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 H14CO3- into chlorophyll a (chl a), gives accurate determinations of both phytoplankton C biomass (Cp) and specific growth rate (.mu.). Cp and .mu., derived from the labeled chl a method, exhibited a diel periodicity and the mean .mu. over 24 h compared well with the dilution rate for an oceanic microflagellate in NH4+-limited continuous culture. The specific activity of carbon in Cp .**GRAPHIC**. and in chl a .**GRAPHIC**. 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 [particulate organic carbon] measured with a CHN analyzer were the same for the microflagellate in continuous culture. Estimates of phytoplankton C 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 .mu. exhibit diel variability through 24 h in the fluorescence maximum layer and overlying water column in the Southern California Bight. Phytoplankton populations in the fluorescence maximum layer grew with generation times of 2 to 3 days, while generation times in the water column above averaged < 1 day.