Mike, Good info. That helps explain the floatation for me. Gas bubbles probably mostly oxygen adhering to the leaves and stems of the mat are causing it to float. Smacking the mat with the paddle dislodges the bubbles in a small part of the mat. Other bubbles quickly take the place of old bubbles as mat remains floating. My explanation is correct if you break loose a small handfull of the Chara and shake it in the water to release almost all bubbles and put the mat back into the water the handfull of Chara should now sink.
