The city’s purchase of the local water company has created a conundrum for city officials: Who will fill the $12,000-a-year seats on Pennichuck Corp.’s board of directors? In previous years the company’s shareholders elected directors, but the city of Nashua is now Pennichuck’s only shareholder. So who will get the vote — the mayor, the president of the Board of Aldermen, or someone else. The problem was discussed at a recent meeting of the Board of Aldermen’s Pennichuck Water Committee. It must be resolved before the first shareholders meeting since the city acquired the company last year, which is scheduled for March 23.