Density-driven current between reed belts and open water in a shallow lake

Differential heating of surface waters of lakes caused by shading from vegetation can induce convection, which could be significant for the ecosystem because the chemistry and biology can differ in the reed belt zone compared to the open lake. This process has been given little attention in the literature. In the present study surface currents just outside a reed belt as well as radiation and water temperature profiles within and outside the vegetated belt were measured in a shallow lake in southern Sweden to determine the lateral flux between the two zones. The measured surface current directed toward the vegetation was about 1 cm/s. Net radiation at the water surface was 85% lower within the vegetation than in the open lake. The water surface temperature during sunny days was on average 0.5 degrees C warmer outside the reed belt, and at maximum 1 degrees C warmer. The lateral heat flux from the open water into the water within the reeds was estimated to similar to 200 W/m(2) during midday on sunny days, which corresponded to a calculated maximum current of 1.5 cm/s. This current between the open water and the reed vegetation is presumably the main water exchange process between the two zones during sunny summer days with low wind speeds.