Abstract
Late copepodids and adults of Acartia clausi Griesbrecht showed no mortality when kept in a very dense culture (109 cells l-1) of a toxic strain of the haptophyte Prymnesium patelliferum Green, Hibberd & Pfenaar, for 48 h at 26-28 PSU. However, the haptophyte had strong negative effects on copepod egestion and reproduction rates, both when the haptophyte was offered as the only food, and when offered in mixtures with Rhodomonas baltica Karsten sensu Zimmermann, an alga known to be ingested and support reproduction in Acartia. The rates were depressed already at concentrations of 0.1-0.3 · 106 cells 1-1 of P. patelliferum, and decreased further with increasing concentration ofthe haptophyte. At typical bloom concentrations (⪰: 4 · 106 cells 1-1) of P. patellijerum, the rates were no greater than those of starved copepods. These results demonstrates that zooplankton feeding and reproduction in coastal waters may be constrained by presence of toxic haptophytes, also when these algae occur in moderate concentrations and mortality is not registered.
