ESTIMATING THE CONTRIBUTION OF MICROALGAL TAXA TO CHLOROPHYLL-A IN THE FIELD - VARIATIONS OF PIGMENT RATIOS UNDER NUTRIENT-LIMITED AND LIGHT-LIMITED GROWTH

Cellular concentrations of chlorophylls and carotenoids were measured in nutrient- and Light-limited cultures of marine microalgae to determ ine the utility of accessory pigments as proxies for the biomass of sp ecific groups of microalgae in the ocean. rn most species, concentrati ons of chlorophyll a (chl a) and photosynthetically active pigments va ried linearly with growth rate in nitrate-limited continuous cultures or with the logarithm of the irradiance in light-limited and light-suf ficient batch cultures, as has been observed before. Rates of pigment- concentration change as a function of irradiance or growth rate did no t covary with rates of maximum growth. Concentrations of carotenoids c ovaried with chl a in most species analyzed; intraspecies variations o f chl a-carotenoid ratios were usually smaller than variations of chl a:b or chl a:c ratios. These results were used to critically evaluate the assumptions underlying iterative methods used to determine the con tribution of different algal taxa to chl a from ratios of chl a and ac cessory pigments. Estimates based on chl a:b or chl a:c ratios are pro ne to error because these ratios can vary by up to an order of magnitu de among species and within species as a function of irradiance, thus violating an assumption of the iterative methods. Instead, such method s should rely on ratios of chl a and photosynthetically active caroten oids. Using simple models and field data from the Chesapeake Bay, USA, we showed that iterative methods are either prone to error when diffe rent populations of microalgae co-vary or do not give discrete solutio ns. As an alternative we suggest methods that rely more strongly on em pirically determined pigment ratios.