IMAR-UNIFESP) and supported by FAPESP.

After the pause, the Queimada Grande Coral Reef, named for a nearby island, resumed growth for several centuries. The resumption was probably due to a brief “window” of favorable conditions, and growth soon ceased once again. The pattern can also be seen in other marginal reefs – structures considered close to the environmental thresholds for survival – such as the Abrolhos reef complex in Bahia state.

“Analysis of samples from the reef structure pointed to a gap in its growth between 5,000 and 3,000 years ago. It grew, stopped growing, grew moderately in a window lasting a few centuries, and stopped growing again. In the study, we show that changes in sea levels aren’t sufficient to explain these growth cycles, as one might expect. The reason is more likely to have been variations in water temperature and sedimentation. The same may apply to other Brazilian reefs,” said Guilherme Henrique Pereira-Filho, principal investigator for the study. Pereira-Filho is a professor at IMAR-UNIFESP in Santos and a researcher at its Marine Ecology and Conservation Laboratory (LABECMar).

Reefs are formed when free-swimming coral larvae attach to rocks or other structures on the seabed and form colonies. The larvae become immobile polyps, with skeletons of calcium carbonate that serve as a foundation for more corals, building reefs that can be several kilometers long and creating biodiversity hotspots by attracting multitudes of other organisms.

The Queimada Grande Coral Reef and associated rhodolith (red algae) bed occupies an area of some 320,000 square meters along the western (leeward) side of the island of the same name, which lies about 30 km off the coast of Itanhaém, a city in São Paulo state. The reef began forming 5,500 years ago and was built basically by a single coral species (Madracis decactis), but is inhabited by fish, algae, invertebrates and microorganisms.

The reef was first described only two years ago by Pereira-Filho and his group while they were working on a project supported by FAPESP.

Most of the structure is 15 meters below the ocean surface. Coral reefs are typically much closer to the surface, where the water is warmer and there is the ample luminosity required for the coral colonies to grow. A sea level rise could therefore explain the gap in growth. However, according to the geological records the sea level was even higher – some 4 meters above the current level – when the reef began.

“The state of this reef is very different from the usual. Even when growth resumed some 2,000 years ago, the sea level was still high. The reefs of Abrolhos, in Bahia, and Anchieta, in Espírito Santo, also display signs of stop-go growth in the same periods, but they were always very near the surface. Temperature variation is a more likely explanation,” Pereira-Filho said.

Marginal reefs

The researchers found that ocean currents converged very near the coast of São Paulo during the period in which the reef stopped growing, about 3,000 years ago, and the temperature of the water there probably fell as a result.

Migration of the confluence between the Brazil Current and the Malvinas/Falkland Current to more northerly latitudes lowered the average temperature along much of the Brazilian coast and affected the growth of coral reefs.

This confluence is now farther south, making areas like Abrolhos warmer and more suited to coral growth. Current conditions at Queimada Grande appear not to do so, and this has been the focus for more research by the group.

“Although sea levels contributed to the pause in growth at Abrolhos and Anchieta, this temperature change is one of the factors that probably explain the pause in both cases as well as that of Queimada Grande,” Pereira-Filho said. “Our conclusion is that these marginal reefs are highly susceptible to climate variations and can rapidly turn growth on or off in response to minor environmental variations.”

The short period (around 300 years) during which reef growth resumed some 2,000 years ago probably resulted from a combination of environmental factors, including rising sea surface temperatures. It came to an end when El Niño intensified at about that time.

Queimada Grande is now senescent, in that its coral does not produce enough calcium carbonate for the structure to grow, as do more active reefs. Nevertheless, it continues to perform the same ecological functions as active reefs, providing a habitat for a wide array of algae, fish, invertebrates and other organisms.

“The community of corals and other calcium carbonate-producing organisms is merely maintaining the reef structure, which is what we would expect of a reef under threshold conditions in the southern Atlantic,” Pereira-Filho said. “The structure of the reef is formed by a single species, but the diversity of other organisms is huge. It’s a unique environment in the Atlantic, where nothing of the kind is to be expected.”

The existence of a coral reef in the region has also drawn attention from society and government, creating the conditions for it to be considered a “locale of interest to tourism” in the São Paulo State Government’s plan for the management of the Central Coastline Marine Environmental Protection Area, which includes the sea around Queimada Grande Island. The plan was signed in March 2021 after around ten years of discussions involving authorities and the public.

The description of the reef and the discussions about its conservation also fueled a documentary launched in March 2021 and featuring researchers, officials and members of organized civil society as they advocate protection of the reef.

The article “Growing at the limit: Reef growth sensitivity to climate and oceanographic changes in the South Western Atlantic” is at: www.sciencedirect.com/science/article/abs/pii/S0921818121000643.
 

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