I'm told that use of aquatic herbicides has a tendency to cause a spike in nutrients due to decaying vegetation but don't have a clear understanding of that.
Aquatic vegetation in sufficient quantities can tie up excess nutrients and prevent an algea bloom, once the vegetation dies off, the decay process releases and recycles those nutrients back into the water which then feed an algea bloom. A heavy enough bloom can also reduce water clarity to the point where it hinders new vegetation growth.