The study reported in this thesis was conducted on Claytor Lake (Virginia) during the late spring, summer, and fall of 1981 and had as its objectives: 1) to assess the role of algae and bacteria in altering the pool of organic compounds, especially those that form trihalomethanes upon chlorination and 2) to evaluate the effectiveness of alum coagulation, followed by clarification, for removing THM precursors from lake water. The weekly grab samples provided evidence of a general relationship between algal population densities and the THMFP of the lake water. Successional shifts in the dominant genera and species of algae had little discernible effect on the THMFP of the water. Diurnal analysis clearly showed that the biota of a water impoundment can have a profound effect on the water's potential to form THM's. Alteration of the THMFP may be brought about by liberation of various types of algal ECP (known THM precursors), bacterial ECP, or modification of algal ECP by bacterial agents. Alum coagulation was effective for the removal of DOC and THM precursors throughout the course of the study, despite shifts in bacterial and algal density and algal community structure. Average removals of DOC and THM-precursors were approximately 24-48 percent and 34-51 percent, respectively. / Master of Science