The importance of crustacean zooplankton in structuring rotifer and phytoplankton communities; an enclosure study

During the spring clear-water phase of 1993, an enclosure experiment was performed in the mesotrophic Schohsee (Plon, FRG) in order to assess the impact of crustacean zooplankton on the rotifer and phytoplankton community. Among the crustacean plankton, calanoid and cyclopoid copepods were abundant, but Daphnia 'longispina' reached the highest densities in this experiment. The colonial rotifer Conochilus unicomis was not affected by crustacean plankton. The two most abundant species, Synchaeta peclinata and Keratella cochlearis, increased exponentially when macrozooplankton had been excluded from the enclosures, but did not increase when crustaceans were present. Birth and death rates of K.cochlearis could be reliably determined in this field experiment, suggesting that this rotifer species was mainly controlled by exploitative competition rather than by mechanical interference or predation. Daphnia 'longispina' generally grazed selectively on the smaller ciliates and algae, thus depriving the rotifers of their phytoplankton resources. The dominant alga, the chrysophycean Dinobryon, increased, whether crustaceans were present or not, but appeared to be grazed upon to a certain extent despite its considerable cell size and colonial organization.