Hawai'i researchers are working with Netherlands scientists to determine whether continuing climate change could disrupt the biology of a crucial region of the ocean — a dense field of plant life that exists at the deepest level that sunlight reaches.

The area is called the deep chlorophyll maximum layer. It exists at a critical interface about 300 feet deep, said oceanographer David Karl of the University of Hawai'i's School of Ocean and Earth Science and Technology.

Most of the ocean's nutrients — the fertilizers of the ocean — are near the sea floor, but the light needed for photosynthesis is near the top. The deep chlorophyll layers are in between, using the food from the deep and the "last few photons" of light from the surface, Karl said.

"A very interesting biology sets up there. Dark-adapted cells have uniquely evolved to thrive in this zone," he said. This region of life is important to ocean productivity since it is the base of the food chain. Tiny animals eat the little plants and photosynthetic bacteria, larger animals and fishes eat them, and bigger creatures eat those.

What happens if something disrupts the ability of nutrients to reach this level? The scientists have developed computerized models of the ocean that suggest that a warming globe could reduce mixing — that if surface waters warm and become less dense, they will be less likely to mix with rich, cold water from the deep. That increases something scientists call stratification, the tendency to form distinct horizontal zones instead of mixing.

"The nutrient entrainment is pretty dependent on mixing. The model suggests that if additional stratification occurs, the integrity of the layers (of photosynthetic life) is destroyed," he said.

Karl said researchers already have noticed that during El Ni–o events, when surface waters become warmer than normal, the amount of mixing is reduced between the surface and the depths.

They are collecting data in the waters north of O'ahu to further test the model, to try to determine whether the deep layers of life become unstable during climate shifts. They are collecting water samples and dangling sediment traps in the near-darkness hundreds of feet below the ocean's surface to try to learn what's going on there.

If the deep chlorophyll maximum is significantly disrupted or disappears, it's not clear what happens.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.