Phytoplankton blooms, depending on the species composition, have three basic requirements 1. proper alkalinity, 2. proper dissolved nitrogen and 3. proper dissolved phosphorus. This assumes temperature is acceptable - usu above 60F. ALTHOUGH, there are some algae species adapted to bloom in cool water and some can even bloom under ice cover if there is enough light penetration or intensity for their "needs". Of secondary importance are macro and micro nutrients which are "USUALLY" adequate in most systems. Various algae species have unique adaptations or requirements for the above parameters. Thus some will bloom while others will wane or decrease because conditions are not just right. Competition (similar to fish communities) among algal species and predators such as zooplankton also can affect how well a bloom progresses.
Thus the nutrient runoff from a field may not have the correct "BALANCE" or ratios of nutrients to stimulate the desired bloom. If one can measure the basic nutrient concentrations to discover the defficiency, then the lacking nutrient can be added and a bloom usually develops. Although it is not ALWAYS quite this simple. Most people usually just GUESS when an intended bloom did not develop.
