Comparing ecological classifications of freshwater lakes based on reference conditions for benthic invertebrates derived from spatial analogues and paleolimnological approaches

A critical step in lake ecological assessment is establishing appropriate reference conditions, which enable the measurement of differences between the current status and these baseline conditions. To achieve this, spatial analogues and paleolimnology are two common approaches, each with strengths and limitations, but few studies have directly compared their performance. This study compares the ecological classification of Swedish lakes based on benthic invertebrate reference conditions derived from these two approaches. Overall, while both approaches yielded similar classifications, the key distinction lies in the emphasis on site-specific (sediment-based paleolimnology) versus type-specific (spatial analogues) conditions. As such, sediment-based estimates are generally more accurate and better suited to establish lake-specific baseline conditions. However, given the ongoing impacts of climate change, restoring lakes to their historical states may no longer be feasible due to climate-induced "shifting baselines". Spatial methods, by contrast, offer a more accurate representation of current conditions by accounting for climate-induced changes and by isolating human pressures. Nonetheless, the challenge of addressing human impacts, often presumed to be negligible, on reference lakes used in the spatial approaches remains unresolved. Our findings highlight the challenges of shifting ecological baselines, where climate change and anthropogenic pressures increasingly overlap. Consequently, there is a risk that current frameworks misclassify lake conditions by attributing climate-induced changes to anthropogenic pressures or overlooking them entirely. We recommend integrating paleolimnological records with current monitoring to improve ecological classification and better account for climate-driven changes in lake ecosystems, and improve assessment tools.