Nutrient status and its assessment by pulse amplitude modulated (PAM) fluorometry of phytoplankton at sites in Lakes Erie and Ontario

Abstract

Variable fluorescence of chlorophyll a (Fv/Fm), as measured by pulse amplitude modulated (PAM) fluorometry, has been proposed as a metric of nutrient deficiency in phytoplankton. However, not all studies use the same make or model of instrument, which may contribute to inconsistent findings. Three different PAM instruments (DivingPAM, WaterPAM, and PhytoPAM) were compared, testing the relationship between Fv/Fm and nutrient deficiency indicators (nitrogen (N) debt, phosphorus (P) debt, and alkaline phosphatase activity (APA)) in natural phytoplankton communities in Lakes Erie (all basins) and Ontario (two sites) in July and September 2011. Varying degrees of N and P deficiency were indicated in both lakes and months, with P deficiency most prevalent, though deficiency appeared to ease prior to the onset of a major cyanobacterial bloom. WaterPAM and PhytoPAM Fv/Fm were positively well-correlated, whereas DivingPAM was negatively but poorly correlated with the others. DivingPAM Fv/Fm was negatively correlated with P debt and APA, consistent with expected Fv/Fm responses to nutrient deficiency. Measurements with the other PAMs designed specifically to measure phytoplankton produced systematically higher values of Fv/Fm, which were uncorrelated to measures of nutrient deficiency. Different models of PAM cannot be assumed to produce equivalent measures of Fv/Fm, and care must be taken in interpreting results.