Oxygen transport rates through mats of Lemna minor and Wolffia sp. and oxygen tension within and below the mat

Abstract

A boundary-layer model has been used to describe the movement of dissolved oxygen through a vegetation mat of Wolffia sp. or Lemna minor. In an environment where light and temperature are held constant, the reaeration rate of the water below the mat is a linear function of mat thickness. In the light, the rate of oxygen transfer into water through a 10.5-mm mat is 4.4 times higher for Wolffia and 2.9 times higher for L. minor than in the dark. In the light the rate of oxygen transfer through Wolffia mats is significantly lower in August and September than in early summer. A seasonal shift in transport rates through L. minor mats has not been conclusively established. The efficiency of gas transfer through washed Wolffia and L. minor mats 8 to 16 mm in thickness varies from 4.0 to 47.0% of the estimated gas exchange rate for a small protected lake. The rate of gas transfer through the mat is higher than might be expected considering the close packing of the component fronds. The shape of the oxygen profile in the upper portion of a photosynthetically active mat indicates that much of the oxygen produced by photosynthesis of the submerged fronds may be lost to the atmosphere.When the light intensity falls below the level required to exceed the compensation point, dissolved oxygen levels in the water trapped between the fronds are quickly reduced to very low tensions despite the proximity of the entire mat to the oxygen-saturated air–water interface.