Comparing Aquatic Environmental DNA, Microscopy and Sedimentary DNA to Investigate Cyanobacterial Community Dynamics Across a Trophic Gradient.

As a result of increasing temperatures and anthropogenic stressors, freshwater biomonitoring indicates that toxigenic and non-native cyanobacterial species are increasing globally. Because of the high-effort nature of traditional microscopy, aquatic Environmental DNA (eDNA) metabarcoding as a form of biomonitoring is becoming commonplace. While eDNA has been found to complement microscopic phytoplankton analysis, it has also been demonstrated that recent eDNA from surficial sediments (sedDNA) can also be a powerful tool to monitor a variety of organisms including benthic and planktonic microorganisms. However, comparisons between all three methods are rare. Therefore, we compared cyanobacterial metabarcoding data derived from aquatic eDNA and sedDNA to cell density and biomass determined by traditional planktonic microscopic analysis on samples taken from more than 20 lakes in the Masurian and Suwałki Lakelands of north-east Poland; an area composed of thousands of lakes with diverse morphologies, land-use histories, and varying trophic states. We found that there was a high degree of within method (eDNA/sedDNA, cell density/biomass) correspondence between the approaches, with trophic changes plausibly driving general patterns in taxa leading to these underlying agreements. Between methods, cell density proved to be more closely associated with eDNA, but was not found to be statistically significant. We postulate that cell numbers are a more similar metric to sequence read numbers than biomass, but that unique qualities of each measure mean that the approaches examined are complementary, rather than redundant. We detected several invasive taxa, which appear to have increased in abundance along the trophic gradients, and postulate that the accumulating nature of sediments may prove useful in detecting invasive taxa, particularly in the low-abundance early introductory stages.