An intercomparison of in vitro chlorophyll a determinations for MERIS level 2 data validation


An investigation of the capability of the MERIS (Medium Resolution Imaging Spectrometer) validation teams to determine chlorophyll a, using the latest measuring protocols and advanced high performance liquid chromatography (HPLC), spectrophotometric and fluorometric method has been performed. Two inter-comparisons, the NIVACal 1 and 2, were made in 2002 as part of the activities of the validation of the MERIS sensor onboard ENVISAT. Eleven validation teams, represented by 20 laboratories, participated in these inter-comparisons. The algal pigment, chlorophyll a, was determined by either HPLC, spectrophotometric or fluorometric methods for samples of algal cultures, for natural samples collected off the coast of Norway, and for prepared pigment extracts. For both the HPLC and spectrophotometric results systematic errors were dominating. A few laboratories showed random errors. The coefficients of variation (CV) were determined after elimination of "outliers". The occurrence of "outliers" were frequent for both intercomparisons. The CV for the determined values of chlorophyll a for algal cultures during NIVACal 1, varied from 10 to 25% for the HPLC and from 5 to 25% for the spectrophotometric determination. The CV for the determined values of chlorophyll a in Case II water for natural samples in NIVACal 2 varied from 10 to 16% for HPLC and from 5 to 20% for the spectrophotometric results. The spectrophotometric results were more consistent between the laboratories. The range for CV was larger for the HPLC-based on chlorophyll a index for Case I water and ranged from 7% to 40%. An intercomparison based on prepared chlorophyll a extracts showed a CV of 8% to 15% for the HPLC and 5% for the spectrophotometric method. Since the CV for analysing the extracts was lower than for extraction and analysis of regular samples, the extraction procedures contribute significantly to the total variation. The recommendation for MERIS validation from this inter-comparison clearly show the importance of proper extraction procedures of the pigments and the use of common and agreed protocols.
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