In biomanipulation, the abundance of planktivorous fish is artificially reduced in order to increase the abundance and grazing activity of herbivorous zooplankton. In a small lake at Graefenhain, Germany, the stocking of piscivorous fish lead to an almost complete elimination of planktivorous fish and to subsequent changes in the zooplankton community. Small cladocerans were replaced by two large-bodied daphnids (DAPHNIA ROSEA and D. PULEX). Simultaneously, the predatory invertebrate CHAOBORUS (CH. FLAVICANS and CH. OBSCURIPES) increased to densities as high as 10 larvae per liter. I investigated the role of CHAOBORUS in the absence of planktivorous fish and its impact as part of a trophic cascade.

Comparing the manipulated lake with a non-manipulated reference lake, I investigated the changes in species compositon of the cladocerean community. In a field experiment within the manipulated lake, I used a newly developed type of enclosure to examine the impact of CHAOBORUS predation on DAPHNIA and on phytoplankton development. Adults of both DAPHNIA species realized a "size-escape" from the gape-limited invertebrate predator by growing to sizes too large to be ingested by CHAOBORUS. The juvenile instars of DAPHNIA spp. were morphologically protected by neckteeth which were only produced in the presence of CHAOBORUS. Nevertheless, CHAOBORUS could be shown to account for severe mortality in DAPHNIA spp. D. ROSEA were found to be more vulnerable, and in turn were strongly favored in enclosures without CHAOBORUS. On the final sampling date, the mean daphnid abundance (+/- standard deviation) in three replicate enclosures with ambient densities of CHAOBORUS was 1.8 +/- 0.6 individuals (ind) per liter, whereas 67.9 +/- 3.9 ind/liter were present in three bags without the invertebrate predator. Phytoplankton and chlorophyll-A concentrations were markedly reduced in enclosures from which the invertebrate predator was excluded. Secchi transparency increased to 4.85 +/- 0.03 meters (m) in the bags without CHAOBORUS, compared to 1.80 +/- 0.37 m in the presence of the predator. Thus, the experiments showed that severe reductions in the abundance of planktivorous fish can allow increasing abundance of an invertebrate predator to numbers which strongly suppress even large, morphologically-defended DAPHNIA and thereby limit grazing impacts on phytoplankton density and water clarity.

Further investigations focused on the mechanisms allowing for competitive coexistence of DAPHNIA PULEX and D. ROSEA. Individuals of D. PULEX showed an increased ability to produce hemoglobin in summer, which enabled them to tolerate an oxygen-depleted medium. Vertical distribution in the water column led to an almost complete spatial separation of the two species. D.PULEX amounted to less than 10% of all daphnids in the surface layer and to more than 96% of all individuals in the metalimnetic low oxygen layer. The D. PULEX were able to maintain high birth rates while Hb appeared to allow them to exploit food resources in the metalimnion.