It has also been observed that the amount of algal growth in streams increases at points where rivers flow through areas that are rich in nitrate and phosphate.
Phosphorus and nitrogen are repeatedly reported as critical factors in algal growth. However, the turnover of inorganic phosphate under natural conditions appears to be caused by aquatic bacteria, but, both bacteria and algae may compete for the dissolved inorganic phosphorus.
It is known fact that many forms of algae thrive below sewage outfalls and also below the outlets of lakes, ponds and dams. The increase caused by sewage is usually attributed to increased supplies of nutrients. The increase below lakes may be caused by nitrogen fixation in the lakes by bacteria or blue-green algae or by the slow release of phosphate from the littoral deposits during warm weather, which is of course, the time when it is most likely to be in short supply in the river.
An example of nitrogen fixation occuring in running water although it may do so as Nostoc and Rivularia species, both of which occur in streams, are known to be able to fix nitrogen. A
All these observations lead to the conclusion that in many waters, one or both of these ions is in inadequate supply for maximum growth of algae.
Studies of the relationship between the production of algal blooms and the nitrogen/phosphorus (n/p) for some algae of 30/1 were found to be optimal. For other algae, ratios of 15 to 18/1 appeared to be most favorable.
A 0.01 mg/l of inorganic phosphorus has been found as the maximum concentration that can be permitted without the danger of supporting undesirable algal growths. If the asseys of inorganic nitrogen and phosphorus from wastes exceed .3 and .01 to .015 mg/1, respectively, at the start of the growing season, nuisance blooms of algae may occur later in the year.
Others have suggested that nutrient elements (other than phosphorus) combined with ferric complexes may be responsible for blooms that occur following the overturn in deeper lakes.