SHORT-TERM EXPERIMENTS ON CALANOID-CYCLOPOID-PHYTOPLANKTON INTERACTIONS

To investigate their potential effects on each other in nature, calanoid (Diaptomus clavipes and D. siciloides) and cyclopoid (Acanthocyclops vernalis and Mesocyclops edax) copepod populations were manipulated in 5 liter aquaria in laboratory experiments of 20-60 days duration. Diaptomus generally had a strongly negative effect on both cyclopoid species. The cyclopoids established populations more successfully when introduced to aquaria before calanoids than they did when calanoids were already present. On the other hand, whether introduced earlier or later than the cyclopoids, Diaptomus populations were unaffected by Acanthocyclops and were strongly depressed by Mesocyclops. Diaptomus effects on the phytoplankton were often strong but varied markedly among experiments. They included reduction of populations of edible algae, such as Chlamydomonas, which are essential for both calanoid and cyclopoid nauplii, and large increases in inedible algae, such as Kirchneriella. Feeding experiments revealed that under conditions of food scarcity Acanthocyclops nauplii survived less well than did Diaptomus nauplii. Competition for edible phytoplankton seemed to be a key factor in the calanoid-cyclopoid interactions, since the survival of herbivorous cyclopoid larvae determined the abundance of the predaceous adults. This indicates that the competitive effects of calanoids on cyclopoids often may exceed the predative effects of cyclopoids on calanoids.