Vast, sheetlike clouds of planktonic marine life occasionally appear in coastal waters off the Islands and then dissipate after just a few days, ocean scientists have discovered.

These "thin layers" of life form at the interface between currents flowing in different directions. They have been known to science only for a decade and have been studied extensively during that time off the West Coast. They were first reported in Hawai'i waters off West O'ahu in April this year, and in addition to microscopic plant life, the dominant inhabitants of the layers were shrimp larvae and other shrimplike creatures.

These oceanic fields of life are much denser than those found above and below the layer, and can be so dense that a diver swimming from the surface disappears when entering the layer from above, and then reappears in the clear water below it, as if flying through a cloud bank.

The layers are amazingly fertile — rich with plant, animal and microbial life — and they are deeply mysterious. Their study may lead to significant new understandings of marine productivity, scientists say. But the science of these oceanic features is so new that researchers haven't come up with consistent ways of talking about what they're dealing with.

"We know a lot about how they're related to physical processes and we're pretty good at predicting where they're likely to occur and when. But we need to know more about what's in them," said University of Hawai'i oceanographer Margaret McManus, who has been among the pioneers in studying them.

Among her UH colleagues in the work are fellow oceanographer Grieg Steward, oceanographic technician Brian McLaughlin and graduate student Chris Ostrander.

Life in the layers is so dense, their study could change the reckoning of how much life is in the oceans, since those calculations have been built on assumptions that oceans are less densely populated.

McManus will lead a session on "Oceanography and Ecology of Thin Plankton Layers" at the 2006 Ocean Sciences Meeting, to be held in February in Honolulu, at which the latest on thin layers will be reviewed.

The layers can be mere films of pale discoloration in the water, or nearly opaque regions of green or brown or red, depending on the dominant life forms in them. They lie between horizontal flows of water the way peanut butter lies between slices of bread. And while they are generally horizontal, the layers can undulate and can even intersect with the surface when "fronts," like underwater versions of atmospheric storm fronts, change the conditions in the sea.

While the layers can be anywhere from 1 foot to 8 feet in thickness, they can spread out horizontally for miles. The largest to date has been measured at nearly eight miles wide. The thin plankton layers can be dense enough that they show up on sensitive sonar equipment.

"They are like a sheet of fabric, some more transparent than others, some like denim and some like lace," McManus said.

The layers form at the bottom of a warm surface current that flows in one direction on top of a cooler, deeper current that flows in an opposing direction. The interface can be anywhere from 10 to 30 feet below the ocean's surface. The warmer water on top tends to be less dense, and the water below more dense.

Microscopic plants called phytoplankton rain down through the less dense water until they hit the more dense layer. There at the interface between bodies of water, they are spread out by the opposing currents and begin to multiply, fed by the constant flow of nutrients from the moving water. Bacteria and viruses — the most abundant form of life in the ocean — also respond to the fertility and develop dense concentrations, said Steward, who specializes in marine microbial ecology.

As the thin layers develop, tiny animals arrive to take advantage of the plant life in the layers. Many may rise from the sea floor to feed at night, and then drop to the bottom again by day.

Fish larvae may dramatically improve their survival by feeding in these rich oceanic pastures, where their chance of finding food is much better than in the open ocean, McManus said. The rich layers may also be a technique for keeping larvae close to their home regions, since they will tend to remain in good feeding grounds close to shore rather than swim long distances to look for food.

Small fishes may show up to feed on both the tiny marine plants and animals, and McManus said she has seen bigger predatory fish cruise through, presumably feeding on the small fish.

"The interaction between species all gets accelerated at the higher concentrations you find in thin layers," Steward said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.