Hydrology of the wetland Naarderemeer: Influence of the surrounding area and impact on vegetation

Abstract

The Naardermeer is a polder located in the central Netherlands at the foot of a sandy ice-pushed ridge. It has been put on the list of.internationally important wetlands. Groundwater flow is directed from the ice-pushed ridge in the east towards the river plain in the west. Seepage from the ice-pushed ridge occurs in the eastern part of the Naardermeer while in the western part infiltration occurs as a result of low water levels in the adjacent polders. During the last decades the water levels in the Naardermeer have become steadily lower due to increasing groundwater extractions in the hill ridge and lowering of the water levels in the adjacent polders. External surface water is suppleted threat to compensate water deficits in summer. These changes form a possible threat to the preservation of the botanical significance of the Naardermeer. In 1985 a survey was started to establish the relation between hydrology and the occurrence of plant species in the Naardermeer. The hydrological data shows that water quantity and quality in the Naardermeer are strongly influenced by the surrounding area. As concerns quantity, the water input by seepage is inadequate to compensate for water loss through infiltration. This results in a mean net infiltration of 0.4 mm/day. The influence of the surrounding area also emerges from the quality of the groundwater. In the seepage area groundwater is generally of the calcium-bicarbonate type. In the most eastern part of the seepage area groundwater occurs with high nutrient load originating from polluted surface water just outside the Naardermeer. Infiltration recorded from piezometric data was confirmed by the presence of water with rain water attributes in the upper groundwater layers. In the area a clear relationship exists between the pattern of hydrological attributes and the vegetation pattern in reed marshes. Vegetations related to the quality of rain water occur in a random pattern in the Naardermeer. Vegetations related to water rich in calcium and bicarbonate occur in the seepage area where they are supplied directly by groundwater of the calcium-bicarbonate type. Furthermore, they occur in the infiltration area where they are supplied by surface water. The surface water originates from groundwater in the seepage area which flows into the infiltration area. This seepage driven flow will prevent the suppletion water from spreading all over the Naardermeer. This is important as most of the species present respond negatively to the quality of the suppletion water. This study illustrates the importance of hydrological information for the conservation of nature in wetlands. The impact of both internal and external (upstream and even downstream) processes has to be considered.