Why do phytoplankton cultures crash if they aren’t harvested/restarted after a while?

[leonardomanzano]

does the photo that die due to lack of nutrients cause a domino affect that starts and ends with an ammonia spike?

If this isn’t it, why can’t the new phytoplankton feed off the released nutrients from the dead phyto and form a sort of closed cycle instead of crashing?

 

[MBruun]

My last culture ran for + 8 months without any sign of crash. Don't think they will crash if the container are cleaned, culture areaeted and feed according to harvesting