One Congress at Bulfinch Crossing courtesy HYM Investment Group

Office vacancies continued to fall in Boston while developers broke ground on projects that will add 2 million square feet of new workspaces in the second quarter.

The Boston office market recorded 500,000 square feet of positive absorption during the quarter, while the vacancy rate declined to 8.1 percent, according to Colliers International’s Market Viewpoint report. The 13.4 million-square-foot Back Bay submarket is the city’s tightest, with a 5.6 percent vacancy rate.

Groundbreaking of the 1 million-square-foot One Congress tower, which is 50 percent preleased to State Street Corp., led the new development activity in the quarter. WS Development and The Fallon Cos. began construction of new towers in the Seaport District for Amazon and MassMutual, respectively. Nordblom Co. and CIM Group kicked off a speculative 235,000-square-foot office building at 321 Harrison Ave. in South End.

Among the larger new requirements for office space are 350,000 square feet by mutual fund provider Eaton Vance and 300,000 square feet by CarGurus, which previously leased 50,000 square feet at developer Urban Spaces’ First Street corridor project in East Cambridge.