Recommendations for obtaining unbiased chlorophyll estimates from in situ chlorophyll fluorometers: A global analysis of WET Labs ECO sensors

Abstract

International audienceChlorophyll fluorometers provide the largest in situ global data set for estimating phytoplankton biomass because of their ease of use, size, power consumption, and relatively low price. While in situ chlorophyll a (Chl) fluorescence is proxy for Chl a concentration, and hence phytoplankton biomass, there exist large natural variations in the relationship between in situ fluorescence and extracted Chl a concentration. Despite this large natural variability, we present here a global validation data set for the WET Labs Environmental Characterization Optics (ECO) series chlorophyll fluorometers that suggests a factor of 2 overestimation in the factory calibrated Chl a estimates for this specific manufacturer and series of sensors. We base these results on paired High Pressure Liquid Chromatography (HPLC) and in situ fluorescence match ups for which non-photochemically quenched fluorescence observations were removed. Additionally, we examined match-ups between the factory-calibrated in situ fluorescence and estimates of chlorophyll concentration determined from in situ radiometry, absorption line height, NASA's standard ocean color algorithm as well as laboratory calibrations with phytoplankton monocultures spanning diverse species that support the factor of 2 bias. We therefore recommend the factor of 2 global bias correction be applied for the WET Labs ECO sensors , at the user level, to improve the global accuracy of chlorophyll concentration estimates and products derived from them. We recommend that other fluorometer makes and models should likewise undergo global analyses to identify potential bias in factory calibration

Hydrography and biogeochemistry dedicated to the Mediterranean BGC-Argo network during a cruise with RV <i>Tethys 2</i> in May 2015

“…Overall, the biogeochemical sensors equipped on the BCG-Argo floats revealed inherent calibration shifts upon deployment. This is in agreement with recent works on ECO fluorometers (Roesler et al, 2017) and oxygen optodes .…”

“…Moreover, the homogeneity of slopes among the series of new sensors (thus recently factory calibrated) gives insight into the gain (between 1.8 and 2) to be applied afterwards to fluorescence data (Roesler et al, 2017).…”

Two databases derived from BGC-Argo float measurements for marine biogeochemical and bio-optical applications

“…The magnitude of this correction within the mixed layer varied between 3 and 50 % (for details see Table 2 in Xing et al, 2017, for the same database). Finally, according to the recommendations by Roesler et al (2017) on the overestimation by standard WET Labs fluorometers, remaining Chl values were divided by a factor of 2 to correct for the global bias in factory calibration.…”

Seasonal dynamics and disturbance of phytoplankton biomass in the wake of Tahiti as observed by Biogeochemical-Argo floats

“…Moreover, the so-called daytime non-photochemical quenching (NPQ) process that results in a decrease of phytoplankton fluorescence per unit of Chl and occurs at high irradiance (Cullen and Lewis, 1995), is corrected according to the method developed by Xing et al (2012), using a depth threshold of 90 % of the mixed layer depth (see details in Schmechtig et al, 2014). Then, the recommended global bias correction factor of 2 is applied to each calibrated fluorescence profile (Roesler et al, 2017). Finally, profiles are also corrected from the fluorescence originated from non-algal matter as 15 recently suggested by Xing et al (2017).…”

“…Then, the recommended global bias correction factor of 2 is applied to each calibrated fluorescence profile (Roesler et al, 2017). Finally, profiles are also corrected from the fluorescence originated from non-algal matter as 15 recently suggested by Xing et al (2017). In situ Chl determinations from High Performance Liquid Chromatography (HPLC) analyses were performed during the FTWake deployment.…”