Evaluation of the <scp>BenthoTorch</scp> using lake periphyton: A comparison of attached cyanobacterial biomass estimates

Abstract Simple monitoring methods for benthic cyanobacteria are lacking, especially in lentic systems. Traditional methods to estimate biomass, such as Chlorophyll‐ a or phycocyanin measurements, are complicated by benthic cyanobacteria's ability to adapt under variable light conditions. In response, benthic cyanobacteria adjust the phycobiliprotein (phycocyanin, phycoerythrin) content in their phycobilisome complex, leading to potential underestimation of biomass if the chosen method does not account for both pigments. Here, we assessed the utility of the BenthoTorch, an in situ fluorometric tool, against in vitro fluorometry, a common benchtop method, in estimating cyanobacterial biomass in periphyton from lakes ranging in productivity and light intensity. We propose a new change point, the first that is specific to cyanobacterial biomass, for the BenthoTorch's comparability to the benchtop method: at 0.5 μ g cyanobacterial biomass cm −2 determined in vitro, above which high variability between estimates was observed. Using a larger dataset of phycoerythrin‐containing periphyton than has been previously assessed, we found that the presence of phycoerythrin did not directly affect the correlation between methods, but evidence suggests higher phycoerythrin content may lead to greater discrepancy. We found that the Turner Trilogy benchtop fluorometer's phycocyanin and phycoerythrin modules are not methodologically independent, and the two pigments are not distinctly quantified by the fluorometer if existing in the same sample. We offer recommendations to consider before applying either of these methods in similar systems when direct measurements (i.e., biovolumes) are not available and suggest that researchers have preliminary knowledge of expected productivity and taxonomy of periphyton communities prior to method selection.