Decoupled Faecal Indicator Bacteria abundances in salinity gradient: field study and experimental approach

Abstract

Abstract Water bodies receive and accumulate pollutants, including faecal pollution. The European Union Directive 2006/7/EC and the Recreational Water Quality Criteria of US EPA (2012) recommend the use of Escherichia coli in freshwaters and Enterococcus in marine/ brackish and freshwaters as Faecal Indicator Bacteria (FIB). Other bacteria such as Thermotolerant coliforms are also used as FIB. Different FIB reacts in particular ways with environmental conditions. The relative abundance of specific FIB at different salinity conditions is still poorly understood in estuarine environments, despite of numerous efforts to determine which FIB should be recommended to assess water quality in coastal environments. We performed an experimental approach and a field survey to evaluate the effect of salinity in the abundance of thermotolerant coliforms and Enterococcus. In the lab, the temporal abundance of both FIBs at different salinity concentrations were measured at 0, 24 and 48 hours. In the field, we estimated the abundance of FIBs in two environmental gradients of Uruguayan coast (beaches of the Rio de la Plata estuary and the Maldonado River basin). We observed that FIBs abundance associate distinctively with salinity gradients in the lab, where estuarine and oceanic conditions decreased the abundance of thermotolerant coliforms, while Enterococcus increased with high salt concentration. At the environment, both FIBs negatively associate with salinity probably denoting the freshwater source of FIBs. The results support the use of coliforms in freshwater environments and Enterococcus in marine and estuarine waters.