A temperature compensation method for chlorophyll and phycocyanin fluorescence sensors in freshwater

The in vivo fluorescence (IVF) of photosynthetic pigments is used widely as a proxy for phytoplankton biomass in fresh and marine waters. Although fluorescence intensity is known to decrease with rising temperature for many fluorophores, temperature quench is rarely accounted for in field studies of plankton IVF. Here, we quantified the effect of temperature on in vivo chlorophyll and phycocyanin fluorescence in the laboratory (∼ 5°C to 30°C), and we derived temperature compensation equations for IVF sensors commonly used in freshwaters. The equations reference measured fluorescence to a standard temperature, and they have the same linear form as the equation derived in an earlier study of chromophoric dissolved organic matter fluorescence: F_r = F_m/(1 + ρ(T_m – T_r)), where F is fluorescence intensity (RFU, relative fluorescence units), T is temperature (°C), ρ is the temperature coefficient at a given reference temperature (°C⁻¹), and the subscripts r and m stand for the reference and measured values. At a reference temperature of 20°C, the temperature coefficients (ρ) for chlorophyll and phycocyanin in Wisconsin lake waters ranged from −0.008°C⁻¹ to −0.012°C⁻¹ and −0.006°C⁻¹ to −0.012°C⁻¹, respectively. For chlorophyll in a pure culture of the green alga Scenedesmus dimorphus, the value for ρ was similar to the value in natural assemblages, averaging −0.018 ± 0.003°C⁻¹; but for phycocyanin in the blue-green alga Synechococcus leopoliensis it was lower (more negative), averaging −0.034 ± 0.003°C⁻¹. This disparity notwithstanding, we conclude that temperature compensation is an important component of IVF monitoring.