The green color results from a plant pigment called chlorophyll. The chlorophyll content is often used to quantify the level of algae in a water sample. Because algae are green plants, they derive energy from the sun through photosynthesis giving off oxygen in the process. At night or when light is limited these plants respire, consuming oxygen from the water. Since warm water holds less oxygen than cold, fish kills due to oxygen depletion by algal blooms commonly occur early in the morning on hot summer days when water the temperature is warm and respiration has occurred all night.
Algal blooms can also cause toxicity, taste, and odor problems.
Many microorganisms engage in a sort of chemical warfare by excreting substances that are toxic to their competitors. Blue-green algae synthesize both nerve and liver toxins that can be fatal to humans, although confirmed human poisonings are rare. The appearance of these toxins is unpredictable. Every year dozens of cattle and hogs die from drinking water containing toxins produced by common blue-green algae while millions of others drinking water containing the same algae suffer no ill effects. What circumstances actually cause these algae to produce such potent toxins is not known.