Field and laboratory studies were conducted to determine the ecological and physiological responses of phytoplankton to environmentally significant concentrations of atrazine, diuron, and 2,4-D. Field results would indicate that all three herbicides have a significant effect on species diversity and species dominance. Atrazine and diuron were particularly effective in decreasing diversity in concentrations of 10 ppb to 1.0 ppm. While total biomass as measured by ATP decreased significantly for both atrazine and diuron, total chlorophyll levels increased significantly in 0.1 mg/l atrazine. Decreases in chlorophyll were noted for all other concentrations tested in atrazine and diuron. Blue-green algae Gloeocapsa sp. and Synecchoccus sp. were the predominant genera in the 0.1 mg/1 atrazine. 2,4-D tests also resulted in significant increases in chlorophyll over control replicates. Additionally, biomass as measured by ATP indicated significant increases over control values in 10 mg/1 2,4-D suggesting that 2,4-D may have stimulated the growth of algae or that select algae within the test were able to take a competitive advantage in the presence of 2,4-D. This result was not consistent however. Laboratory tests using continuous cultures of Chlorella sorokiniana indicated that both atrazine and diuron were effective toxicants in low concentrations. Diuron was an effective growth inhibitor in a range of concentration from 0.0014 to 0.01 mg/l. Atrazine was an effective growth inhibitor from 0.014 to 0.1 mg/l. 2,4-D stimulated the growth of continuously cultured Chlorella sorokiniana in a range of concentration from 8.6 to 20 mg/l as measured by cell counts, total organic carbon, in vivo chlorophyll fluorescence, and ATP. The data indicated that the effect was not due to the utilization of 2,4-D as a carbon source but to an actual enhancement of the photosynthetic process. The uptake of radioactively labeled carbon (¹⁴c added as H¹⁴CO₃) was significantly increased in the presence of 2,4-D. The literature contains several reports of enzyme activity stimulation by 2,4-D (108). The data supported the suggestion that 2,4-D may influence the Hill reaction and respiration by stimulating an enzyme (or enzymes) involved in these processes. The net result may have been an increase in the rates of reactions in both the Calvin cycle and photorespiration. / Ph. D.