An alternative method for correcting fluorescence quenching

Abstract

International audienceUnder high light intensity, phytoplankton protecttheir photosystems from bleaching through nonphotochemicalquenching processes. The consequence ofthis is suppression of fluorescence emission, which mustbe corrected when measuring in situ yield with fluorometers.We present data from the Southern Ocean, collectedover five austral summers by 19 southern elephant sealstagged with fluorometers. Conventionally, fluorescence datacollected during the day (quenched) were corrected using thelimit of the mixed layer, assuming that phytoplankton areuniformly mixed from the surface to this depth. However,distinct deep fluorescence maxima were measured in approximately30% of the night (unquenched) data. To account forthe evidence that chlorophyll is not uniformly mixed in theupper layer, we propose correcting from the limit of the euphoticzone, defined as the depth at which photosyntheticallyavailable radiation is 1%of the surface value. Mixed layerdepth exceeded euphotic depth over 80% of the time. Underthese conditions, quenching was corrected from the depth ofthe remotely derived euphotic zone Zeu, and compared withfluorescence corrected from the depth of the density-derivedmixed layer. Deep fluorescence maxima were evident in only10%of the day data when correcting from mixed layer depth.This was doubled to 21% when correcting from Zeu, moreclosely matching the unquenched (night) data. Furthermore,correcting from Zeu served to conserve non-uniform chlorophyllfeatures found between the 1% light level and mixedlayer depth

An optimized method for correcting fluorescence quenching using optical backscattering on autonomous platforms

“…The net result is a decrease in fluorescence yield per unit chlorophyll in surface waters during daytime periods of increased irradiance. If left uncorrected, daytime fluorescence will generate approximations of chlorophyll concentration that under‐represent the true values (Biermann et al ).…”

“…On the vertical scale, quenched fluorescence profiles are also problematic as they have the same shape as profiles that exhibit deep chlorophyll maxima (DCM). DCM are important to pelagic food webs and integrated rates of net primary production (Biermann et al ), and their existence varies from persistent to rare in regions such as the North Atlantic and Southern Ocean, respectively (Holm‐Hansen and Hewes ; Poulton et al ). Even in regions where DCM are rare, recent studies have indicated that highly dynamic mixing and settling regimes of heavily silicified species alongside chlorophyll packaging effects with depth (Behrenfeld and Boss ), mean that any assumption of homogeneity is potentially problematic.…”

“…A number of methods for correcting fluorescence quenching have been proposed in the literature (e.g., Sackmann et al ; Xing et al ; Biermann et al ; Hemsley et al ; Swart et al ). Each of these methods relies on a number of assumptions (e.g., homogeneity in oceanic regimes, both horizontally and vertically), which when applied to the data sets from their respective studies held true.…”

Subsurface algal blooms of the northwestern Arabian Sea

“…The glider fluorescence data were first corrected for quenching using a time and depth dependent night-time ratio of optical backscatter to fluorescence corrected from the 3 channels of the ECO puck as per Hemsley et al (2015;our Fig. 2); no quenching could be detected below the euphotic depth (1% surface light), indicating that the process was able to accurately identify quenching-affected portions of the water column (Biermann et al 2015). Quenchingcorrected fluorescence was then converted to chl a values using a scale factor determined by regressing the light attenuation profile from the PAR sensor to the theoretical light attenuation profiles calculated from the fluorescence profiles and attenuation coefficients.…”

“…The sensors emit light in the blue to excite chl a fluorescence and measure the resulting emissions in the red. Phytoplankton adapts to specific light regimes and suffers from quenching when exposed to excessive light (Biermann et al 2015). The glider fluorescence data were first corrected for quenching using a time and depth dependent night-time ratio of optical backscatter to fluorescence corrected from the 3 channels of the ECO puck as per Hemsley et al (2015;our Fig.…”