Stock cultures of each species were maintained in 50 milliliters 

 of the f/2 medium (salinity range 5.5 to 7.5 parts per thousand) con- 

 tained in 125-milliliter Erlenraeyer flasks either plugged with sterile 

 cotton wads or tightly covered with aluminum foil. Cultures (1 

 milliliter of stock) were transferred biweekly into fresh sterile 

 (autoclaved) medium. No attempt was made to maintain axenic 

 (bacteria-free) cultures. The cultures were checked periodically to 

 (a) detect possible contamination by other species of phytoplankton, 

 and (b) monitor growth rate of the stock (cells were counted at lOOX 

 on an improved Neubauer hemacytometer) . Motile forms were fixed 

 before counting with 6o5 percent glutaraldehyde (pH = 7.4) buffered 

 with sodium cacodylate (0.14 molar). 



Cultures were grown at 20 ± 0.5° Celsius under continuous cool 

 white fluorescent illumination (0.8 x 10** ergs cm"^ sec"-^) in a constant 

 temperature incubator. Under these conditions, an exponential growth 

 rate (Sorokin, 1969) of 0.35 to 0.40 day"-^ could be maintained for 

 these species at cell concentrations between 5 x 10^ and 5 x 10^ 

 cells ml"-"-. 



Before starting an experiment, approximately 10 milliliters 

 (depending on cell concentration) of stock culture in exponential 

 growth phase were transferred into 1.5 liters of fresh, sterile (auto- 

 claved) f/2 medium (enriched, filtered Patuxent River water) in 

 sterile Erlenmeyer flasks (2-liter capacity). The increase in cell 

 numbers under culture light intensity was monitored until a cell con- 

 centration of approximately 50,000 cells ml"-^ was attained. The exper- 

 imental run was then conducted with minimum dilution of these cells by 

 fresh medium. This cell concentration (50,000 cells ml~^) may be 

 representative of natural concentrations in certain areas of the 

 Chesapeake Bay system (Flemer, 1972, personal communication) „ Carbon- 

 12 concentrations during experimental work and in stock cultures were 

 monitored by the method of Karlgren (1962) reported in Sherk (1969). 

 Dissolved inorganic carbon concentrations were maintained at 15 to 20 

 milligrams q i~1 in cultures and experimental media. 



b. Sediments . A graded series of concentrations of silicon 

 dioxide (Fisher NOo S-153; see App.) was used for all assimilation 

 experiments . 



c. Determination of the Saturation Curves . Saturation curves 

 were determined for phytoplanktonic photosynthesis under the illumi- 

 nation of six 40-watt cool white 'fluorescent lamps. The light was 

 passed through Pyrex infrared reflecting glass (Kaufmann Glass Company, 

 Wilmington, Delaware) before reaching the Plexiglass top of a constant 

 temperature shaking-type incubator. The light intensity recorded 

 inside the incubator at the incubation bottle location was 1.45 x 10** 

 ergs cm~2 sec"-^. All measurements of light intensity were made with 



a Y.S.I. Model 65A radiometer. 



Two sets of six ground glass-stoppered bottles (130 ± 1.5 milli- 

 liters capacity) were used for each determination. These bottles were 



