Chlorophyll content and fluorescence responses cannot be used to gauge reliably phytoplankton biomass, nutrient status or growth rate

$\bullet$ To consider the relationship between chlorophyll a (Chl a) content and phytoplankton growth and nutrient status, four phytoplankton species were grown in nitrogen (N)-limited [and, for one species, phosphorus (P)-limited] culture and measurements were made of CNP biomass, in vivo and in vitro Chl a content, the ratio of variable to maximum fluorescence ($F_{V}/F_M$) and the performance index for photosynthesis, $PI_{ABS}$ (a derivative of the O-J-I-P analysis of photosystem II functionality). $\bullet$ Interspecies differences plus the development of intraspecies differences during nutrient stress produced c. 10-fold variations in Chl : C. Estimates of C from in vivo Chl content were better than those from extracted Chl content, as the decline in Chl : C during nutrient stress was offset in part by increased Chl fluorescence. $\bullet$ $F_{V}/F_M$ was not a robust indicator of nutrient status or relative growth rate. Responses of $F_{V}/F_M$ in cells re-fed the limiting nutrient showed no consistent pattern with which to gauge nutrient status. $PI_{ABS}$ showed some promise as an indicator of nutrient status and relative growth rate. $\bullet$ Chl a content and fluorescence parameters do not deserve the unquestioned status they usually enjoy as indicators of biomass and physiological status.