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Groundwater ecology merged during the second part of the 20 th century with modern ecological practice after having adopted the ‘ecosystem concept’. The latter was first applied to karstic systems and separately for alluvial non-consolidated aquifers along surface running waters. Today groundwater ecosystems are studied within a multi- and transdisciplinary framework at various spatial and temporal scales by experts dealing with microbiology, the ecology and systematics of meio- and macro-fauna, geochemistry, hydrogeology and mathematical modelling. A further paradigmatic change occured with the recognition that subterranean assemblages of organisms are formed by both hypogean and epigean taxa. The biological diversity in subterranean ecosystems can be much higher than earlier thought and may even exceed surface diversity in some taxa. This largely unrecognized biodiversity in many cases deserves environmental protection. A third phase in the development of groundwater ecology has occured over the last 15 years with the incorporation of socio-economic research topics within groundwater ecology (GIBERT et al., 1994a) and in this sense today we have the “New Groundwater Ecology”.

An improved assessment for the enumeration of aquatic sediment bacteria is introduced. Sediment samples were stained with DAPI and counterstained with SYBR Green II. This, for the first time, allows the researcher to clearly distinguish between bacteria and nonspecifically stained bacteria-like particles. A strong linear correlation was observed between total number of attached bacteria and sediment grain size in sandy fractions of lotic, lentic and subsurface environments. This trend did not continue into the silt and clay fraction. Ratios of attached to free-living bacteria in our studies spanned three orders of magnitude and varied with sediment type. Sediment habitats which were exposed to anthropogenic disturbance and/or contamination always had lower biofilm-porewater ratios than obviously undisturbed sites. Biofilm-porewater ratios are therefore suggested as valuable indicators for the characterization of sediment systems.