Estimating chlorophyll a abundance from the ‘phytoplankton colour’ recorded by the Continuous Plankton Recorder survey: validation with simultaneous fluorometry

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Abstract
The green colour (measured with reference to standard colour charts) of sections of the Continuous Plankton Recorder (CPR) filtering silk was compared with estimates of chlorophyll a concentration derived from a fluorometer mounted on the CPR during seven tows in the North Sea between February and May 1991. After the green colour was assessed, the abundance of phytoplankton cells on the filtering silks was quantified by microscope analysis. Data were collected for 115 10-nautical-mile samples over a total of seven cruises. For these 115 samples, there was only a weak ( F1.113 = 3.8, P = 0.05, r2 = 0.03) positive relationship between the colour of the filtering silk and the chlorophyll a concentration. However, when this comparison was restricted to four tows (68 10-nautical-mile samples) where the recorded phytoplankton cell abundance on the silks was very low, there was a highly significant ( F1.66 /,69.1, P < 0.001, r2 = 0.51) positive relationship between the silk colour and the chlorophyll a concentration. By measuring the relative colour intensity of CPR standard colour categories and quantifying the individual variation in the assessment of colour, a theoretical model was developed which pedicted that if the silks were coloured in direct proportion to the chlorophyll a concentration in the water, then the expert r2 for the relationship between silk colour and chlorophyll a concentration would be 0.62. The green colour recorded by the CPR survey was therefore identified as a quantitative index of chlorophyll a concentration, but only when numbers of phytoplankton cells on the CPR silks are not high.

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