Unhappy with his limited play at the Latvian club team Vilki Rīga, Karsums elected to transfer to the Moncton Wildcats of the QMJHL. He enjoyed a very good rookie season for the Moncton Wildcats, finishing in the top five of both team and league rookie scoring with 53 points, and accumulating 76 PIM. Karsums also dominated the 2004 World Junior Ice Hockey Championship Division I tournament, scoring 7 goals and adding 6 assists for 13 points in five games. Karsums had problems with injury that limited his play with Moncton for the 2004-05 season, but he still managed to be productive. In 30 games, he scored 26 points and had 31 penalty minutes. During the 2005–06 season, which was his most successful season so far, Karsums scored 65 points and helped his Moncton Wildcats to win the Jean Rougeau Trophy and the President's Cup. He also received the Guy Lafleur Trophy as playoffmost valuable player, scoring 27 points in 22 games and was the first European born player to do so. Eight of his 16 post-season goals have proven game-winning goals, including three of the four game-winners in the championships series vs. the Quebec Remparts. He played in the 2006 Memorial Cup and scored one goal and had three assists in five games.
Professional leagues
On May 22, 2006, Karsums signed a three-year contract with Boston Bruins. He spent the majority of that time playing for the Providence Bruins of the American Hockey League, the top farm team of the Bruins. In his most successful 2007/08 season with Providence Karsums scored 20 goals and had 63 points in 79 games. On December 13, 2008, Karsums debuted in NHL with the Boston Bruins against Atlanta Thrashers in a 4–2 win. On March 4, 2009, on the NHL Trade Deadline, he was traded by the Bruins, along with Matt Lashoff, to the Tampa Bay Lightning in exchange for Mark Recchi. While playing for the Lightning, Karsums scored his first career NHL goal on April 3, 2009, against the New Jersey Devils' Martin Brodeur.
Return to Europe
On January 14, 2010, after playing the first half of the season with affiliate, the Norfolk Admirals of the AHL, Karsums was reassigned by the Lightning to HC MVD of the Kontinental Hockey League. However, he didn't join the team, and was instead loaned to Dinamo Riga. After struggling in the first season, he became a major contributor and one of the scoring and on-ice leaders in his next three seasons with Dinamo Riga. He played at least 50 games in all three seasons finishing above 30 points. Mid-through 2012–13 season Karsums was named as captain of the team. After four seasons with Dinamo Rīga, Karsums signed a contract with HC Dynamo Moscow. Following the 2017–18 season, his fifth with Dynamo, Karsums left the club as a free agent and agreed to a one-year deal to continue in the KHL with HC Spartak Moscow on May 4, 2018.
