Diel periodicity of nitrogen and carbon assimilation in five species of marine phytoplankton: accuracy of methodology for predicting N-assimilation rates and N/C composition ratios*

Diel patterns of carbon and nitrogen assimilation were investigated in 5 taxonomically diverse species of unicellular algae grown in NH:-limited cyclostats (12 .12 L: D). Calculated 24 h average rates of N-assimilation based on C-14 incorporation into protein overestimated the actual rate by an average of 24 ? 27 %. Overeshmation of carbon assimilation by the C-14 method (16 i19 "h) accounted for most of the observed discrepancy in the estimation of N-assimilation rates. Estimates of actual N-assimilation rates were more accurate at high relative growth rates (104 f 20 %) than at low relative growth rates (143 * 22 o4,). The N/C assimilation ratio provided an accurate estimate (104 * 16 %) of the N/C composibon ratio after a 12 : 12 L: D cycle. The percentage decrease in particulate C-14 acbvity at night due to respiration was 105 ".I greater than the predicted decrease estimated from the loss in particulate carbon concentration. Dark respiration of carbon with an average specific activity equal to that of the inorganic carbon could not entirely account for thls phenomenon. Excretion of dissolved organic carbon (DOC) dunng the photopenod and uptake of low specific activity DOC at mght may have been responsible for the decrease m specific activity of particulate C-14 at night.