Moerasmelkdistel (Sonchus palustris L.) en Grote engelwortel (Angelica archangelica L.) langs het Twentekanaal: indicatoren van gebiedsvreemd water

In the Netherlands Sonchus palustris is a species characteristic of the borders of mostly oligohaline waters. Its optimum is in the western part of the country. In 1934 Angelica archangelica was found for the first time in the Netherlands along the river Waal. Since that time this species has expanded its area, mainly along the lower reaches of the main rivers and the borders of the IJsselmeer. This species grows in the same habitat as Sonchus palustris. In 1990 new pockets of both species were found in the eastern part of the Netherlands, along the Twentekanaal (fig. 3, 7). Here they grow in a community of tall perennials (table 1, fig. 8). In dry periods water from the river IJssel (a branch of the Rhine), loaded with salts and other pollutants, is pumped into this canal (fig. 4, 6). This causes high concentrations of chloride-ions in the canalwater, even at a distance of 20 km from the intake-point (fig. 5). Both species are concentrated along the western part of the canal, which is connected with the river IJssel by means of locks. The relation between the water-intake and the occurrence of the species was discussed. It is supposed, that the waterintake is responsible for the transportation of seeds (especially of Angelica archangelica) and because of its high concentration of chloride it might have influenced their germination and settlement