Abstract
Chlorophyll 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 matchups 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.
Highlights
The point-by-point comparisons for paired High Pressure Liquid Chromatography (HPLC) chlorophyll a (Chl) and in situ factory-calibrated Chl fluorescence observations yield large scatter in the data distribution (Fig. 1A)
We describe the sources of uncertainty, provide evidence for the bias, quantify the bias, and make recommendations to the community of users so as to both improve the accuracy and reduce the uncertainty in global estimates of chlorophyll a (Chl) concentration and phytoplankton biomass
There is a large source of natural variability which may be viewed as an impediment to quantifying Chl concentration and phytoplankton biomass
Summary
The point-by-point comparisons for paired HPLC Chl and in situ factory-calibrated Chl fluorescence observations yield large scatter in the data distribution (Fig. 1A). This suggests that the in situ fluorometric estimate of Chl obtained by WET Labs ECO fluorometers using the factory-provided calibration overestimates Chl throughout most of the ocean by a factor of 2, the polar regions diverge from the global median by at least a factor of 4 in the subarctic Atlantic and 6 in the Southern Ocean.
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