Because algae does not care about nutrient levels. That is not what triggers algal blooms. A combination of light and ammonia production rate are the prime causal factors. Under very high lighting it only takes a small amount of an ammonia transient to trigger the blooms. Under lower lighting the tank can withstand larger ammonia production transients. There may be other contributory factors such as poor O2 or low Redox potential levels but these are not well understood and they are usually accompanied by, or are indicative of the prime factors such as high organic waste or low nitrification bacterial populations. Algae respond to unhealthy environmental conditions. You'll find natural systems with algal blooms where the nutrient levels are low and/or ammonia levels are high, such as in stagnant pools.
Nutrient deficiency combined with high light is a deadly combination for higher order Submersed Aquatic Macrophytes. The algal species that plague our tanks are those that evolutionary forces have optimized to respond to a decaying/unstable/transient environment.
Everywhere in this society people have been programmed to automatically associate eutrophic environments with algal blooms. It's in all the textbooks. It's standard material at universities. So it seems intuitive to run around blindly blaming nutrients for the problems in one's tank. The truth of the matter is just the opposite. S.A.M's require thousands of time more nutrient levels than algae. In fact, they require millions of times more nutrients than algae. When they don't receive these required levels they decay and algae attacks feeding on what little remaining nutrients that spew out from ruptured plant cells.
If we would only take the time and energy to understand the biochemistry of plant growth it would become obvious that the textbooks and the universities that proclaim the evils of nutrient levels are wrong. Algae are opportunists. They attack when the environmental system is broken. Most peoples tanks have a broken environmental system - because they don't understand what's happening in that tank, so they get algae so they run to the nearest respected source of information and are immediately told to get rid of PO4 or to reduce NO3.
Let me clarify another point. The water changes associated with EI have nothing to do with lowering the nutrient levels. They have everything to do with removing toxic buildup of organic waste. The more plants grow the more waste they produce. The more you feed plants the more organic waste there is. In exactly the same way, the more you feed your fish the more organic waste they produce. Organic waste interferes with nutrient uptake. Organic waste subsequently breaks down and causes increased ammonia production. The massive water changes associated with EI are all about keeping the tank clean by removing organic waste. In the EI scheme the same day that you remove the water you immediately dose more nutrients, so how can this possibly have to do with removing nutrients? If you wanted to reduce nutrient levels all you would have to do is to stop dosing, one week on on week off based on the uptake rate and buildup rate. The various EI dosing schema would look totally different if nutrient buildup were an issue. Barr uses this nutrient buildup concept and the expression "Tank reset" in order to assure those who are paranoid that there is little danger of the toxic buildup that they worry about. He had to say this because he had to allay fears of toxic buildup. This was a convenient explanation at the time but the real issue is simply that plants don't grow very well in dirty water caused by their own waste and that this dirty water contributes to algal blooms.
Algae an plants do not inhabit the same ecological niche. That is another reason why eutrophic waters do not automatically grow algae just because they are eutrophic. Algal spores are not using nutrient levels as a trigger mechanism, they are using decay as their trigger because they "know" that in a decaying environment they will get all the nutrients they need by scavenging those very same plants that are decaying. This is like the difference in ecological niche between and elephant and a vulture. With insufficient food the elephant dies and then the vulture attacks. The vulture will not attack the elephant while it's alive, so these are entirely different environmental imperatives.
Now check the results of a hyper-eutrophic environment. This tank was dosed with over 3X the EI values - over 60ppm NO3 , and over 10ppm PO4 weekly. These levels far exceed any conceivable uptake rates, leaving plenty for algae, yet there were no algal blooms - unless I failed to dose for a while or unless flow/distribution/CO2 was impaired. The tank was kept scrupulously clean. I've done this a few times with the same results. Look at the evidence. Instead of algal blooms this is a case of S.A.M's gone wild. The only deterrent to this maniacal dosing strategy is the required pruning and maintenance. So anyone complaining about high nutrient levels
causing algae is mistaken. Now, high nutrient levels
will absolutely exacerbate any algae that is there - make no mistake about that, but there is a huge difference between
exacerbation and
causality. Find the
cause of your algae and fix it. If that cause is flow/distribution/CO2 then fix that. If the cause is poor maintenance then fix that as well. If the cause is too much light, reduce the light. If the cause is poor nutrient levels then absolutely fix that, but one never needs to worry about nutrients
causing algae, regardless of concentration levels.
Cheers,