Debbie Arnold’s attempt to block a colleague’s appointment to a minor office was disappointing. As the newest member of the Board of Supervisors, Arnold should be building bridges — not deliberately alienating her colleagues.

In case you missed it, one of Arnold’s first official acts after her swearing-in was to cast the lone “no” vote against appointing Bruce Gibson vice chair of the board. That post is largely ceremonial, though it does put Gibson in line to be chair next year.

True, Arnold wasn’t alone in objecting to what is usually a routine appointment. A couple of members of the audience groused that Gibson shouldn’t hold any leadership position because he had an affair with his legislative assistant, who has since been transferred to another department.

That we expected, as Gibson’s detractors rarely miss an opportunity to publicly chastise him. But Arnold’s protest vote — along with her reasoning — came out of left field.

She said nothing about Gibson’s affair when she opposed his appointment. Rather, she argued that it wasn’t his turn to be vice chair.

Based on her research, she believed it should be Frank Mecham’s turn. (It’s a longstanding tradition for board members to take turns serving as chair and vice chair.)

Under Arnold’s convoluted logic, the board should return to the rotational schedule it abandoned in 2009, when it was decided that Gibson would serve as chair rather than the newly elected Mecham. That gave Mecham a year to familiarize himself with the way the board operates; he then served as chair in 2010.

That decision made perfect sense then, and it still does. It also seems wise to show a little flexibility and simply follow the new rotation. Yet Arnold would rather fuss over a technicality than start the year on a collegial footing.

We understand that her politics are far more conservative than those of her predecessor, Jim Patterson, and that some clashes between liberals (Gibson and Adam Hill) and conservatives (Arnold, Mecham and Paul Teixeira) are going to be unavoidable.

But Arnold should pick her battles. Going to bat over something as insignificant as a vice chairmanship is silly, and sets the wrong tone for a celebratory swearing-in ceremony.

Let’s hope her conduct Monday was an aberration. Otherwise, we’re in for a long four years.