ROCKPORT — Contemporary music has been integral to the Juilliard String Quartet’s identity since its founding seven decades ago, when it championed Bartók and the Second Viennese School composers. So it was fitting that a new work was at the center of Sunday’s Rockport Music concert, its final local appearance before the departure of Joel Krosnick, its cellist since 1974. Advocacy doesn’t take a day off, even (or especially) at a moment when others might opt for nostalgia.  The new piece in question was the Ninth String Quartet of American composer Richard Wernick, his fourth work written for this group, which gave its premiere in November. Wernick’s program note calls it “a response to our oversimplified populist culture with its allure of easy, immediate reward.” Violinist Joseph Lin, speaking from the stage, compared it to Rockport’s craggy, unforgiving terrain, partially visible through the back window of Shalin Liu Performance Center.  I sometimes wonder whether such apologias, meant to prepare an audience for something tough and unfamiliar, do more harm than good, hyping the experience into something more forbidding than it really is. Wernick’s quartet, which opened with a tightly built movement of angular riffs and motifs, was a case in point: The landscape may have been unfamiliar, but Wernick is a skilled cartographer, and his writing offered listeners plenty of reference points to track the music’s progress.  The Juilliard’s bracing, rhythmically adept performance evoked a mechanism of interlocking parts, meticulously built to create a single, strong gesture. The second movement, “per una selva oscura. . .” (“in a dark wood,” a quote from “Inferno”) built chromatic textures atop repeated notes passed among the strings. The internal tension occasionally erupted into brief, explosive outbursts. Their emotional caliber needed no explanation.  Get The Weekender in your inbox:  The Globe's top picks for what to see and do each weekend, in Boston and beyond.  Framing the Wernick were favorites by Mozart (the “Dissonance” Quartet) and Debussy. The lean, driving sound of the Juilliard’s old days has evolved into something warmer yet less distinct. In each piece the slow movement was the standout. In Mozart’s darker moments, the ensemble played with a breathy, almost raspy sound, as if to undermine the composer’s undeserved reputation for constant high spirits. In the Debussy, the sound was shadowy and recessive, seeming to disappear without fully asserting itself.  In both, Lin’s stylish, polished playing was a delight. Krosnick was also assertive if sometimes inexact, his playing tinged with rubato, as if to hold off just a bit his departure from this storied group. It will be fascinating to hear what its next instantiation brings.  Presented by Rockport Music. At Shalin Liu Performance Center, Rockport, March 20
In its final area performance with longtime cellist Joel Krosnick, the Juilliard Quartet covered musical terrain both unknown and familiar.