motility greatly reduced the need for shaking the incubation bottles 

 during experiments. These studies measured the radioactive uptake 

 by zooplankton that were not yet producing radioactive fecal material. 

 The method is outlined in Rigler (1971) and was introduced by Nauwerck 

 (1959) from a slightly modified technique originally used by Marshal 

 and Orr (1955). This method eliminates three problems usually asso- 

 ciated with feeding rate determinations: 



(1) It is suitable for measurement of feeding rates of organisms 

 that do not produce coherent fecal pellets. 



(2) It eliminates the need for estimates of the leaching of 

 tracer carbon out of fecal pellets. 



(3) It reduces the error associated with excretion of tracer 

 carbon (some ingested food cells remain undigested at the 

 end of the short-feeding period) . 



Two ground glass-stoppered bottles (130-milliliter capacity) con- 

 taining high cell concentrations of actively growing M, _ lutheri were 

 each injected with approximately 10 microcurie NaHC^^Os-" ^*^®^®,. 

 cells were incubated under cool white fluorescent illumination at 20° 

 Celsius for 4 hours. After incubation the ceils were centrifuged at 

 6,000 revolutions per minute for 10 minutes. Supernatant water was 

 decanted and the cells were resuspended in enriched, sterile, filtered 

 Patuxent River water (Section I). The cells were then recentrifuged, 

 the supernatant decanted, and the cells suspended a second time. This 

 procedure removed most of the unassimilated dissolved inorganic 

 carbon- 14 and excreted fixed carbon compounds which had accumulated 

 in the bottles during the incubation period. After the second resus- 

 pension, cell concentration was counted at lOOX on an improved 

 Neubauer hemacytometer. The radioactivity of a 10-milliliter aliquot 

 of this suspension was determined by the acidification and bubbling 

 procedure of Schindler, Schmidt, and Reid (1972) outlined in Section I. 



During the phytoplankton incubation period, replicate pairs of 

 ground glass-stoppered bottles (130-milliliter capacity) to contain 

 the zooplankton during the feeding experiment were prepared. Each 

 pair of bottles represented a specific time series interval of feeding 

 activity for zooplankton exposed to radioactive phytoplankton cells 

 (food). These intervals were 5, 10, 15, 20, 30, 45, and 60 minutes 

 or longer depending upon the number of adults of either species avail- 

 able for experimental use. Each bottle was rinsed twice with 0.01 

 normal HoSO^ to remove any carbon-14 from previous experiments, rinsed 

 twice with deionized glass-distilled water, rinsed twice with filtered 

 (IVhatman GFC glass-fiber filters) Patuxent River water, and filled 

 with 50 milliliters of the filtered river water (salinity range 5.5 to 

 7.5 parts per thousand). 



Adult zooplankters (both sexes) of the species being investigated 



19 



