[Chlamydomonas] Clumping

[Lutz Becks]

Hi,
we are culturing Chlamydomonas with rotifers in chemostats. After 20 days
the Chlamy starts to clump causing some problems for sampling and others.
First i thought it was some kind of defense against the grazing, but
Chlamy starts clumonig in chemostats without the predator too. Has anybody
a idea why Chlamy starts to clump and how i can avoid it?
Thank you for your help

Lutz