Premium
Manipulation of the Pelagic Food Web by Stocking with Predacious Fishes
Author(s) -
Benndorf Jürgen,
Kneschke Helmut,
Kossatz Kathrin,
Penz Eckhard
Publication year - 1984
Publication title -
internationale revue der gesamten hydrobiologie und hydrographie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.524
H-Index - 52
eISSN - 1522-2632
pISSN - 0020-9309
DOI - 10.1002/iroh.19840690308
Subject(s) - pelagic zone , zooplankton , food web , biomass (ecology) , predation , phytoplankton , stocking , grazing pressure , trophic level , population , environmental science , biology , trophic cascade , ecology , ecosystem , fishery , nutrient , grazing , demography , sociology
Two water bodies, which are quite different with respect to nutrient load and hydrophysical conditions, are used to perform long‐term experiments in the whole water on the manipulation of the pelagic food web. Experimental water 1: Bautzen Reservoir (Dresden County, GDR); hypereutrophic, mean depth=7.4 m; extremely exposed to wind. Experimental water 2: Small pond in a former quarry (Dresden County); mesotrophic; mean depth=7.0 m; extremely protected against wind. Only the results of Experiment 2 are given in detail. Experiment 1 is not yet finished. Experimental water 2 was investigated in 1979 and 1980 when no predatory fish species were present, and in 1981 after introduction of predators (mainly Salmo gairdneri ). The response of the ecosystem can be summarized as follows: (1) The biomass of the zooplankton‐eating fish (mainly Leucaspius delineatus ) decreases rapidly. (2) The biomass of the herbivorous zooplankton increases to nearly 400%. (3) This finding reveals that the dense population of an invertebrate predator ( Chaoborus flavicans ) is not able to compensate for the feeding pressure of the small fish. But the intensive feeding activity of the young larvae of Chaoborus leads to a strong increase (200 to 300%) in the mean individual body size of the crustaceans during late summer and autumn, which supports the “balanced predation hypothesis”. (4) The remarkable enhanced grazing pressure of the herbivorous zooplankton on the phytoplankton does not exert any effect on the total phytoplankton biomass. This result is interpreted as a consequence of growth limitation of the algae due to low nutrient (Fe, P) supply in that mesotrophic water body. But the phytoplankton composition does reveal a strong response to the enhanced grazing pressure. The Secchi depth increases as a consequence of this change in the phytoplankton composition. The conclusion is drawn that, when using biomanipulation as a means of water quality management, it is obviously necessary to take into account the complex interrelationships between fish stocks, predacious invertebrates, herbivorous zooplankton, phytoplankton as well as nutrient load and hydrophysical processes in the particular water.