The main objective of BIOCLASS-FRESH is to develop and validate biological indicators suitable for classification of ecological status in rivers and lakes in line with the EU Water Framework Directive. The project will build upon the results of recent pro jects and expand further by focusing on still missing indicators for eutrophication and acidification of rivers and lakes, such as phytoplankton blooms, benthic algae, littoral invertebrates and fish. The project will also include indicators for hydromorp hological pressures on macrophytes, benthic fauna and fish. Existing and new monitoring data will be compiled and analysed, including paleo-ecological data to validate reference conditions for selected indicators. A series of different statistical and mod elling techniques will be used to identify potential thresholds and to quantify the uncertainty in the response curves of the indicators along the different pressure gradients. The results will be used to recommend boundaries between the different classes of ecological status for the selected biological quality elements, and to estimate uncertainty of various ways of combining the different metrics into a holistic classification of water bodies.