Correction of profiles of in‐situ chlorophyll fluorometry for the contribution of fluorescence originating from non‐algal matter

Abstract

In situ chlorophyll fluorometers have been widely employed for more than half a century, and to date, it still remains the most used instrument to estimate chlorophyll‐a concentration in the field, especially for measurements onboard autonomous observation platforms, e.g., Bio‐Argo floats and gliders. However, in deep waters (> 300 m) of some specific regions, e.g., subtropical gyres and the Black Sea, the chlorophyll fluorescence profiles frequently reveal “deep sea red fluorescence” features. In line with previous studies and through the analysis of a large data set (cruise transect in the South East Pacific and data acquired by 82 Bio‐Argo floats), we show that the fluorescence signal measured by a humic‐like DOM fluorometer is highly correlated to the “deep sea red fluorescence.” Both fluorescence signals are indeed linearly related in deep waters. To remove the contribution of non‐algal organic matter from chlorophyll fluorescence profiles, we introduce a new correction. Rather that removing a constant value (generally the deepest chlorophyll a fluorescence value from the profile, i.e., so‐called “deep‐offset correction”), we propose a correction method which relies on DOM fluorometry and on its variation with depth. This new method is validated with chlorophyll concentration extracted from water samples and further applied on the Bio‐Argo float data set. More generally, we discuss the potential of the proposed method to become a standard and routine procedure in quality‐control and correction of chlorophyll a fluorescence originating from Bio‐Argo network.

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

“…As Proctor and Roesler (2010) and Xing et al (2017) stated, the observed Chl increase at depth is due to very high CDOM (which is a consequence of the anoxic conditions prevailing at depth in the Black Sea) and non-algal matter concentrations that can affect the chlorophyll fluorescence signal. After correcting the profile according to Xing et al (2017), Chl concentrations below 100 m are zero. The profile from the South Atlantic subtropical gyre mostly exhibits a nonzero dark offset, which is removed in quality control (Fig.…”

“…Profile-byprofile analysis-visualization, number of invalid points, and related origin are available on http://seasiderendezvous.eu. Profiles collected in areas such as the Black Sea and subtropical regions were further corrected for the contribution of fluorescence originating from non-algal matter following procedures described in Xing et al (2017). The correction was applied when Chl and FDOM concentrations were positively correlated below the depth at which Chl was supposed to be zero (for equations and quantitative metrics see .…”

“…The correction was applied when Chl and FDOM concentrations were positively correlated below the depth at which Chl was supposed to be zero (for equations and quantitative metrics see . 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.…”

“…However, a comparison between the method by Xing et al (2012) used here and based on the calculation of the mixed layer depth, and an alternative correction developed by Sackmann et al (2008) based on the use of particulate optical backscattering, showed similar performances for BGC-Argo Chl measurements (X. Xing, unpublished data). As discussed by Xing et al (2017), the correction of Chl profiles for non-algal matter disturbance by using alternative procedures with respect to the one applied here may also introduce errors, which vary regionally and are the highest in the Black Sea area (∼ 0.1 mg m −3 ), while the lowest are observed in the subpolar North Atlantic Ocean and Mediterranean Sea (∼ 0.007 and 0.004 mg m −3 , respectively). A main challenge in quality-assessing fluorescence Chl measurements relies on the assumption of what is measured and what is actually phytoplankton biomass .…”

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

“…An additional dark count, a deep‐offset correction (Schmechtig et al 2014), generated from the mean deep fluorescence between 300 m and 400 m was subtracted from the profiles. As our seagliders did not have FDOM sensors attached during deployment, we are unable to determine the contribution of deep sea red fluorescence on our in situ dark counts (Xing et al ). Cleaning, despiking, and smoothing steps were performed on the fluorescence data.…”