Meiofauna assemblages discharged by springs from a phreatic aquifer system in the Netherlands

Groundwater meiofauna washed out of springs was studied by means of spring water filtration. The principal interest was a quantitative analysis of the number and diversity of outwash fauna in relationship to hydrological conditions. In addition, a comparative analysis was made of outwash fauna of different outlets of the same hydrological system. The spring complex studied is part of the Centraal Plateau in the southern-most part of The Netherlands. This area is characterized by, for The Netherlands, relatively high hydraulic heads and large permeability of aquifers. Influence of human activities (agriculture) in the recharge area was demonstrated by elevated concentrations of nitrate, chloride, and sulphate in the spring waters. No traces of groundwater pollution by heavy metals or pesticides were found. Temporal variation of meiofauna outwash was studied in a single rheocrene spring during one year. This rheocrene had a relatively large and constant discharge (1300±8 ¦h−1). Meiofauna numbers fluctuated between 110–240 ind. m−3. Considerable differences in meiofauna numbers in a one-off analysis of different springs (rheocrenes and helocrenes) were found. Numbers ranged from 140 to 5800 org. m−3 and an inverse relationship with the amount of water discharged was shown. Organisms in the water filtrates were of multiple origin: aquifer(e.g. Niphargus, Parastenocaris), benthic spring head habitats (e.g. Gammarus, Chironomidae), and (saturated) soils near the outlets (e.g. Criconomatidae). Meiofauna abundance and composition is discussed in relation to hydrology and spring head morphology. A semi-quantitative analysis of outwash aquifer fauna was masked by the presence of epigean elements or elements of unclear origin. The outwash fauna of rheocrenes of large discharge supposedly is the most representative for the aquifer. Adaptations of hypogean populations to oligotrophic porous environments and the consequences for drift of juvenile stages is discussed.