"A lot of it is generated from the defensive zone," Kelly said. "We were coming back and playing stingy in our end."
Thomas has two shutouts this season, and New York was blanked for the fourth time this season. This was the Islanders' worst effort of the season, and coach Jack Capuano let his anger and frustration spill out.
"My thoughts are that when you play this game you need to play with fire, you need to play with passion, you need to play with determination, you need to play with desperation," Capuano said.
Bergeron made it 1-0, 7:08 in by redirecting Tyler Seguin's pass past goalie Rick DiPietro. Horton doubled Boston's lead to 2-0 just 5:12 later when he roofed DiPietro's clearing attempt from the top of the right circle.
"No excuse for that," DiPietro said of his ill-advised clear. "It's not a play I want to repeat too often."
Kelly pushed the lead to 3-0 with a left-circle slap shot at 18:45 after New York defenseman Travis Hamonic lost the puck in his zone.
The Islanders didn't record a shot until Frans Nielsen's short-handed drive with 8:52 remaining in the first period. Boston outshot New York 13-2 in the frame and 30-23 overall. The two shots tied a season low in a period for the Islanders.
"I don't know if we came out flat, but we had no execution," forward P.A. Parenteau said. "We didn't have much poise with the puck. If you don't control the puck, this is the NHL, you're going to get beat like that if you don't execute.
"There's definitely not much positive you can take from this one. There's only bad stuff, there's nothing good about this game."
DiPietro was pulled between the first and second periods and replaced by AHL call-up Anders Nilsson, who was brought up from the minors Saturday because of injuries to Evgeni Nabokov (groin) and Al Montoya (hamstring).
Nilsson, a third-round pick of the Islanders in 2009, stopped 14 of 17 shots in his NHL debut. He gave up third-period goals to Ference with 9:35 remaining, Kelly at 12:54, and Chara at 18:15. DiPietro stopped 10 of 13 shots in the first period.
Thomas didn't have to work for this shutout as the Bruins clogged the neutral and defensive zones in the final 40 minutes. New York was mostly limited to shots from the perimeter, except for Nino Niederreiter's backhander from the slot early in the third, Nielsen's power-play backhander with 12 minutes left in the game, and two shots by Matt Martin and Jay Pandolfo when the Islanders were short-handed.
"It's hard to get a shutout in this league," Thomas said. "When you have a big lead in the third period, the human tendency is to let up.
"I did the best I could to be ready when they got their chances."
Islanders RW Kyle Okposo was a healthy scratch for the third straight game. ... Ed Westfall was inducted into the Islanders' Hall of Fame in a pregame ceremony. Westfall was the Islanders' first selection in the 1972 expansion draft and was their first captain. Westfall, who had 231 goals and 625 points in an 18-year career with New York and Boston, called the second with Jiggs McDonald on television. They were the Islanders' announcing duo from 1980 until 1996. ... Thirteen Bruins were plus-one or better, led by Joe Corvo's plus-four. Ten Bruins recorded at least one point. Kelly finished with three points, two goals and an assist.
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UNIONDALE, NY - NOVEMBER 19: Gregory Campbell #11 of the Boston Bruins takes a shot during warm-ups before a game...