MIT scientists find first evidence that rivers form coral reef passes
In a paper published in the peer-reviewed journal Geophysical Research Letters, the MIT team described reef passes as "deep, navigable channels dissecting coral reefs around volcanic islands." These are basically wide channels that cut through the coral and serve as conduits for ocean water and nutrients to filter in and out.
As many reef passes are found to form near large island river basins, researchers had previously assumed a relationship existed between the two. However, the theory remained unproven until now.
The authors of the latest study showed "how coral reefs line up with where rivers funnel out from an island's coast." These water channels play a direct role as a freshwater body meets the sea, providing circulation throughout a reef and maintaining the health of corals by allowing saltwater and transporting key nutrients, a media release by MIT explained.
The study impacts scientists' understanding of how coral reefs form and function, as much discussion has been brewing about this symbiotic interaction in recent years.
MIT researchers focused on the Society Islands, a chain in the South Pacific Ocean around Tahiti and Bora Bora. However, the pandemic limited their ability to work onsite, needing to rely on satellite images and maps.
Lead study authors professor Taylor Perron and graduate student Megan Gillen found that "reef passes are deep channels cutting through reef barriers and flats that facilitate wave- and tide-driven circulation. These passes regulate flow between lagoon and ocean."
"It is well known that rivers discharging freshwater and sediment off islands affect reef structure and composition," study authors added. However, was there a direct connection between outlets of large island rivers and reef passes?
Thanks to geospatial analysis of the archipelago, the researchers discovered that river basins drain to parts of the reef, essentially creating an underwater highway. River erosion actually shapes these coral formations. Rivers running towards the ocean created these eroded depressions in the coastline over time.
The study authors identified two river-driven reef pass formation mechanisms: reef incision and reed encroachment. Incision refers to how "rivers cut passes into reefs exposed when the sea level is lower." Encroachment refers to where "old river channels on land are preserved as passes when sea level rises."
"Reefs migrate toward the islands as sea levels rise, trying to keep pace with changing average sea level," observed Gillen. The study further states that the results "show how rivers may support reef health over geologic timescales by enhancing water circulation between lagoons and oceans."
"A lot of discussion around rivers and their impact on reefs today has been negative because of human impact and the effects of agricultural practices," said Gillen. "This study shows the potential long-term benefits rivers can have on reefs, which I hope reshapes the paradigm and highlights the natural state of rivers interacting with reefs," she added
Now, with this knowledge proven, Gillen will be exploring how rivers could be engineered to promote circulation and reef health.
You can view the full study in the journal Geophysical Research Letters.
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