Extending monitoring with sediment archive approaches: Comparison of biomonitoring, metabarcoding, and biomarkers to assess past phytoplankton dynamics

Sedimentary archives can provide valuable insights into the study of anthropogenic impacts on marine and limnic ecosystems over centennial and millennial timescales, potentially extending the temporal breadth of observation-based biomonitoring. Sedimentary archives allow for the tracking of biodiversity changes over long time periods, potentially including periods before human-induced changes. However, evaluations of biodiversity reconstructions using sedimentary approaches through comparisons with existing observation-based biomonitoring data are limited. Here we compared sedimentary ancient DNA metabarcoding and several biomarkers with >50 years of phytoplankton biomonitoring data from the Baltic Sea. Our findings indicated that both sedimentary ancient DNA metabarcoding and biomarkers reveal historical trends in phytoplankton communities. Sedimentary ancient DNA data was strongly correlated with biomonitoring data, while biomarkers showed weaker correlations, particularly for dinoflagellates. In addition, the sedimentary ancient DNA data indicated the past prevalence of ecological communities with no present-day analogs, highlighting the challenges of using modern observational data to infer historical biodiversity trends. The study underscores the importance of validating sedimentary approaches against observation-based data and calls for further research to improve the taxonomic resolution of metabarcoding and the specificity of biomarkers. These advancements could significantly enhance our ability to reconstruct historical biodiversity trends and inform future conservation strategies.