previously washed in 0.01 N H2SO4, rinsed twice with deionized glass- 

 distilled water, and rinsed twice with filtered, sterile (autoclaved) 

 Patuxent River water enriched with f/2 medium. Each bottle was filled 

 with approximately 5 x lO'* cells (differences between bottles could 

 not be distinguished from counting error, p<0.1). 



Replicate pair of bottles was wrapped in black plastic window 

 screen to attenuate light. The first pair was left unwrapped; the 

 second pair was wrapped in one layer of screen; and the third, fourth,, 

 fifth, and sixth pairs were wrapped in 2, 3, 4, and 8 layers of 

 screen, respectively. 



After the bottles were filled with cells and placed within the 

 screen layers, 1.0 microcurie NaHC^'^03 (1-milliliter solution 

 adjusted with NaOH to pH 9.5 in distilled water) was injected into 

 each bottle with a Hamilton gas-tight syringe. For work with M. 

 lutheTi, . 10.0 microcurie NaHC^^03 was injected into each bottle. 

 The syringe was rinsed once with water from each bottle after initial 

 injection. The bottles were stoppered and incubated in the constant 

 temperature (20° Celsius) shaking (2 cycles per second) incubator for 

 4 hours under a light intensity of 1.45 x lO't ergs cm"^ sec"-^. 



After incubation, the replicate pairs of bottles were removed from 

 the incubator. Carbon assimilation of the phytoplankton was deter- 

 mined immediately by the acidification and bubbling technique of 

 Schindler, Schmidt, and Reid (1972), Two 20-railliliter samples were 

 withdrawn from each incubation bottle and placed into two short Allihn 

 tubes (30-milliliter capacity) each containing 0.5 ml 0.1 N HCL to 

 ensure complete stripping of dissolved inorganic carbon. Air was 

 bubbled through the tubes for 20 minutes to remove the dissolved 

 inorganic carbon. After bubbling, two 2-milliliter samples were with- 

 drawn by pipet from each tube and placed into liquid scintillation 

 vials. Fifteen milliliters of Bray's (1960) dioxane-base scintillation 

 fluid were added to each vial. 



All samples were counted at 9° Celsius using a Packard Tricard 

 3320 liquid scintillation spectrometer„ After background subtraction, 

 sample counts were corrected for quenching by an external standard 

 quench curve constructed from counts of standard toluene carbon-14 

 with increasing amounts (0 to 400 microliters) of chloroform in 15 

 milliliters of Bray's solution. Counting efficiency of most of the 

 samples was approximately 85 percent. Sample counts were averaged for 

 each set of screens. Results are reported as gross counts per minute 

 resulting from the carbon-14 assimilated per milliliter of sample per 

 hour of incubation (cpm ml~^ hr~^). 



d. Determination of Suspended Solids Effects . Two sets of 

 assimilation experiments were conducted with each of the four phyto- 

 plankton species exposed to graded concentrations of silicon dioxide. 

 Both the assimilation effects and light saturation curves were deter- 

 mined for each species on the same day. Monoahrysis lutheri was 



