The D-A-(C) index: A practical approach towards the microbiological-ecological monitoring of groundwater ecosystems
Groundwater is not only a vital resource, but also one of the largest terrestrial aquatic ecosystems on Earth. However, to date, ecological criteria are often not considered in routine groundwater monitoring, mainly because of the lack of suitable ecological assessment tools. Prokaryotic microorganisms are ubiquitous in groundwater ecosystems even under the harshest conditions, making them ideal bioindicators for ecological monitoring. We have developed a simple, inexpensive approach that enables ecological groundwater monitoring based on three microbiological parameters that can be easily integrated into existing routine monitoring practices: prokaryotic cell density (D) measured by flow cytometry; activity (A) measured as prokaryotic intracellular ATP concentrations using a simple cell-lysis-luminescence assay; and, as an optional parameter, the bioavailable carbon (C) measured as the concentration of assimilable organic carbon in a simple batch growth assay. We analyzed data for three case studies of different disturbances representing some of the main threats to groundwater ecosystems, i.e. organic contamination with hydrocarbons, surface water intrusion, and agricultural land use. For all three disturbances, disturbed samples could be reliably distinguished from undisturbed samples based on a single index value obtained from multivariate outlier analyses of the microbial variables. We could show that this multivariate approach allowed for a significantly more sensitive and reliable detection of disturbed samples compared to separate univariate outlier analyses of the measured variables. Furthermore, a comparison of non-contaminated aquifers from nine different regions across Germany revealed distinct multivariate signatures along the three microbial variables, which should be considered when applying our approach in practice. In essence, our approach offers a practical tool for the detection of disturbances of groundwater ecosystems based on microbial parameters which can be seamlessly extended in the future by additional parameters for higher sensitivity as well as flexibility.
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German Federal Ministry for Education and Research ( BMBF ) for financial support of the project consortium ‘ GroundCare ’ ( 033W037A ), via the call ‘Regional Water Resources Management for a sustainable protection of waters in Germany’ (ReWaM) and the funding scheme ‘Sustainable Water Management’ (NaWaM). Thanks in this respect go also to the water suppliers of the cities of Augsburg (E. Sailer, R. Asam), Würzburg (A. Lanfervoss, N. Jäger, C. Schiller, H. Reith), Hannover (enercity; W. Raue, T. Alex), and Mittenwald (M. Pöll, J. Gschwendtner). Further data were collected in the framework of the UBA-project ( FKZ 3708 23 200 ) funded by the German Federal Environment Agency and the DBU project No. 33252/01-33/2 funded by the Deutsche Stiftung Umwelt. We thank the colleagues from the Stadtwerke Düsseldorf (C. Wagner, H.P. Rohns, B. Droste, P. Eckert) for support and access to the river bank filtration site and the former gas works site. I. Engehardt and D. Knabe from the TU Berlin are acknowledged for their support in the DBU project.