DETERMINATION OF PHYTOPLANKTON ABSORPTION-COEFFICIENT IN NATURAL SEAWATER SAMPLES - EVIDENCE OF A UNIQUE EQUATION TO CORRECT THE PATHLENGTHAMPLIFICATION ON GLASS-FIBER FILTERS

Estimations of the spectral absorption coefficients of phytoplankton i n the sea, based on the spectrum of absorption of particles retained o n a filter, require a correction for pathlength amplification in the g lass fibre filter (beta factor). The relationship between the optical density in suspension and 'on filter' is misleading due to the hystere sis effect. This effect is reduced on eliminating the absorption due t o non-pigmentary material, minimising the dependency of the beta facto r on wavelength. A general quadratic equation, ODsus(lambda) = 0.38 OD filter(lambda) + 0.42 ODfilter2(lambda) (r(2) = 0.97, n = 15600), was obtained on comparing 52 pairs of spectra of 9 marine phytoplankton sp ecies. The algorithm was validated against natural samples, obtaining a 1% average error. Comparisons between Cleveland & Weidemann's equati on (1993, Limnol Oceanogr 38:1321-1327) and the equation obtained in t his study provided a 2% average difference, suggesting that a unique e quation can be useful in determining the phytoplankton absorption coef ficient in seawater samples. We compared the absorption coefficients o btained by spectra reconstructed from the pigment content in seawater samples with those obtained with spectra on GF/F filters and beta-corr ected. Our results indicate that the spectra-reconstructed approach is only suitable when the sample consists of small quasi-spherical cells . For coastal-upwelling samples which are mainly composed of large cel ls the approach overestimates the package effect.