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

0 , Denmark ABSTRACT: The influences of light and nutnents on ratios of pigments/chlorophyll a (chl a) were investigated for several species of different phytoplankton groups from estuaries and coastal areas lor the purpose of calculating the biornass of individual phytoplankton groups as chl a based on pig-menWch1 a ratios. Pigment ratios were constructed for varying Light intensities and qualities and for nutrient-starved algae cultured in the laboratory. The pigrnent ratios were tested on 4 data Sets obtained in estuanes and coastal areas using the CHEMTAX program, for calculating phytoplankton group abundance as chl a. Field samples were collected over a vanety of time and spatial scales as well as being subjects to variations in light and nutnent conditions. The pigmentkhl a ratios derived frorn the different treatments in culture experiments generally had a minor effect on the CHEMTAX biomass calculations, although the biomass of cyanobactena 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 ali 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 pigrnent ratios in the CHEMTAX prograrn, corresponded well to rnicroscopic determinations of the biomass of phytoplankton. One of the data sets, where small algal cells dominated, was counted under the microscope by 2 different laboratones. 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.