According to Natural Environmental Systems (www.naturalenviro.com), algae growth can upset the normal balance of a body of water. “Normally a small amount of algae is ok because pond plants like algae do release oxygen back into the water. It is when algae outbreaks get severe, that they too affect the pond’s oxygen level. During daylight hours, algae consume carbon dioxide from the water and release oxygen, but when night falls, this process is reversed and the algae begin utilizing the ponds oxygen to survive. So when there is a large algae population, oxygen levels can be significantly reduced in the over night hours leading to increased algae growth during the day. Another byproduct of the cycle of poor organic material breakdown is the increased presence of odors. Material that is slow to breakdown can become anaerobic and emit pungent and noxious gases like ammonia and hydrogen sulfide. Again, under normal conditions where a pond has a healthy oxygen level, the matter is digested aerobically and no odor is produced, but when there is not enough oxygen to do the job, anaerobic microbes begin digesting the waste, creating the odors.”