Russ, L Strain and S Strain are different species of Rotifers. L-Strain is smaller than S-Strain. Both will eat Nannochloropsis and do fine, but to use them as a nutritional bridge for larvae, it will lack DHA (or was it EPA?).
Also, I think that mixing different cultures of the same phytoplankton can cause mutations. I'll have to look that up though.