Chlorophyll a Versus Adenosine Triphosphate as Algal Biomass Indicators in Lakes1

SUMMARY The relation between phyloplankton chlorophyll a and adenosine triphosphate (ATP) was investigated during a yearly cycle in Lake Tahoe, California–Nevada, and a variety of North American and New Zealand lakes of ranging trophic state. Since cellular concentrations of ATP have been shown as acceptable indicators of live biomass among natural microbial populations, the ratio of chlorophyll a to cellular ATP reveals the extent of pigment production per unit biomass under diverse environmental conditions. In general, mixed systems, i.e., epilimnion of most lakes sampled, showed good consistency of cellular chlorophyll a:ATP ratios. Under thermally stratified conditions at Lake Tahoe extensive modifications of the ratio occurred, leading to a 10‐fold increase in the ratio at the bottom of the euphotic zone by late summer. Thus, the applicability of chlorophyll a as a quantitative algal biomass indicator is greatly restricted in lakes having thermally stratified euphotic zones.