Colored dissolved organic matter (CDOM) is a mixture of organic compounds that give brown water lakes their characteristic color. Common constituents of CDOM include organic acids such as humic and fulvic acids. CDOM may affect algal growth in several ways. A major negative effect is the reduction of algal growth by shading from absorbance of light by CDOM. In addition, CDOM may change the availability of inorganic nutrients directly, since it contains nitrogen and phosphorus. CDOM may also affect nutrient availability indirectly by stimulating bacterial growth, which in turn may change the availability of nitrogen, phosphorus, and carbon. This dissertation explores the complex effects of CDOM on algae in several ways. In Chapters 1 and 2, I describe results from short-term experiments conducted with phytoplankton from West Long Lake. Chapter 1 contains experiments designed to discern between the positive and negative effects of CDOM on algae. Results from Chapter 1 suggest that positive effects of CDOM may outweigh negative effects for nutrient limited phytoplankton if CDOM contains nutrients which are available to phytoplankton. Chapter 2 contains experiments and a model designed to test whether interactions between algae and bacteria affect algal response to CDOM. Experimental results showed that the effect of CDOM on algal growth depended on the amount of phosphorus sequestered by bacteria, suggesting strong competition between algae and bacteria. Both of these chapters highlight the importance of the source and composition of CDOM in determining whether the net effect of CDOM on algal growth is positive or negative. Chapter 3 investigates the effects of the interaction between changes in nutrient loading and changes in CDOM on the dynamics of phytoplankton communities in Long Lake. During the 1990’s, Long Lake was part of a whole-lake experiment in which changes in CDOM were accompanied by changes in nutrient loading. The interaction between CDOM and nutrients suggests that predicting community responses to multiple drivers cannot be achieved by simply adding up the effects of single drivers.