The threat of groundwater pollution for petrifying springs; defining nutrient threshold values for an endangered bryophyte dominated habitat

Abstract

Eutrophication by human activities is increasingly affecting ecosystem functioning and plant community composition. So far, studies mainly focus on the effects of atmospheric nitrogen deposition, surface water eutrophication or soil nutrient accumulation. Groundwater pollution of spring habitats, however, has received much less attention, although numerous papers report groundwater nutrient enrichment worldwide. This study presents a survey on groundwater pollution (with emphasis on nitrate and phosphate) and bryophyte composition in 51 ambient petrifying springs in 5 NW European countries, which were compared to published data from 173 other sites in 11 European countries. The reviewed dataset covers a broad range of unpolluted to heavily polluted springs with nitrate concentrations between 0.7 and 3227 μmol l^-1. Most petrifying springs in the rural lowlands of NW Europe were found to have elevated concentrations of nitrate and phosphate with the most polluted springs occurring in The Netherlands. The cover of individual characteristic bryophyte species significantly correlates with groundwater nutrient concentrations indicating that nutrient pollution of spring waters affects bryophyte composition. Palustriella commutata, Eucladium verticillatum and Brachythecium rivulare prefer unpolluted petrifying springs whereas Cratoneuron filicinum and Pellia endiviifolia show a much broader tolerance to groundwater pollution. In order to sustain at least the basic conditions for the typical bryophyte composition of petrifying springs habitats, threshold values of 288 μmol (18 mg l^-1) NO₃^- l^-1 and 0.42 μmol (0.04 mg l^-1) ortho-PO₄^3- l^-1 were defined. Data analysis of the spring water composition indicates that the main source for nutrient and nutrient induced base cation enrichment are nitrate losses from intensively used agricultural fields. The anthropogenically induced but regionally different chemical processes in subsoil and aquifers can result in different levels of nutrient pollution in springs. Further regulations for nitrate and phosphate application are required to conserve and restore groundwater fed ecosystems in Europe.

Keywords: Ambient springs; Base cation leaching; EU-Habitat directive; Lycopodo-cratoneurion; Nutrient threshold values.