photosynthetic algae - photosynthesis takes place during daylight, CO2 is used and O2 is released. At night, aerobic respiration occurs, O2 is used and CO2 is given off.
If these two processes are in balance with each other (or are they?), how would an algal bloom decrease the O2 level in water? There would be more algae using up oxygen at night but also more would be producing oxygen during daylight?