Apartment rents fell almost nine percent in the the Seaport during the third quarter, a new report from CoStar Group found, thanks to competition from two new towers.

The Boston Business Journal reports that rents dropped once leasing began for nearly 450 apartments in two apartment buildings in the Seaport: Drew Co.’s 236-unit Waterside Place and Gerding Edlen’s 202-unit 315 on A.

In order to maintain their tenant base, other Seaport landlords lowered their rent, a CoStar real estate economist told the BBJ.

During the third quarter, Seaport rent increases rate fell to just 1.9 percent, down from as much as 12 percent year-over-year increases in the past, CoStar Group found. Average rents in the Seaport in the third quarter dropped to $1,952, down from $2,136 a year ago.

CoStar’s neighborhood grouping of the West End, Beacon Hill, North End, and Charlestown, saw average rents fall from $2,256 in 2012’s third quarter, down to $2,133 in 2013.

Back Bay and South End average rents also fell from $2,599 to $2,539 in the same period.

Fenway’s $2,113 average rent was stagnant.

Chinatown and the Financial District have the highest average rent of $2,961.

CoStar found that Boston’s overall apartment vacancy rate has remained between two and three percent since 2011.

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