Colacium vesiculosumEhrenberg (Euglenophyceae), infestation of planktonic copepods in the Western Baltic

Colacium vesiculosum, a common epizoic freshwater euglenoid occasionally was found in the Western Baltic infesting planktonic copepods. Its occurrence coincided with low salinity, high temperature and large biomass of cope pods in the area. Infestation was most pronounced on Acartia sp. living in the upper part of the water column, but decreased in Centropages hamatus, Temora longicornis and Pseudocalanus elongatus (in that order), probably reflecting the vertical migrational behaviour of these copepods. Survival and egg production in the copepods were not affected by Colacium infestation, and as its biomass only amounted to 1.5% of phytoplankton biomass, it is concluded that C. vesiculosum only plays a minor role in the area.