The ski equipped airplane nosed over while landing on a frozen lake. The pilot reported that when he departed another seaplane base, the surface was very rough due to frozen slush created by snow mobiles. He reported that when he touched down on the frozen lake, he heard a "loud grinding" noise and the airplane slowed down rapidly. The pilot stated that the grinding noise stopped then started again only much louder. He reported that the nose of the airplane dropped and the propeller contacted the ice. The pilot reported that the airplane then dropped to the right and flipped over onto its back. The pilot reported that the ground track from the left ski was "rounded" at initial touchdown followed by a space then a much narrower and deeper track. He reported that the four quarter-inch bolts sheared off between the axle and the gear leg on the left ski.