LOS ANGELES—Sgt. Daniel Gonzalez has broken up bar fights, chased armed gangbangers and even apprehended a blood-soaked murderer after a car chase.

But nothing in his 22 years with the Los Angeles Police Department could have prepared him for his latest responsibility: art critic.

Since January, the clean-cut cop has patrolled the beachside neighborhood of Venice's famous boardwalk, passing judgment on painted porcelain skulls, henna tattoos and scrap-metal Star Wars sculptures offered for sale. On a recent Thursday, he told a dread-locked artist dabbling in a variety of media that he should "get more into the wood stuff, less into the hats."

"Now this is what people are looking for," Sgt. Gonzalez told another artist, offering what he called "positive reinforcement" for selling on-the-spot spray paintings. "Do you really take Visa? That is awesome."

Sgt. Gonzalez inspects wares on the boardwalk in Venice Beach. Hannah Karp/The Wall Street Journal

Sgt. Gonzalez's new duties are part of one of the LAPD's more unusual mandates: keeping Venice weird.

Famous for its funky mix of performers, skateboarders and runaways, Venice Beach is one of California's biggest tourist draws in part because day-trippers are all but guaranteed to see something strange. But increasingly the surfside community is battling to preserve its character as big businesses and wealthy individuals move in, threatening the very quirkiness that helped attract them in the first place.

Google Inc. recently opened a Southern California office in Venice. Hollywood producer Joel Silver is converting Venice's old post office into his corporate headquarters. And a Canadian company recently won approval to install a controversial zip-line ride along the boardwalk, infuriating locals who fear it will only exacerbate the area's traffic and commercialization.

The L.A. City Council last December passed a new ordinance that effectively banned anyone but local artists from engaging in commercial activity on the boardwalk's beach-facing side. Ordinance violators are subject to fines and repeat offenders can end up in jail.

That has left it to officers like Sgt. Gonzalez to routinely weigh in on a debate more suited to the Museum of Modern Art or the Guggenheim: What constitutes art?

Residents say making the cops police art isn't ideal, but there is little alternative.

"You could have volunteer boardwalk walkers, but then what?" says Matt Kline, director of outreach for the Venice Neighborhood Council. "It is a tough job for the police to do, but this is an ordinance, so they're really the only people who can do it."

Venice, a former epicenter of Beatnik culture that now attracts 16 million visitors a year, according to the Venice Chamber of Commerce, has long struggled with how to regulate the freedom of expression on the wide, concrete path that borders the beach, officially known as "Ocean Front Walk."

Sgt. Daniel Gonzalez

There were virtually no rules governing commerce on the boardwalk until two decades ago, when the city banned unlicensed vending there at the behest of local merchants who complained that vendors were stealing their business.

But in 1997 a federal appeals court overturned the ban, arguing it trampled First Amendment rights. Since then the city has tried implementing a slew of different rules to avoid sheer chaos—for example requiring artists to enter a lottery for "Public Expression Participant Permits." But each version of the law proved either too restrictive or too vague to hold up in court, deterring some police from issuing citations altogether.

Now, though, Sgt. Gonzalez says he thinks the city has finally hit the nail on the head. The ordinance that took effect earlier this year gives police more explicit guidelines than ever to determine what qualifies as art. Pottery doesn't count as art because it has a utility apart from its artistic message, according to the ordinance, nor do any goods that appear mass produced, like home appliances and auto parts.

Still, he says there is plenty of gray area. Hula-hooping might be performance art, but selling hula hoops is not. Hand-drawn henna tattoos can pass for art, but those made with mass-produced stenciled designs are another story. Mass production is difficult to determine, too, since the ordinance gives no specific number of items that constitutes a "mass."

Sgt. Gonzalez says he is no art aficionado—he studied business and economics in college, and the 45-year-old Southern California native says most of his art education came from working for several years as a part-time security guard for Sotheby's in Beverly Hills, shortly after he joined the police force in 1990.

While working at the art auction house during his off-duty hours, he says he "saw a lot of cool stuff" and developed a particular appreciation for Impressionist artists like Paul Cezanne.

"I don't know how they came up with that oil-based paint—how thick and real it looks," says Sgt. Gonzalez.

J. Scott Smith, a homeless man, sells cardboard "bum signs" that qualify as art. Hannah Karp/The Wall Street Journal

Still, he isn't afraid to give pointers to the boardwalk artists about what he thinks will sell—and spends much of his time encouraging ordinance violators to find their hidden talents. This year he says he inspired one crystal vendor to make beach-themed snow globes from her precious stones, since selling plain crystals is a no-no, and he says he spurred another woman to melt crayons onto canvases with a blowtorch.

This spring, J. Scott Smith, a homeless man who has lived in Venice for several years, says he asked Sgt. Gonzalez: "What do I have to do to stay here?"

"I said, 'Create something, perform,' " Sgt. Gonzalez recalls. "If you're out here and you do have some creative juices flowing inside you, figure it out."

Now, Mr. Smith makes and sells cardboard "bum signs" with messages like "Need $ To Bail Mom Out of Jail" and "Couldn't think of Anything Snappy So Just Give Me a Dollar And Beat It."

Art policing isn't for everyone: Sgt. Gonzalez says that since he joined the beach patrol in January he has seen three officers quit the patrol. But he has no plans to leave. "It's a little bit of higher calling," he says.

Write to Hannah Karp at Hannah.Karp@wsj.com