Freshwater blooms are the result of local inputs of nutrients exceeding natural processing capacity (i.e., there is not enough algae to process the nutrients). These can happen naturally with blooms usually occuring in the spring and early summer. During these times, cladocerans, rotifers, and other algivores are low in abundance due to the winter die off which allows the algae to explode.

Under normal circumstances, there is a limit to how much algae can bloom. And we are talking about the unicellular type. Ponds and lakes have a continuum called stable states equilibrium where at any time you can have either a high macrophyte (rooted and real plants) densities or high algal densities but not both. This is usually dependent on the time of year and also on the nutrient loads. Higher nutrient loads tend to favor algae blooms which cause explosions in algae populations, shading the water, and constricting if not eliminating growth of macrophytes. Under lower nutrient conditions, the macrophytes are better suited for those conditions and algae die off while macrophytes prosper. When algae dominates the water tends to turn a characteristic green color in favor of diatoms and green algae. This doesn't last long however, because as the algae grow, the algivores grow too, so after a little while you see an explosion of the algivore community which in turn fuels the secondary consumers, teritiary, quaternary and up to the big predators.

That's the normal bloom cycle. When artificial inputs of nutrients are considered, there's a slight issue. Algae explodes, but the algae growth is not constrained necessarily by the algivores... mainly because the prey algae are not the only algae blooming in these circumstances. Under normal conditions, what we see blooming are diatoms, green algaes and dinoflagellates with some cryptophycae. These compete with filamentous algaes and cyanobacteria (the so-called blue-green algae) and control their growth. The cyanobacteria especially tends to be the most harmful because they are toxic. Interestingly under high artificial inputs, it's very possible for cyanobacteria to bloom up and take over the community... which is bad because nothing really eats the cyanobacteria and their respiratory activities can drop water quality very quickly.

There's a lot more to it than that, but that's a simplified version of bloom dynamics in freshwater ecosystems.