Undersea canyons nourish isles’ deep-water life, study finds

Surveys of the undersea canyons around Hawaii show high levels of bio­diversity among small invertebrates in seafloor sediments, making the canyons the equivalent of oases in the desert, Hawaii scientists report.

And the vitality of these small organisms are likely key to the livelihood of larger fish, they conclude.

“Canyons may be particularly important in the Hawaiian islands, in part because they supply organic matter to the typically food-limited deep sea,” Fabio C. De Leo, a doctoral graduate in oceanography from the University of Hawaii at Manoa, said in a statement last week. “When there’s more food, there’s more life.”

De Leo is lead author on a paper recently published in the scientific journal Deep Sea Research Part II.

De Leo and colleagues, including oceanography professor Craig Smith, the study’s principal investigator, conducted 34 submersible dives into six underwater canyons and their nearby slopes. The study area ranged across the Hawaiian archipelago, from the main Hawaiian Islands through the Papahanaumokuakea Marine National Monument.

The Pisces manned submersibles collected sediment core samples from the seafloor down to 5,000 feet.

The scientists evaluated and mapped each canyon habitat, including the roughness of the seafloor and the steepness of canyon walls.

From these samples, they sorted out and identified 

the marine organisms called macrobenthos — including worms, clams and shrimp-like crustaceans — ranging in size from a fraction of an inch to a few inches long. The scientists then correlated the species data with the landscape metrics.

The scientists found that the canyons can serve as oases in the sea by channeling ocean currents, capturing and trapping sinking particles, funneling migrating animals, and providing a varied physical landscape.

Canyons near the main Hawaiian islands tended to collect and hold much more land-based organic matter than those in the Northwestern Hawaiian Islands, the scientists found. Branches, leaves, nuts and algae were abundant off Molokai and Oahu, washed into the ocean by rain and carried out by currents.

These decomposing materials, scarcer in the islands of Nihoa and Maro Reef, are a valuable food source for the invertebrates, themselves food for larger fish.