When we think about the vast diversity of life in the ocean, we automatically picture pristine coral reefs teeming with life. This is especially true for rich, tropical locations like Hawaii. What we don't think of are the deep, dark depths of the canyons that lie just beyond the shallow paradises we know and love. Scientists have known for years that these deep water locations may contain a wide variety of species, but don't talk about them as much because no one had ever explored them to see what species lived there. Now, thanks to researchers from the University of Hawaii's School of Ocean and Earth Science and Technology (SOEST) and Hawaii Pacific University, someone has.
After 36 submersible dives into canyons off of four Hawaiian islands, scientists have a much better picture of what lives in Hawaii's depths. What they found is that there is much more in the deep than they previously thought! Moreover, these habitats may be key in replenishing biodiversity in nearby areas.
Submarine canyons extend 1,000 to 5,000 feet deep between the islands of the Hawaiian Archipelago. These deep ditches are only 50 to 300 feet wide and lie just a few miles from shore. To study the biodiversity in these deep depths, the researchers sent down UHM's subermibles Pisces IV and Pisces V to videotape and collect samples from the canyons. Canyons were examined off of Oahu, Molokai, Nihoa and Maro Reef (see the image on the right for exact locations).
Researchers found that despite their small size and lack of light, the canyons were filled with life. One of the reasons for this is that the canyons concentrate nutrients. As leaves, wood, algae and anything that falls into the canyon decays, they release nutrients that serve as food sources for a variety of small organisms. These creatures then feed the macroorganisms, including shrimp and invertebrates, which in turn attract fish and even larger creatures.
Compared to similar depths of the continental slops, the canyons contained not only more life but a higher diversity of it. This was particularly true for fast-moving, mobile species like sharks, fish, shrimp and squid. These large megafauna were twice as abundant in the canyons as on the slope, and there was a much wider variety of species in the canyons. They even encountered a rare sixgill shark during one of their dives off of Molokai! Many of these species likely feed in the canyons, and thus the canyons may be particularly key in ensuring the abundance of several commercially important fish species.
But there weren't just big critters from the surrounding reefs hiding in these canyons. One of the most intriguing finds of the research was that these canyons contained all kinds species not found anywhere else in Hawaii. During their surveys, the researchers found 41 species that were completely unique to the canyons, ranging from algae to fish, and may be endemic to specific canyons they were found in. Here's a table that summarized what they found:
Another thing that was surprising to the researchers was the abundance of land-derived artifacts. Kukui nuts, for example, littered the floor of the canyons off of Molokai (on right), even though they were miles from shore. The scientists hope to continue the research by looking at the stable carbon and nitrogen isotopes in the tissues of shrimp and other canyon species. By doing so, they hope to determine how important the land-based organic matter is in providing a food source for the canyon's inhabitants.
The researchers also plan to use their new knowledge to help formulate conceptual models that will allow scientists to predict which features hold the keys to increased biological abundance and diversity.
And this is just the first attempt scientists have made into understanding the biology and ecology of Hawaii's submarine canyons. Who knows what else they'll find on subsequent dives into these dark depths!
Vetter, E., Smith, C., & De Leo, F. (2010). Hawaiian hotspots: enhanced megafaunal abundance and diversity in submarine canyons on the oceanic islands of Hawaii Marine Ecology, 31 (1), 183-199 DOI: 10.1111/j.1439-0485.2009.00351.x
Images c/o SOEST's Press Release
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