Distribution of littoral macrophytes in a North Swedish riverside lagoon in relation to bottom freezing

The relationships between the distribution of aquatic macrophytes and the winter habitat conditions of the littoral zone in a river in northern Sweden were studied during a 6-year period. Because the water level gradually decreases during the winter, the ice cover freezes solid to the shore substrate, i.e. bottom-freezing occurs. This is suggested to be a fundamental factor affecting the distribution of the macrophytes. The vertical frequency patterns of Isoetes setacea Lam., Eleocharis acicularis (L.) Roemer and Schultes and Subularia aquatica L. approached normal distribution around an intermediate level of frost disturbance. The vertical movements of these and other species followed the annual changes in the extent of bottom freezing ( P < 0.05). The various adaptations developed to obviate such disturbance by freezing are discussed.