Conclusions and recommendations for the use of sedDNA in palaeolimnology
Validation of sedDNA against concurrent lake monitoring is crucial to further its development and evaluate its performance as a palaeolimnological tool. Our comparison with long-term microscopy-based monitoring of phytoplankton in the lake surface water revealed broadly similar trends in the diversity and relative abundance and occurrence of chlorophytes, dinoflagellates, ochrophytes and bacillariophytes, and up to 20% of genera detected by microscopy were also detected by sedDNA. These results support the use of sedDNA as an effective tool for the reconstruction of past phytoplankton communities, at least within the time period investigated in this study. However, DNA degradation may occur in older sediments which could limit the reliability of reconstructions over longer time periods, and a substantial proportion of the phytoplankton community detected by microscopy were missed by sedDNA. Based on these results, we recommend that sedDNA reconstructions over time periods exceeding 100 years or of groups such as cryptophytes that were poorly resolved with sedDNA are treated with caution, and future research should focus on identifying the key determinants of variable DNA degradation and deposition among taxa. Furthermore, due to incomplete reference databases, it is important that future studies consider the fact that phytoplankton sedDNA reconstructions may only represent a subset of the total community in lakes. Continued improvements to reference database coverage, in addition to the combined use of multiple targeted primers may enable the wider phytoplankton community to be captured with sedDNA.