Multibeam seabed mapping and the unexpected reef
When researchers studying the Amazon River bloom in 2012 collected multibeam echosounder data, little did they know that they had revealed something unique. "When we processed the multibeam data, we saw what looked like reefs at the shelf edge," says Dr Alex Bastos, who is based at the Universidade Federal do Espírito Santo. Four years later, further survey work confirmed what Bastos had seen – an extensive reef system hidden beneath the thick, murky plume of the Amazon River.
An unexpected reef
The idea that there may be something interesting lying off the Amazon River mouth has been floated around long before the reefs were first detected with multibeam. In the late 1970s, US-based researchers presented evidence of reef fish and sponges off the mouth of the Amazon River at the Third International Coral Reef Symposium. Fishers from nearby coastal communities had successfully caught fish normally associated with reefs in the vicinity. However, nobody imagined that the reef could be so extensive or support such a variety of life.
Some 20% of the global riverine discharge that flows into the sea comes from the Amazon River, bringing an estimated 1.1 billion tonnes of sediment and 270 million tons of dissolved organic matter every year. Clear is not the word anyone would use to describe the area around the mouth of the Amazon River. Neither is the word hospitable, particularly when talking about sponges, rhodoliths (algae that form calcareous nodules), coralline algae, or hard and soft corals. Yet here, in the Amazon Reef System, they can be found supporting a host of other reef-associated marine life.
"It's not a reef as you would normally imagine," says Bastos, highlighting that while tropical coral reefs form more or less continuous structures, the Amazon reefs are more like a series of systems - sponge gardens and rhodolith beds, for example, spread out in depths ranging from fifty metres to a few hundred metres. Much of the System is also entirely mesophotic, meaning it exists where light levels are low. This depth, combined with wind and currents, seems to have allowed the Amazon reefs to survive. For the most part, the Amazon River discharge plume sweeps northwestwards and above the delicate gardens and beds that would otherwise be smothered.
The topography of the seabed also plays a role in supporting life. In the northern section of the system, for example, the tops of prominent high-relief structures standing between 110 and 165 metres in depth support coralline algae, sponges, bryozoans, and other species. In the central section, which feels the greatest impact of the plume thanks to the lack of a shelf break and proximity to the mouth of the Amazon River, life thrives on rocky outcrops. With a shallow shelf break, canyon, and lower sedimentation rates resulting from the plume's northerly flow, the south is the most biologically diverse part of the Amazon Reef System.
When oil meets reef
The Amazon Reef System "is a story of love and blood for parts of the Brazilian scientific community," says Bastos. The area is under increasing pressure for oil and gas exploration, and the presence of a unique reef system could hamper that development. By 2018, some politicians, oil industry professionals, and scientists were asking questions. Some argued that the system isn't really a reef. Some suggested that the reef was likely dead. Later research, which demonstrated that the system was not only alive but growing, answered the second point. As for the first, "people are still discussing if, conceptually, this is a reef," says Bastos.
Based on bathymetric and seismic data from oil companies, one study, which has not been published in a scientific journal, estimated that the Amazon Reef System spans around 13,500 square kilometres. This estimate is substantially larger than initial estimates of about 9,500 square kilometres. However, the authors argue that the area of the mesophotic part of the reef itself is smaller than previously estimated. This conclusion, as well as several opinions raised by the researchers who conducted the study, has attracted the criticism of other scientists, who "call for a scientific consensus about the extension and vitality of the [Amazon Reef System] based on replicable open-access data obtained through independent research."
"There is a huge discussion about the connectivity of the reef. We need to understand more about the biodiversity there. We know nothing about the deep-sea sections, especially in the canyons and upper slopes," says Bastos, adding that despite the size estimates, "we still haven't mapped the entire system, so we don’t really know for sure what is there."
More discoveries to be openly shared
The Amazon Reef System isn't the only area in Brazil mapped by Bastos and his fellow researchers. "Apart from the Amazon, we've been mapping the southeast Brazilian margin and the Abrolhos Archipelago," says Bastos.
"The Abrolhos Archipelago is one of the most important coral reef systems in the South Atlantic. We're mapping the submerged reefs and the mesophotic reefs. We're finding things we didn't know existed before, and that's all because of the high-resolution mapping we're able to do."
For Bastos, sharing seabed data is a priority. In 2022, the Nippon Foundation-GEBCO Seabed 2030 Project and the Brazilian Program of Marine Geology and Geophysics (PGGM) announced a new partnership to share seabed data for the Nippon Foundation-GEBCO Grid. Bastos is the coordinator for the PGGM.
Even if the data shared isn't the raw data, "it's better that it is available for everyone," says Bastos. "You can generate more information out of the data and have more people working and discovering things with that data…then we can make better, more sustainable decisions."
