Impact of riparian trees shade on aquatic plant abundance in conservation islands.

Temperature, acidity, light conditions, total dissolved salts, conductivity, dissolved oxygen, submerged macrophytes and shade and sun path directions were measured at 23 sites along the River Nile banks with Acacia nilotica growing at water's edge around the First Cataract Conservation Islands. Ceratophyllum demersum and Potamogeton crispus were common in the shaded and unshaded zones, Myriophyllum spicatum and Vallisneria spiralis were found only in the unshaded zone and Azolla filiculoides only in the shaded zone. Banks of the sites surveyed were oriented to five directions (NW, SE, NE, SW, N). There is a significant difference in both the type and density of submerged plants growing under the shade of riparian trees (Acacia nilotica) as compared to unshaded areas. Water column irradiance is the most influential variable dictating the distribution of submerged plants. The area of the shade provided by riparian trees was affected by environmental and/or plant variables. Environmental variables comprised the daily course of the exposition to sun; and plant variables included the area of the tree crown, the height of the tree and geographical position of the tree in relation to sun exposition. Trees on the west bank of the islands at the SW-NE direction have the highest shading effect. The management of tree vegetation might control incoming solar radiation affecting submerged macrophytes. [ABSTRACT FROM AUTHOR]