Unusual Toxic Cyanobacterial Blooms in a Tannin Stained Lake

Cyanobacterial harmful algal blooms (cyanoHABs) are increasingly detected in new environments not expected to have them. CyanoHABs are generally not expected to occur in acidic tannin - stained lakes. Tannins (polyphenols) have been shown to inhibit the growth of cyanoHAB species and used as a mitigation measure for cyanoHABs. Here we describe unusual toxic cyanoHABs caused by Microcystis and Dolichospermum in a tannin - stained acidic lake (pH 6.8) in Wisconsin. In collaboration with the Tug Lake Community Association a study was undertaken to describe the toxicity of these blooms. Microcystins (MC), anabaenopeptins (AP) and cyanopeptolins (CP) were measured by liquid chromatography tandem mass spectrometry in samples collected throughout the open water season. MCs occurred in all but two samples at an average concentration of 26.8 µg/L at the deep hole whereas nearshore scum samples contained a maximum of nearly 4000 µg/L. The MC congener MC-LA was most abundant followed by MC-LR > MC-LY > MC-LF > [Dha⁷]MC-LR. AP’s and CP’s occurred less frequently with an average of 10.2 and 8.5 µg/L. High frequency in situ chlorophyll and phycocyanin measurements detected two major bloom events. These bloom events coincided with fall mixis and entrainment of soluble reactive phosphorus in the photic zone. Watershed land usage is primarily forested which may suggest these cyanoHABs are driven by internal loading. Further work is needed to understand the existence of these blooms in an acidic tannic environment.