Groundwater nitrogen and the distribution of groundwater-dependent vegetation in riparian areas in agricultural catchments

Abstract

Eutrophication is a major threat for several species-rich habitat types e.g. fens and meadows in riparian discharge areas throughout Europe, but few data are available to support the supposition that nutrient-contaminated groundwater from agricultural land is the direct cause for the continued decline of these formerly widespread habitat types. In the present study we investigated the role of nitrogen for the distribution of groundwater-dependent vegetation types in discharge areas in Denmark. Specifically we hypothesised (1) that the nitrogen availability in riparian discharge areas is linked to agricultural intensity in the topographic catchment and (2) that the spatial distribution of vegetation in riparian areas is influenced by nitrogen in the groundwater e.g. high productive reed beds dominate in areas with high levels of groundwater nitrogen, whereas species rich fens occur in areas with lower levels of groundwater nitrogen reflecting that incoming nutrients influence the productivity within the areas. We find evidence that the percentage of agriculture in the topographic catchment directly influenced the amount of nitrate in the discharging groundwater in the study areas and that groundwater nitrogen is closely coupled to the distribution of groundwater-dependent vegetation. Meadow and reed bed vegetation being highly productive were associated with higher levels of nitrate in the groundwater compared to low productive rich fen vegetation. From these findings we recommend that protected groundwater-dependent vegetation is mapped in groundwater discharge areas and that only those without presence of protected habitats are considered in rewetting plans to mitigate nutrient loss from agricultural production.