Nick Carter, Diving Into a Cause

Nick Carter has dolphins, not ex-squeeze Paris and her predicament, on his mind. (Dan Steinberg -- AP)

A pileup of celebrity advocates in town yesterday presented some hard choices. Did we want to hear a dignified and beloved two-time Oscar winner describe her struggles with osteoporosis? Or watch a scruffy boy-bander known for rehab, family squabbles and starlet girlfriends try to save the dolphins?

Sorry, Sally Field. You couldn't have known Nick Carter would come to D.C. the same day.

The lead singer of the hot-in-'99 Backstreet Boys was officially appointed a "special ambassador of the Year of the Dolphin" yesterday by the United Nations Environment Program Convention on Migratory Species. The 27-year-old Carter showed up at the National Press Club in a black suit and blue patterned tie, his blond bangs brushed down on his forehead.

Why Carter? He explained that he grew up in Florida and "from a very young age enjoyed the ocean and all its wonderful creatures. . . . It saddens me deeply to learn that something that gives me such pleasure is in danger." Then he read off a bunch of stats about dolphins, dying by the hundreds of thousands lately through hunting, pollution, habitat loss and climate change.

Would we like a moment to talk to Carter one-on-one? Oh, yeah. Why dolphins, Nick? "I've been a scuba diver since I was little and saw dolphins all the time. . . . You know, it's like living with a pet almost, but one that you can't necessarily touch. But that's fine, you know, because they're around you and you see how happy they are."

More advocacy work on his horizon? "Yeah, but one thing at a time. I'm going to deal with the dolphins first."

Okay, gotta ask: Any thoughts on ex-love Paris Hilton's jail sentence? No. "I, uh, want to leave that negativity and that lifestyle behind and pursue things that actually mean something, you know?" And since you ask: New Backstreet Boys album on the way this summer.

HEY, ISN'T THAT . . . ?

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• Gwen Stefani, looking buff on the penthouse balcony of the Ritz-Carlton yesterday. The pop star (in town for last night's show at Nissan Pavilion) was instantly recognizable from neighboring office buildings in a red bra top, black pants and a ponytail; she was accompanied by year-old son Kingston in purple stripes, a friend (nanny?) and a young girl.

SURREAL ESTATE

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Buyers: Doug and Laura Gansler

Price: $1.2 million

Details: Gansler has moved into a new job -- Maryland's attorney general -- and a new house. "He likes to joke that during the campaign, unbeknownst to him, his wife got a new home and a new dog," said spokeswoman Raquel Guillory. The couple sold their Chevy Chase home of eight years and bought a slightly larger (3,300 square feet) four-bedroom, two-bath in Bethesda; the dog is a Maltepoo (Maltese-poodle mix) named Max.

THIS JUST IN . . .

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• Paris Hilton will serve just half of her 45-day jail sentence, Los Angeles sheriffs said Wednesday. The socialite heiress got a break for good behavior (including showing up for her last court date -- just like a normal person) and will spend 23 days at the Century Regional Detention Center beginning June 5. While in the slammer, Hilton will be in a "special needs housing unit" reserved for high-profile inmates and will get at least one hour a day outside her cell to shower, watch TV or yak on the phone.