Silvertip sharks congregate in large groups potentially numbering in the hundreds in the Chagos Archipelago in the Indian Ocean. Now researchers working in this remote archipelago think they know why. Roughly every 12 hours, tides set in motion waves of cold, dense water that slosh over two of the region’s seamounts. This regular movement transports important nutrients like nitrate up the water column, fueling photosynthesis and biomass production that support marine life ranging from zooplankton to sharks and tuna.
This work highlights how ocean food webs are established, said Elliott Hazen, a marine ecologist at the Southwest Fisheries Science Center in Monterey, Calif., who is not involved in the research. “The aggregation of zooplankton [can] increase foraging opportunities for marine predators.”
A Remote Archipelago
The Chagos Archipelago, located 500 kilometers south of the Maldives and also known as the British Indian Ocean Territory, is home to a 640,000-square-kilometer marine protected area, the world’s second largest. A lot of science is being done to understand this region, thanks to its protected status, said Phil Hosegood, a physical oceanographer at the University of Plymouth in the United Kingdom, but it’s still “massively undersampled,” given its remote location.
Hosegood and his collaborators recently completed fieldwork around two seamounts known as Sandes and Swart, both with summits roughly 70 meters below the sea surface, to determine why the mountains’ vicinity is such a hot spot for top predators. “There are loads of sharks around Sandes and Swart,” said Hosegood.
Sloshing Delivers Nutrients
With funding from the Bertarelli Foundation, the team installed a roughly 65 meter long mooring dotted with instruments on the flank of Sandes in 2015. The next year, the researchers placed the mooring on the top of Swart. These instruments measured current, temperature, and particles suspended in the water column. The researchers found that cold, nutrient-rich water driven by tides periodically sloshes up the flanks of the seamounts, delivering chemicals like nitrate that phytoplankton consume.
Because the summits of Sandes and Swart are both located in the so-called euphotic zone—a shallow layer from the sea surface downward that receives enough light to permit photosynthesis—this delivery is particularly efficient at promoting the growth of phytoplankton that animals like zooplankton and fish then feed on, said Hosegood, who reported these results last month at the 2018 Ocean Sciences Meeting in Portland, Ore.
Confined by Rock
At the same time, the presence of Sandes and Swart bolsters the local food chain in yet another way, the researchers found. In the open ocean, zooplankton typically migrate several hundred meters each day, moving up and down in the water column: During the daytime, they move down into the darkness of the deep water to escape being seen by predators, and they move back up the water column at night.
But zooplankton like copepods floating above Sandes and Swart can’t go very deep, noted Hosegood. “Unfortunately for them they’re over a seamount.” These trapped zooplankton—observed by the scientists in a roughly 10 meter thick layer of water stirred upward by the tides—make a ready meal for small fish. Those, in turn, become food for larger predators. “Sandes and Swart do have some special properties, from an environmental perspective, that clearly make them an attractive location for sharks,” said Hosegood.
There are likely other localized hot spots of biodiversity in the Chagos Archipelago, Hosegood said, and he and his team hope to return to the Indian Ocean to study them. “We want to understand the role that these seamounts may have in sustaining shark populations.”