Cyanobacteria, including the genera Anabaena, Aphanizomenon, Microcystis, and Oscillatoria, are known to release or have the potential to release phycotoxins into the water. Indeed, there are documented cases of both animal and human intoxication. Data obtained from field observations and laboratory experiments demonstrate a correlation between the development of the cyanobacterial population and the level of phycotoxin present in the water, though it should be noted that not all cyanobacterial blooms are toxic. The development of cyanobacterial populations is described by a dynamical model which accounts for factors that include algal growth, degradation, and light-driven buoyancy under the assumption of an isothermal, calm, and nutrient-abundant lake. A semiempirical mathematical model for water-column toxicity is developed and is used in conjunction with an improved result for the density of the cyanobacteria population obtained from the dynamical model. Light transfer through the water column, light absorption, the implication of the differentiated attenuation of light by the water, and its diurnal effect on the cyanobacteria population is considered. The result of which is a plausible description of the seasonal development of cyanobacteria populations and of the toxicity within the water body.