Law & Order Losing Longtime Lieutenant S. Epatha Merkerson

Merkerson joined the drama in 1991, making her its most veteran castmember

Like the tide, the Law & Order cast comes and goes in waves.

This season, Dick Wolf's 20-year-old mothership is poised to lose S. Epatha Merkerson, currently the most veteran member of the L&O team, E! News confirmed Thursday.

Merkerson, whose character, Lt. Anita Van Buren, has been battling cancer, joined the series in 1991, replacing Dann Florek, whose Capt. Cragen reemerged on Law & Order: SVU in 1999.

A source tells us that the 57-year-old actress, who just won a NAACP Image Award for her supporting-role efforts, apparently gave the word that she wanted out before the season finale was shot, allowing Van Buren to get a proper send-off, and not one of those next-season "whatever happened to?" explanations.

Then again, there's actually no guarantee right now that Law & Order itself is returning to NBC next season, so we're on hold as to who might be in the pipeline to replace Merkerson.

The news comes as sister show Law & Order: Criminal Intent lost original castmembers Vincent D'Onofrio and Kathryn Erbe, and more recent addition Eric Bogosian, in one fell swoop on its ninth-season premiere Tuesday.