The map shows the ecological status or potential of European rivers, expressed using a specific biological quality element (BQE), namely macroinvertebrates, reported by EEA member countries via WISE. The ecological status class is assessed by each country according to their national classification system, following the principles established by the Water Framework Directive (WFD). The assessment may be based by one or more samples taken during the year of reporting. The map displays the latest reported information under the WISE SoE reporting, so the year of reference may vary. Note that under the WFD reporting, the ecological status or potential based on all BQEs combined is represented by the BQE with the worst class - the "one-out-all-out" principle - which does not apply here, given that only one BQE is represented and that the reporting is done annually. The purpose of the map to enable the user to compare values per country or per monitoring station, depending on the scale of visualisation.