The use of crustaceans as sentinel organisms to evaluate groundwater ecological quality
Résumé
Criteria for the evaluation of groundwater quality are essentially based on the physical and chemical characteristics of the water, but biological and ecological indicators are needed to estimate groundwater ecosystem disturbance correctly. Such ecological evaluations may use communities (of micro- or macro- organisms) as disturbance indicators, but the density and diversity of groundwater fauna can be too low to permit effective evaluation. In these cases, the use of sentinels (i.e., caged organisms in situ) may complement physical and chemical indicators in the assessment of subterranean ecosystems. We tested the use of aquatic crustaceans (Amphipoda and Isopoda) as sentinel organisms by caging and exposing them in piezometers. In a first step, four species were tested in six piezometers located in the east Lyon aquifer, located upstream and downstream of three urban storm-water infiltration basins. In a second step, we used two species: the epigean Amphipoda Gammarus pulex for a short-duration exposure (one week) and the stygobite Niphargus rhenorhodanensis for a long-duration exposure (one month). Sentinels were tested in four infiltration basins, using upstream (control) and downstream (impacted) piezometers, on three occasions in 2010 and 2011 and in the laboratory using three types of water with increasing pollution. Infiltration of storm water induced a decrease in dissolved oxygen (DO) and an increase in dissolved organic carbon (DOC) between control and impacted piezometers. We therefore proposed a Water-Quality Index (WQI) based on the ratio of DO to DOC concentrations in groundwater. We measured the survival rates and the levels of body stores (glycogen and triglyceride) at the end of the exposure period. The survival rates of both species, when significantly different, were lower in impacted than in control piezometers, but body-store levels did not change with location. We propose an Ecophysiological Index (EPI) that combines the survival rate and the state of body stores. The EPI of sentinels at the end of each exposure period was negatively correlated with DOC concentrations and positively correlated with WQI for both species; this measure was also positively correlated with DO concentrations for N. rhenorhodanensis. Short-term exposure (i.e., one week) of an epigean species (such as G. pulex) may be used to assess acute toxic disturbance, while a longer exposure (i.e., one month) of a stygobite organism (here N. rhenorhodanensis) may be used to assess diffuse organic pollution and for a global evaluation of groundwater ecological quality if the appropriate ecophysiological indicators are used to estimate stress during exposure.