Nutrients and Algae
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 critical factors in algal growth in water.
The turnover of inorganic phosphate under natural conditions appears to be caused by aquatic bacteria. Both bacteria and algae may compete for the dissolved inorganic phosphorus.
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 and by the slow release of phosphate from the littoral deposits during warm weather. This is the time when nutrients are most likely to be in short supply in the river.
Observations are that in many waters, one or both of these ions are in inadequate supply for maximum growth of algae.
Studies of the relationship between the production of algal blooms and the nitrogen/phosphorus (N/P) ratios 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 inorganic phosphorus has been found as the maximum concentration permittable without the danger of supporting undesirable algal growths.
If the assays of inorganic nitrogen and phosphorus from wastes exceed .3 and .01 to .015 mg/1, respectively, at the start of the growing season then nuisance blooms of algae may occur later in the year.
Controlling Factors of Algae Growth are briefly discussed.
Grazing Aquatic Animals