368 ZEEMAN AND GRUNEWALD 



Carbon Uptake 



Photosynthetic rates of phytoplankton were estimated by the 

 radiocarbon method described by Steeman-Nielsen (1952). Individ- 

 ual samples were mixed well, and subsamples were siphoned into sets 

 of two light and one dark bottle (125-ml glass-stoppered bottles). 

 For each site, three sets were used in the size-fractionated procedure, 

 one set from each of three different samples. In addition, one or two 

 other sets were filled to measure the total unfractionated carbon 

 uptake. 



Each bottle was inoculated with 5 [jlCi of Na2H''*C03 and 

 incubated for 4 hr in a constant temperature (ambient) and light 

 incubator. All incubations were started between 1000 and 1100 hr 

 local time to minimize any effects of diurnal rhythms (Doty and 

 Oguri, 1957). 



Light was provided by 40-W cool-white fluorescent tubes. 

 Intensities were checked before each experiment and adjusted to give 

 not less than 150 microeinsteins m~^ sec"' (~900 ft-c). 



Samples were filtered immediately after the incubation period, at 

 a vacuum of /^ atm. 



The radiocarbon and chlorophyll samples were fractionated 

 sequentially. Bothwell (1975) showed that this was superior to 

 estimating from differential titrations. The apparatus, based on a 

 design by Schubel and Schiemer (1969), used filters of 10- and 

 64-jum pore size Nitex netting (Tetko, Inc.) and 0.45-jum HA 

 Millipore filters. 



After filtration, all filters were fumed with concentrated HCl and 

 placed in vials containing 15 ml of Aquasol (New England Nuclear). 

 Samples were later counted in a liquid scintillation counter. 



Total inorganic carbon in the water samples was determined 

 potentiometrically as described by Golterman and Clymo (1969). 



Chlorophyll 



Chlorophyll a and phaeopigments were measured fluorometrically 

 (Strickland and Parsons, 1972). After filtration, the filters were 

 placed in screw-cap vials containing 10 ml of 90% acetone, and the 

 vials were wrapped in aluminum foil and kept refrigerated for 24 hr. 

 No grinding was attempted since it was impractical with the Nitex 

 filters. Thus there was a possibility of incomplete extraction 

 (Yentsch and Menzel, 1963), but Bothwell (1975) indicated that this 

 was not a serious problem. 



