As far as I remember it the bloom/die off thing is entirely natural and expected. The size is regulated by water temperature and the number of nutrients in the water, not by the presence or lack of predators. It can have affects on broader populations though. We had big problems with this in my home town back in the 80’s and 90’s. An increase in sod farming lead to fertilizer heavy run off leaching into shallow estuaries, where you’d get frequent algae blooms. Blooms in shallow enclosed bodies can lead to the water getting heavily oxygenated, and subsequently large die offs in other organisms in the system. So in that case the blooms were killing off the fish and shellfish rather than die offs leading to the blooms.
