TROPHIC CASCADES AND COMPENSATION - DIFFERENTIAL RESPONSES OF MICROZOOPLANKTON IN WHOLE-LAKE EXPERIMENTS

Head webs in three lake basins were manipulated by altering fish commu nities to either reduce or increase the abundance of Daphnia. These ba sins were subsequently fertilized with nitrogen and phosphorus for two years. We tested three predictions about the response of heterotrophi c flagellates, ciliates, and rotifers (collectively, microzooplankton) derived from prior studies. We: predicted that (I) microzooplankton w ould increase with lake fertilization, (2) lakes with abundant Daphnia would have lower increases in microzooplankton, and (3) both increase s in resource availability and suppression by Daphnia would determine microzooplankton dynamics. Contrary to the first prediction, microzoop lankton did not increase with fertilization relative to the reference lake, except in the low-Daphnia system, The second prediction was supp orted, as Daphnia prevented microzooplankton from increasing in the fe rtilized lakes with the strength of the Daphnia effect being greater t han anticipated, Because of this strong effect, microzooplankton dynam ics were: in all but one case most strongly related to suppression by Daphnia rather than to a combined effect of resources and suppression, The microzooplankton communities were differentially affected by the trophic cascade. Heterotrophic flagellates appeared to be limited by a variety of predators. Even in the low-Daphnia fertilized lake, mortal ity was apparently high, Ciliates and rotifers increased in the low-Da phnia fertilized lake and were strongly suppressed otherwise, These ex periments indicate that small-scale, short-term experiments and larger -scale comparative analyses may be inadequate for assessing the streng th of trophic interactions, The potential for community-level response s, not well assessed except at the ecosystem scale. may alternatively dampen or enhance the impacts of trophic cascades in food webs.