Phytoplankton carbon biomass and specific growth rates determined with the labeled chlorophyll a technique

Abstract

The labeled chlorophyll a technique, based on the incorporation of H14C0; into chlorophyll a (chl a) gives accurate determinations of both phytoplankton carbon biomass (Cp) and specific growth rate (p). Cp and p, derived from the labeled chl a method, exhibited a diel periodicity and the mean p over 24 h compared well with the dilution rate for an oceanic miaoflagellate in NHi- limited continuous culture. The specific activity of carbon in C (R&) and in chl a (R& a) were equal after only 2 h and remained the same through 24 h. This supports the main assumption of the technique that within a useful time scale, the specific activities are equal. Values of Cp and phytoplankton POC measured with a CHN analyser were the same for the microflagellate in continuous culture. Estimates of phytoplankton carbon derived from several indirect methods were either greater than (methods based on measurement of adenosine triphosphate and microscopic examination of cell counts and volumes) or less than (methods based on measurement of particulate organic carbon and chlorophyll a concentrations) values determined with the labeled chl a technique. Time course 14C incubations indicate that both Cp and p exhibit diel variability through 24 h in the fluorescence maximum layer and overlying water column in the Southern California Bight. Phytoplankton populations in the fluores- cence maximum layer grew with generation times of 2 to 3 d, while generation times in the water column above averaged less than 1 d.