MEPS 329:13-21 (2007)  -  doi:10.3354/meps329013

ABSTRACT: CHEMTAX is a computer program that allows researchers to allocate chl a, a proxy for phytoplankton biomass, into different algal groups defined by a suite of pigment markers. The program uses an iterative process to find the optimal pigment:chl a ratios and generates the fraction of the total chl a pool belonging to each pigment-determined group. A major difficulty is the choice of initial or seed ratios, which largely determine the results obtained. To test the sensitivity of CHEMTAX to initial ratios and its effectiveness to quantitatively discriminate the existing phytoplankton groups, 8 different initial matrices of pigment ratios were used on an artificial pigment database of 40 samples built from known pigment ratios. Successive runs of CHEMTAX using the output from each run as the input for the next one led to the correct adjustment of most ratios, greatly improving initial values and, therefore, biomass estimates. The convergence of different pigment ratios with successive runs was always directed towards the true value. This procedure also correctly rejected groups that were not used to build the pigment database but whose pigment ratios were included in the pigment-ratio matrices. Therefore, matrices including most pigment-ratios can be used without the need to check for the presence of all those phytoplankton groups in the natural sample. Lack of convergence also occurred with some of the 8 initial matrices for some phytoplankton groups. Because some indeterminations could not be removed for all initial ratios, the use of several initial seed values is proposed.

KEY WORDS: Phytoplankton · Pigment ratios · HPLC · CHEMTAX