Macrophyte development and habitat characteristics in Sweden's large lakes.

The four largest Swedish lakes, Vänern, Vättern, Hjälmaren, and Mälaren, were surveyed by means of aerial IR-color photography in the 1970s. Along small slope gradients in all lakes the emergent communities extended to a width of hundreds of meters, while the extent on steeper slopes was a few meters. Because of the fractionate shape, L. Mälaren has a very long shoreline, which promotes vegetation growth in the transition zone between land and water. Sheltered conditions are numerous and water lilies and other floating leaved and freely floating plants are abundant. Emergent as well as submerged macrophytes grow vigorously in the northernmost part of the oligotrophic lake Vättern. In L. Vänern exposed shores pre-dominate, but sheltered bays and archipelagos are overgrown with reeds. In the shallow lake Hjälmaren monospecific stands of Phragmites australis are dominant. Recurrent surveys of L. Mälaren and L. Vänern in the 1990s showed changes that could be connected to the watertable fluctuations, which have been stabilized since the first investigation. Reeds and water lilies expanded in sheltered parts of the lakes, while the distribution of reed stands growing at their deep limit decreased.