(14)C-labeling of the compounds excreted by phytoplankton for employment as a realistic tracer in secondary productivity measurements.

Preparations of the dissolved organic compounds released by photosynthesizing marine phytoplankton have been obtained with(14)carbon activities as high as 1.5 × 10(5) dpm/ml. The radioisotope content of the preparations resides wholly in dissolved organic compounds of low molecular weight (MW<3500), which are uncontaminated by residual(14)C-labeled inorganic carbon. The labeled compunds arise through photosynthetic fixation and do not appear to be products of cell lysis during the incubation or to originate from cell damage during the filtration step employed.