Nick McGowan-Lowe, national organiser for Scotland at the National Union of Journalists, described the move to cut the Aberdeen programme as an act of “cultural vandalism”.
He accused the company of financial mismanagement and of making a “rushed through, panicked” decision.
“This does not give the impression of a company who is thinking long term,” he told MSPs on Holyrood’s culture committee.
“They’re doing a knee-jerk reaction and what they are proposing is going to have a catastrophic effect on local democracy and the culture of the north east if it is allowed to go through.”
STV recently signed a 10-year deal for north and central Scotland TV licences, which came into force in January.
However, the company is now seeking to amend that deal so it can share material from both regions.
Mr McGowan-Lowe likened cutting spending while setting up a new radio station to “buying a sports car and then trying to pay less on the mortgage to afford it”.
He added that he was “astonished” STV was not reviewing its plans for a radio station.
Paul McManus, negotiations officer at Bectu, said STV had spent millions of pounds on bonuses for senior executives, refurbished studios and on the radio station.
He said committing to such outlay shortly before announcing major cuts was not “rational”.