Harmonised biological indicators for rivers and lakes: Towards European assessment of temporal trends in ecological quality

• Ecological Quality Ratio (EQR) data are reported to the European Environment Agency. • EQR data representing biota in surface waters in 28 countries are available online. • Temporal trends were analysed for 1800 time series (2015–2021) from 18 countries. • Results show positive trends for sites initially in poor/bad status, and vice versa. • Linking the EQRs to water quality data can enable analysis of underlying causes. Freshwater biological indicators are essential and required by the EU Water Framework Directive (WFD) to assess ecological impacts of human pressures and measures to restore degraded rivers and lakes. The level of degradation and how this level changes over time can be assessed by using the ecological quality ratio (EQR) for each major group of biological communities. The EQR is measuring the deviation from reference conditions found in pristine waters on a harmonized scale from 1 (undisturbed water bodies) to 0 (fully deteriorated water bodies). EQR values have been reported by many European countries to the European Environment Agency (EEA), for four biological quality elements (BQEs): phytobenthos and benthic invertebrates in rivers, and phytoplankton and macrophytes in lakes. Following internal data processing, normalization, quality checking and publication of the dataset, the EQR data can be used to analyse trends to assess whether biological communities are recovering. Here we report on trend analysis based on consistent time series of normalised EQR values (nEQR) for years 2015–2021 from 2500 rivers and 700 lakes. For each of the BQEs, the time series were grouped according to their initial status (i.e. ecological quality of the BQE in 2015). For the subset of rivers where phytobenthos and benthic invertebrates initially indicated poor or bad status, the overall trends were positive (Sen’s slope S = 0.027 and 0.014, respectively). Correspondingly, positive trends were also found for phytoplankton and macrophytes in lakes with initially poor or bad status ( S = 0.009 and 0.014, respectively). Conversely, for water bodies with initial status high or good, negative trends were indicated for all BQEs ( S in range −0.027−−0.011), although statistically significant only for phytobenthos and invertebrates. These preliminary results demonstrate the potential of these indicators in assessing effectiveness of European water policies. Nevertheless, longer time series will be needed for more robust assessment of trends in ecological quality of water bodies.