A benthic barrier in which a polygonal shaped mat having straight sides is mounted on a similarly polygonal frame, but in which the frame sides bow out slightly between corners, causing the mat to stretch slightly and become taut. The frame comprises a plurality of straight leg members and a number of corner members as required by said polygonal shape. The corner members encompass interior angles which are from about 1 to about 5 degrees greater than the interior angle of the polygon defined by the straight sides of the mat. The frame member legs are rigid, but slightly flexible such that when assembled into the mat as an assembled frame, the straight legs are forced to bow out slightly between the corners, pulling the mat taut.
