ATP and chlorophyll a as estimators of phytoplankton carbon biomass1

Thalassiosira fluviatilis, Amphiprora paludosa, and Dunaliella tertiolecta were grown in continuous culture at 20°C on a 12:12 L:D cycle under nutrient and light limitation. Diurnal periodicity was clearly evident in both ATP:C and Chl a :C ratios, but the magnitude of the diurnal changes was relatively small. The greatest range of ATP:C and Chl a :C ratios was found at low growth rates, the highest ratios being characteristic of light limitation and the lowest associated with nutrient limitation. Under strictly nitrogen limitation the range of ATP:C ratios was about 2.5–5.5 mg·g −1 ; Chl a :C ratios were more variable. Under phosphate limitation both ATP:C and Chl a:C ratios varied by about a factor of six. Under light limitation ATP:C ratios were insensitive to growth rate, as were Chl a :C ratios at low growth rates. Interspecific differences in both ATP:C and Chl a:C ratios were great enough to suggest that even a cursory examination of phytoplankton species composition could improve estimates of phytoplankton carbon based on ATP or Chl a measurements.