Comparison and validation of biomarkers and sedaDNA with observation- based monitoring data in the Baltic Sea

Sedimentary archives provide valuable insights into the long-term impacts of human activities on marine and freshwater ecosystems, beyond the temporal scope of traditional biomonitoring. These archives allow the reconstruction of biodiversity changes over centennial and millennial timescales, potentially capturing ecological conditions prior to human influence. However, direct comparisons between sediment-based biodiversity reconstructions and long-term observational biomonitoring data remain limited. In this study, we compared sedimentary ancient DNA (sedaDNA) metabarcoding and multiple biomarker analyses with over 50 years of phytoplankton biomonitoring data from the Baltic Sea. Our results show that both sedaDNA and biomarkers can reveal historical trends in phytoplankton communities, with sedaDNA showing a strong correlation with biomonitoring data, whereas biomarkers showed weaker correlations, especially for dinoflagellates. In addition, sedaDNA data provided evidence for past ecological communities with no present-day analogues, highlighting the limitations of relying solely on modern observations to infer historical biodiversity trends. These results highlight the importance of validating sedimentary approaches against observational data, and underscore the need for further research to improve the taxonomic resolution of metabarcoding and the specificity of biomarkers. Improving these methods could significantly enhance our ability to reconstruct past biodiversity dynamics and inform future conservation strategies.