What impact would an increase in the algae population in a lake have on the lake ecosystem

The increased algae population (algal bloom) can kill some ecosystems.

Decrease oxygen level: Increase of bacteria due to more dead plants
Kills micro-organisms (Starves fish that eat it, starves fish that eat that fish, and screws the food chain)
A sign of pollution (increase of nitrogen/phosphorus) : Further killing plants/animals