Comparison between fluorometry and microscopy-based phytoplankton assessments in the Laurentian Great Lakes

Photosynthetic pigment fluorescence is commonly used in limnology and oceanography as a proxy for phytoplankton biomass. Fluorometry has been used to detect subsurface algal blooms, characterize dynamics of the deep chlorophyll layer, and to provide greater vertical resolution to phytoplankton monitoring. However, more studies are needed to better understand how fluorometric data relate to phytoplankton microscopic assessments across a wide range of natural conditions. We combined data generated using pigment fluorometry with simultaneously collected phytoplankton taxonomic data, assessed via microscopy, from the Laurentian Great Lakes to analyze the correspondence between the two approaches. Across the Great Lakes, chlorophyll a concentrations estimated using fluorometry were significantly correlated with the total phytoplankton biovolume from microscopy-based assessment. Stronger correlations between the two approaches were observed in spring samples compared to summer samples, with the weakest relationships for summer deep chlorophyll layer samples. For phytoplankton group-specific correlations, brown-pigmented algae (mostly diatoms) were the most consistent between the two methods, whereas only weak correlations were observed for green algae, cyanobacteria, and cryptophytes. Weak relationships for some taxa may reflect poor accuracy across the gradient of phytoplankton abundance observed in the Great Lakes. We further discuss potential reasons for discrepancies and implications for fluorometric data interpretation. Fluorometry may enable greater spatial, vertical, and temporal resolution of phytoplankton data critical for assessing lower food web dynamics, but it may not closely correspond to microscopy-based approaches in terms of predicting specific phytoplankton groups.