Hydrological and Biogeochemical Controls on Calcareous Fen Ecosystems

Abstract

<p>Calcareous fens are base cation-rich peatland ecosystems that support a diverse assemblage of vegetation. This study furthers our knowledge on these unique wetlands through the determination of the hydrological and biogeochemical processes governing calcareous fen ecosystem function. The growing season hydroperiod of three calcareous fens in close proximity to one another was found to be very different: some streamside locations \Vere subject to water table fluctuations ofjust 10 cm. whereas other sites to > 1 rn fluctuation. Only a small area of the three fens was subject to appreciable groundwater upwelling. contradicting previously held concepts. The Riparian Fen derived most of its \Yater from the stream: with groundwater recharge and stream discharge the principal outlets. The Trough Fen was precipitation and evapotranspiration dominated. with moderate overhank flooding. and small groundwater discharge. The Basin Fen received half of its growing season water through groundwater discharge. with most water leaving through cvapotranspiration. The subsurface flowpath direction and magnitude were variable among the calcareous fens and were subject to between-year flow reversals. Phosphorus was very limiting across most of the fen areas. and this was principally related to an association with calcium and magnesium in the fen_pore-waters. Gradients of redox-sensitive solutes were detected at all sites. and were organized along the dominant hydrological flowpaths. Plant species richness and diversity within the calcareous fens \Vere controlled by differential responses of individual species to in situ hydrological and biogeochemical gradients. Organic matter content in the surface peat and the duratfon of the initial saturation period to start the growing season proved to be the most useful metrics for determining plant species ranges/tolerances within the calcareous fens.</p>