Flooding and groundwater dynamics in fens in eastern Poland

Abstract

Abstract. Dynamics in hydrology and water chemistry in the Biebrza mires (Poland) were examined by means of a sampling survey that was repeated four times between 1987 and 1992. The dynamics in the vertical stratigraphy of water types in the peat profile are considerable from close to the mire surface to a depth of 50 cm. Water composition in the root zone correlated best with vegetation types during extremely dry or wet conditions. In the root zone of groundwater‐fed rich fens with Caricetum limoso‐diandrae and Calamagrostietum strictae vegetation, specific groundwater types evolve from the interaction of discharging groundwater from below the root zone and the temporal influence of precipitation and evapotranspiration. The Caricetum limoso‐diandrae is fed by the continuous discharge of nutrient‐poor, relatively mineral‐rich water. The site conditions in the Calamagrostietum strictae are determined by occasional flooding and the presence of discharging mineral‐poor groundwater in the lower part of the root zone. In the Caricetum limoso‐diandrae and the Calamagrostietum strictae the maximum variations in water level were 56 and 86 cm, respectively. The composition of shallow groundwater of the Betuletum humilis/Caricetum rostratodiandrae fen is diluted most compared to other vegetation types by rainwater in wet periods. In periods of prolonged drought it has a water type that is affected by evapotranspiration and peat mineralisation. The water level varies by only 33 cm. In the Magnocaricion and Glycerietum maximae in the floodplain the water composition is determined by spring flooding of the river and the natural draw‐down that occurs in the following summer. Here, maximum variations in water level were 108 and 117 cm, respectively.