WAIKIKI — The postcard-perfect beaches of Hawai'i attract millions of people every year.

And it's no surprise: The state has some of the cleanest waters — and sand — around.

But that doesn't mean these beaches aren't teeming with microscopic critters.

While there haven't been a lot of studies done on Hawai'i's sandy habitats, researchers who do look at the inner life of beaches say it's a fascinating world of nature at its smallest and finest.

"Like in any animal kingdom, you have predators and you have prey," said Watson Okubo, Monitoring & Analysis Section chief of the state Health Department's Clean Water Branch. "They're all feeding on each other. It's a mad world."

Many of these microscopic organisms are less than a millimeter long and sometimes as small as one-twentieth of a millimeter. A bucket of sand may contain thousands of these tiny creatures.

To survive the constant — and forceful — movement of the ocean, many have adhesive glands so they stick to the sand and aren't washed away. Some use tiny claws or suction cups to hold on; others secrete a sticky substance to stay in place.

But even smaller stuff — bacteria — also live in Hawai'i's sandy habitats.

According to David Karl, UH oceanography professor, there are probably 1 billion bacteria in each cubic centimeter of sand and about 1 million in each cubic centimeter of seawater. (One inch equals about 2.5 centimeters.)

"Although most people fear marine bacteria because of their association with human disease and infection, we owe our very existence to them, since they harvest sunlight and provide much of the oxygen we breathe," Karl said. "There are probably 100,000 'good' bacteria — or more — for every 'evil' one, yet the public knows only about the bad ones."

That's because it's the bad bacteria we all worry about.

IT'S ALL RELATIVE

Wea Huang dug her toes in the sand at Kuhio Beach without worrying about what microscopic critters live between the grains.

She said the beaches here are "way cleaner" than those in California, where she lives.

"The beaches aren't like this," the 28-year-old — pregnant with her first child — said last week. "It's not even half as nice as this."

Like many beach-goers, Huang knows about the bacteria that breed in polluted waters.

But what about in the sand?

"There's no germs, right?" she asked, her feet back on the beach mat.

Sure there are.

The sand on beaches is bristling with microscopic creatures, from nematode and sand worms to staph bacteria.

A recent study of the coral rubble and sand at Hanauma Bay, which had 1.6 million visitors in 2005, found more than 10,000 kinds of invertebrates living on the beach.

Most of the time these tiny invertebrates, parasites and protostomes don't cause any harm.

But sometimes certain strains of bacteria, such as enterococcus or E. coli, can enter an open wound and cause serious infections.

Still, experts say that Hawai'i's water and sand are among the cleanest.

"The water quality in Hawai'i is actually very good, especially compared to other parts of the country," said Dr. Roger Fujioka, a research professor at the University of Hawai'i's Water Resources Research Center and leading expert in bacteria in recreational waters in Hawai'i. "We have a higher standard that we're meeting at most beaches than the rest of the United States."

REGULAR TESTING

State officials regularly conduct bacteria testing and water-quality monitoring because the ocean can become polluted by runoff from streams, storm drains and sewage spills.

Researchers plan to launch a study of beach sand by the end of the year to find the most effective methods to disinfect the sand in the event of a sewage spill.

In March 2006, city officials diverted 48 million gallons of raw sewage into the Ala Wai Canal, which wound up contaminating some south-shore beaches, including Kuhio Beach in Waikiki.

That same month, about 10 million gallons of untreated sewage poured into storm drains that lead to Ka'elepulu Stream and Kailua Bay.

Beaches were closed and signs went up to discourage people from heading into the water.

With added nutrients in the water after a sewage spill, bacteria flourish.

But it doesn't take a major spill to create ideal environments for bacteria, said Okubo.

"If the beach is not by a stream or storm drain entering the ocean, bacteria levels are very, very low," Okubo said. "But when you do have rain, bacteria levels jump up ... Streams are flowing, the earth is being rinsed off. Everything is going through mountains and dirt and storm drains. The water is high in nutrients, so the bacteria population takes off. That's natural."

If the water has high levels of bacteria, it can be assumed the sand does, too.

"Anything in the ocean water will be in the sand," Fujioka said.

Some of the bacteria in the sand come directly from the ocean.

Others, such as fecal coliforms E. coli and enterococci, can enter the water from land, whether through storm drains, streams or even runoff from public showers, which can send bacteria-rich soil into the ocean.

In fact, there's more bacteria in the dirt in your backyard than in the sand, Okubo said.

Enterococci, indicators of contamination, are found in the fecal material of animals — including humans. But it's also prevalent in Hawai'i's soil.

"So you don't know where it's coming from," Fujioka said.

Not to mention we're all walking around with bacteria on our bodies already.

"Everybody has staph, but 30 percent has Staphylococcus aureus, the dangerous one, and they're not sick," Fujioka said. "We don't know how long staph lives in the sand."

Staph, Okubo points out, is present everywhere, including on doorknobs, restaurant menus and shopping-cart handles.

"It's not something that's (specifically) water-related," Okubo said. "There's a possibility, sure, but you have to keep it in perspective."

TROUBLE SPOTS

Some beaches are more susceptible to higher bacteria levels than others, researchers say.

Beaches near stream mouths, such as Kailua Beach and Waimea Bay, or near storm drains, such as the waters off the Kapahulu groin, tend to have higher bacteria counts, particularly during heavy rains, Fujioka said.

Still, Hawai'i beaches meet standards higher than ones set by the federal Environmental Protection Agency.

According to the BEACH Act of 2000, which was passed by Congress in 2004, the EPA recommends the geometric mean level of 35 colony-forming units of enterococci per 100 milliliters of marine water as the maximum allowable.

That equals 19 people per 1,000 getting sick, Fujioka said: "That's way too high."

So in Hawai'i, which gets millions of visitors at its beaches every year, the standard is much higher: just 7 colony-forming units per 100 milliliters of water.

(The state Health Department also uses clostridium as a secondary indicator. High enterococci levels with low clostridium counts usually means no sewage spill, Okubo said.)

"If you leave Mother Nature alone, and you don't harm her, she takes care of everything," Okubo said about how nature will find a way to regulate itself, even after a sewage spill. "It's when man gets involved we have problems."

Sheila Meadows, a 29-year-old from San Francisco, basked in the sunshine last week at Kuhio Beach with her husband.

She, too, said the beaches in Hawai'i are much cleaner than those in California, which she frequents often.

"The water is way bluer here, and there aren't any thorns in the sand," said a pregnant Meadows, lounging in a beach chair. "Sometimes I see bugs here and there, but it doesn't freak me out."

But what about the critters — like nematodes, rotifers and sand worms — she can't see?

"Well," she said, surveying the sand, her manicured toes safely on the beach chair, "that makes me a little more cautious."

Reach Catherine E. Toth at ctoth@honoluluadvertiser.com.