Pigment ratios and phytoplankton assessment in northern Wisconsin lakes

Nine lakes in northern Wisconsin were sampled from February through Septemb er 1996, and HPLC analysis of water column pigments was carried out on epil imnetic seston, pigment distributions were evaluated throughout the water c olumn during summer in Crystal Lake and Little Rock Lake. The purpose of ou r study was to investigate the use of phytopigments as markers of the main taxonomic groups of algae, As a first approach, multiple regression of mark er pigments against chlorophyll a (chl a) was used to derive the best linea r combination of the main xanthophylls (peridinin, fucoxanthin, alloxanthin , lutein, and zeaxanthin), A significant regression equation (r(2) = 0.98) was obtained for epilimnion data. The good fit indicates that the chi a:xan thophyll ratios were fairly constant in the epilimnion of the nine lakes ov er time. Chlorophyll a recalculated from the main xanthophylls in each samp le showed good agreement with measured chi a in epilimnetic waters, A secon d approach used the CHEMTAX program to analyze the same data set. CHEMTAX p rovided estimates of chi a biomass for all algal classes and allowed distin ction between diatoms and chrysophytes, and between chlorophytes and euglen ophytes. These results showed a reasonably good agreement with biomass esti mates from microscope counts, despite uncertainties associated with differe nces in sampling procedure. Changes of pigment ratios over time in the epil imnetic waters were also investigated, as well as differences between surfa ce and deep samples of Little Rock Lake and Crystal Lake. We found evidence that changes in the ratio of photoprotective pigments to chi a occurred as a response to changes in light climate. Changes were also observed for cer tain light-harvesting pigments. The comparison between multiple regression and CHEMTAX analyses for inferring chl a biomass from concentrations of mar ker pigments highlighted the need to take account of variations in pigment ratio, as well as the need to acquire additional data on the pig ment compo sition of planktonic algae.