Assessing sedimentary proxies to reconstruct the occurrence and extent of harmful algal blooms

During the recent decades, human activity led to vast and rapid changes in algal communities through pollution, alteration of catchment areas and climate change. As a consequence of such stressors, lake conditions can offer favourable conditions for the formation of harmful algal blooms (HABs) of cyanobacteria. The rise of bloom-forming taxa and toxin-producing taxa are threatening ecosystem services provided by lakes. Understanding the dynamic nature of algae and these stressors is crucial for developing effective mitigation strategies to preserve or potentially restore the integrity of aquatic ecosystems. Monitoring data is limited to a relatively recent period and thus sedimentary proxies facilitating the reconstruction of past cyanobacteria abundances are necessary to provide historical context for modern observations.

Recently, the phytol:sterol index (PSI), which corresponds to the ratio of phytol (produced by all algae including cyanobacteria) and specific phytosterols (produced only by eukaryotic algae) was proposed as a proxy for the relative abundance of cyanobacteria within the overall algal community (Klatt et al., 2025). Here, we aim to test the suitability of the PSI alongside other emerging sedimentary proxies to record cyanobacteria abundances and the abundance of HABs. To this end, we extracted short cores covering the past ~250 years from two of the Finger Lakes, Owasco and Skaneateles, in Upstate New York (USA) using a universal coring system in spring 2024. While Owasco Lake has experienced progressive eutrophication since the 1960s, nearby Skaneateles Lake with a much smaller watershed to lake surface area ratio remains oligotrophic. HAB occurrences in Owasco began several years before Skaneateles, but since 2017, HABs have occurred in both lakes, with greater prevalence in Owasco.

Bulk sediment analyses (% total organic carbon, C/N ratios) indicate an increase of algae productivity coinciding with the reported progressive eutrophication of Owasco Lake, while Skaneateles Lake shows rather stable conditions. This stability is also reflected in the PSI, which is about 0.25 in Skaneateles Lake throughout our record, indicating relatively low abundance of cyanobacteria. In Owasco Lake, on the other hand, PSI values reveal a marked increase to 0.45 after ~1950, consistent with a shift in community composition towards Cyanobacteria mid-20^th century. We compare these results to compound-specific hydrogen isotope measurements of fatty acids and phytol, and the offsets between them, to further distinguish ecological changes in the lakes. Finally, we use sedimentary DNA (sedDNA) metabarcoding to validate our lipid data by assessing changes in the phytoplankton community and to identify the presence of bloom forming taxa. Overall, the combined results of our proxies are in accordance with observations, and further extend our knowledge of algal community composition prior to the monitoring period. This emphasises the potential of this proxy and the strength of multiproxy approaches.

 

Klatt, A., De Jonge, C., Nelson, D.B., Reyes, M., Schubert, C.J., Dubois, N., Ladd, S.N., 2025. Algal lipid distributions and hydrogen isotope ratios reflect phytoplankton community dynamics. GCA 394, 205–219. https://doi.org/10.1016/j.gca.2025.02.013