2001-02: Karsums played for Prizma Riga’s second team and at the junior level in his native Latvia. He played five games for Latvia’s U-18 team, scoring 1 goal with 14 PIM.
2002-03: Karsums played two games at the pro level with Prizma Riga (Latvia) in the EEHL as well as playing with their U-20 squad and with Vilki Riga in the Latvian league. He drew the attention of scouts at the at the Division 1 U-18 and U-20 tournaments. Karsums scored twice in four games at the U-18 tournament and showed another side of his game in the U20 event when he had 1 assist with 35 PIM in five games.
2003-04: Karsums left Vilka Riga to play with the Moncton Wildcats of the QMJHL. He enjoyed a very good rookie season in Moncton, finishing in the top five of both team and league rookie scoring with 30 goals and 23 assists and was +6 with 76 PIM. Karsums scored 8 goals with 9 assists and was +8 with 14 PIM in 20 playoff games as the Wildcats reached the QMJHL finals. Karsums dominated the WJC U-20 Div. 1 tournament, scoring 5 goals and adding 7 assists in five games for Latvia. Karsums was drafted by the Boston Bruins in the second round (64th overall) of the 2004 NHL Draft.
2004-05: Karsums was limited to 30 games due to injury in his second season with Moncton but he still managed to be productive. He scored 14 goals with 12 assists and was +9 with 31 PIM. Karsums scored five times on the power play with two short-handed goals and was scoreless in two playoff games. Karsums scored 3 goals with 3 assists in six games for Team Latvia at the WJC U-18 Division 1 tournament.
2005-06: Karsums rebounded a nagging ankle injury to help Moncton Wildcats to the finals of the Memorial Cup tournament. He scored 34 goals with 31 assists and +25 with 89 PIM in 49 regular season games. Karsums was particularly effective on the power play with 16 of his goals coming with the man advantage. Karsums continued his hot scoring in the QMJHL playoffs with 15 goals (eight on the power play) and 11 assists and was +10 with 22 PIM in 21 games. Karsums scored seven game-winning goals in the playoffs for the Wildcats and added 1 goal with 3 assists in Moncton’s five Memorial Cup games (The Wildcats fell to league rival Quebec in the championship game). Karsums was a member of Team Latvia in the U-20 WJC tournament as Latvia competed in the main draw. He scored 3 goals with 3 assists in six games for Latvia. Karsums signed a three-year, entry-level contract with the Bruins during the summer of 2006.
2006-07: Karsums got off to a slow start as a rookie with the Bruins’ AHL affiliate Providence and also missed some time due to injury but he was playing his best hockey by the end of the season. Karsums scored 13 goals with 22 assists and was +1 with 41 PIM in 54 games. Five of his goals came on the power play and his intense play impressed P-Bruins coach Scott Gordon. Karsums scored 3 goals with 1 assist and was -1 with 2 PIM in 12 playoff games as the P-Bruins defeated Hartford in the first round before falling to Manchester.
2007-08: Karsums was the second-leading scorer (behind Pascal Pelletier) and arguably the best forward on a Providence team that had the best record in the AHL. He scored 20 goals with 43 assists and was +25 with 57 PIM in 79 games for the P-Bruins. Karsums scored seven power play goals and three times short-handed. In the AHL playoffs, he had 7 goals with 3 assists and was +5 with 6 PIM and two power play goals in 10 games as Providence were upset by Portland in the second round. Karsums played for Latvia in the World Championships, scoring 1 goal with 2 assists and 2 PIM in two games.
2008-09: Karsums made his NHL debut in December with the Bruins after opening the season in Providence. He appeared in six games with the Bruins before being returned to Providence. Karsums was traded in March along with Matt Lashoff to Tampa Bay in a deal that sent Mark Recchi to the Boston Bruins and spent the rest of the season with the Lightning. He had 1 assist and was -3 with the Bruins. He appeared in 18 games with Tampa Bay, scoring his first NHL goal against New Jersey on April 3rd, and finished with 1 goal with 4 assists and was -5 with 6 PIM. In 43 AHL games with Providence, he scored 9 of his 17 goals on the power play and had 24 assists and was +8 with 20 PIM.
2009-10: Karsums spent the first half of the season with Tampa Bay’s AHL affiliate Norfolk. He was loaned to Dynamo Riga in January in preparation for the Olympic Games. Latvian national team coach Oleg Znarok was also the Dynamo coach and and 16 of the 23 national team players were with that club. Karsums struggled in 36 games with Norfolk as the Admirals got off to a slow start that led to the firing of coach Darren Rumble. He scored 4 goals with 12 assists and was +1 with 6 PIM. In 12 KHL games, he scored 4 goals with 4 assists and was +6 with 16 PIM. Karsums played for Latvia at both the Olympic Games and the World Championships. He had two assists in Vancouver as Latvia was winless in four Olympic games and scored 3 goals with 1 assists and was +5 with 2 PIM in six games as Latvia finished 11th in the tournament. Karsums received a qualifying offer from the Lightning in June. He signed a two-year contract with Dynamo Riga in July.
2010-11: Karsums skated in 52 games for Dynamo Riga in the KHL and was second on the team with 17 goals. He had 15 assists and was -3 with 46 PMs as Dynamo finished fourth in the Bobrov Division; reaching the second round of the playoffs. In 11 playoff games Karsums scored 1 goal with 2 assists and was -1 with 8 PMs.
Karsums has good overall skill with his hockey sense and scoring instincts being his strongest assets. Karsums is not a fast skater but has good balance that makes him difficult to knock off the puck with a wide base. His soft hands give him good puckhandling and passing skills, especially while moving. Does not have a powerful wrist or slap shot, but has a quick release. He has a solid build and plays much bigger than his size. Karsums aggressively fights for loose pucks along the boards and shows no hesitation in using his body to make a play. He is popular with his teammates due to his enthusiasm and attitude. Karsum was a big scorer in junior hockey but struggled with injuries and inconsistent offensive production at the pro level.
Karsums figured to be one of the forwards competing for a spot with the Lightning under new coach Guy Boucher in training camp but signed a two-year contract with Dynamo Riga in the KHL in July. There are doubts that he's interested in returning to hockey in North America. Karsums has the talent to be a lower line winger in the NHL with the ability to play physical, intense defense with his strong, if not flashy, skating.