Nitrogen is a chemical that occurs in amino acids and proteins of organisms. Nitrogen along with phosphorus are the elements in greatest demand by plants and microbes. It is very significant in determining primary production of aquatic systems, and microbial cycling of organic matter. It is added to lakes by way of precipitation, surface or groundwater drainage and from the fixing of nitrogen from the atmosphere. Nitrogen levels in lakes vary greatly depending on the amount of human activity on the land surrounding lake.