干旱区生态环境知识资源中心

Man-made change in wetland ecosystems is mainly a 20th century process. Here, we report the changes over the last 55 years in the hydrology and botany of a Spanish semi-arid wetland, Las Tablas de Daimiel, a Ramsar site and a National Park. The landscape is valuable because of interesting waterfowl and plant species (particularly, Netta ruffina and Cladium mariscus) brought about by the fluctuations in water table arising from the interaction of surface flooding due to stream- and groundwater discharges and water retention by watermill dams. However, drainage reduced the wetland area to one seventh of its original Value in 8 years between 1965 and 1973, and watermills were destroyed in the 1960s. Water pollution, mainly organic matter, nitrogen and phosphorus, coming from point (towns and agroindustry) and non-point sources (agricultural practices in the watershed) started in the late 1970s and peaked by the middle of next decade, decreasing later as a result of newly-implemented, water treatment plants in the catchment area. Water availability was reduced greatly between the late 1970s and the 1990s because of irrigation programs in the catchment area, which exhausted the groundwater aquifer, and their effects on plants were mediated by decreasing yearly average flooding and its variability. The combined effect of increased eutrophication and decreased water inputs reduced the Cladium cover 10-fold, reduced the extent of Charophyte meadows and three quarters of hydrophyte species were lost. The reed (Phragmites communis) cover increased 22-fold. Later, a sudden increase in water inputs coming from a nearby aquifer raised the number of hydrophytes and the Cladium cover? and reduced the reed cover. Thus, decreases in water quantity and quality have acted together on wetlands over the last 35 years affecting plant species richness and cover. Wetland survival is endangered if no remedial actions, such as those we propose, an implemented. (C) 2000 Elsevier Science Ltd. All rights reserved.