ESTIMATING CHLOROPHYLL-A ABUNDANCE FROM THE PHYTOPLANKTON COLOR RECORDED BY THE CONTINUOUS PLANKTON RECORDER SURVEY - VALIDATION WITH SIMULTANEOUS FLUOROMETRY

The green colour (measured with reference to standard colour charts) o f sections of the Continuous Plankton Recorder (CPR) filtering silk wa s compared with estimates of chlorophyll a concentration derived from a fluorometer mounted on the CPR during seven tows in the North Sea be tween 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 concen tration. However, when this comparison was restricted to four tows (68 10-nautical-mile samples) where the recorded phytoplankton cell abund ance 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 c olour and the chlorophyll a concentration. By measuring the relative c olour intensity of CPR standard colour categories and quantifying the individual variation in the assessment of colour, a theoretical model was developed which predicted that if the silks were coloured in direc t proportion to the chlorophyll a concentration in the water, then the expected r2 for the relationship between silk colour and chlorophyll a concentration would be 0.62. The green colour recorded by the CPR su rvey was therefore identified as a quantitative index of chlorophyll a concentration, but only when numbers of phytoplankton cells on the CP R silks are not high.