The use of phytoplankton pigments for identifying and quantifying phytoplankton groups in coastal areas: testing the influence of light and nutrients on pigment/chlorophyll a ratios

The influences of light and nutrients on ratios of pigments/chlorophyll a (chl a) were investigated for several species of different phytoplankton groups from estuaries and coastal areas for the purpose of calculating the biomass of individual phytoplankton groups as chi a based on pigment/chl a ratios, Pigment ratios were constructed for varying light intensities and qualities and for nutrient-starved algae cultured in the laboratory. The pigment ratios were tested on 4 data sets obtained in estuaries and coastal areas using the CHEMTAX program, for calculating phytoplankton group abundance as chi a. Field samples were collected over a variety of time and spatial scales as well as being subjects to variations in Light and nutrient conditions. The pigment/chl a ratios derived from the different treatments in culture experiments generally had a minor effect on the CHEMTAX biomass calculations, although the biomass of cyanobacteria and prymnesiophytes was significantly influenced by the ratios chosen. In addition, interspecies variations in pigment/chl a ratios within the individual phytoplankton groups were even more pronounced than variations caused by the different growth conditions, indicating that the ratios chosen for CHEMTAX calculations should, if at all possible, reflect the dominant phytoplankton species present in a given area. For 2 of the data sets, where larger algal cells dominated, the composition and the biomass of the individual phytoplankton groups, using our pigment ratios in the CHEMTAX program, corresponded well to microscopic determinations of the biomass of phytoplankton. One of the data sets, where small algal cells dominated, was counted under the microscope by 2 different laboratories. Their biomass estimates were not consistent, and both disagreed with the CHEMTAX results. This probably reflects the subjectivity of microscopic analysis, which is greatest when small phytoplankton cells dominate. In a fourth data set, owing to the high sensitivity and reproducibility of the pigment analysis, differences were detected between phytoplankton groups over a transect of 5 km, which might have been unresolved using standard microscopic techniques.