Diel and spatial variability in cyanobacterial composition, gene abundance, and toxin concentration: a pilot study

We designed a pilot field study to assess relations between sunlight, cyanobacteria, and cyanotoxins. In 2021, we collected day (07:00 h, 10:00 h, 13:00 h, 16:00 h) and night samples (19:00 h, 22:00 h, 01:00 h, 04:00 h) at two locations in Kabetogama Lake, MN, USA. One sample set was collected from the lakeward end of a boat dock and the other on the nearby shoreline. Cyanobacterial phylogenetic eDNA differences over 24 h (pseudo F = 2.0938, p = 0.127) were not significant. Copies of anatoxin (anaC) and microcystin (mcyE) synthetase genes varied significantly over the sampling times at the dock (Friedman Χ² = 15.01, df = 7, p = 0.036; Friedman Χ² = 19.22, df = 7, p = 0.008) and the shoreline (Friedman Χ² = 19.33, df = 7, p = 0.007; Friedman Χ² = 20.56, df = 7, p = 0.005), with the highest anaC counts occurring during the night for both sites. Additionally, the highest total and dissolved microcystin concentrations occurred at night. Despite the proximity of the sampling locations, cyanobacterial phylogenetic eDNA results indicate that the variability between sites (pseudo-F = 27.547, p = 0.001) were greater than temporal differences over 24 h (pseudo F = 2.0938, p = 0.127). Understanding the effect of diel and spatial variability may help researchers and resource managers make informed decisions about sampling and potential exposure.