Setting safe ecological boundaries for nutrients and exploring climate impacts on biological resilience in lowland lakes and rivers in the Nordic and central European regions

Boundary concentrations for phosphorus and nitrogen set by EU countries and Norway are intended to support good ecological status in lakes and rivers. Yet these boundaries vary by more than an order of magnitude within comparable waterbody types, raising concerns that some limits may not ensure good status. This study estimates nutrient boundaries compatible with good ecological status for lake phytoplankton and river phytobenthos by using binomial logistic regression (BLR) and examines how climate variables affect their ecological status. Using datasets on total phosphorus, total nitrogen and ecological quality ratios for lake phytoplankton and river phytobenthos from three Nordic and three Central European/Baltic countries, we estimated nutrient boundaries that are likely to support good ecological status for nutrient-sensitive biota in common lake and river types within each of these regions. The boundaries estimated with BLR correspond well to previous estimates based on other regression methods. Most of the currently used national boundaries are also in line with our estimates. However, some national boundaries exceed the upper 95% confidence limit of the estimated values-indicating that they may require further validation. Climate effects on ecological status were variable and much weaker than the strong influence of nutrients. Nonetheless, ecological status for phytoplankton in lakes (expressed as normalized ecological quality ratio) showed a small but consistent negative relationship with summer air temperature for all the stratified lake types included. Consequently, river basin managers may need to adopt more stringent nutrient boundaries and/or implement additional measures to mitigate climate-driven risks to freshwater ecosystems.