Original paper
Confounded impacts of planktivorous fish on freshwater biomanipulations
McQueen, D. J.; France, R.; Kraft, C.
Archiv für Hydrobiologie Volume 125 Number 1 (1992), p. 1 - 24
46 references
published: Jul 9, 1992
DOI: 10.1127/archiv-hydrobiol/125/1992/1
ArtNo. ESP141012501000, Price: 29.00 €
Abstract
A factorial study conducted in large enclosures (8 m dia x 14 m deep) was used to assess the relative impacts of fish (1+ yellow perch) and nutrients (nitrogen and phosphorus) on; zooplankton biomass and size structure, chlorophyll-a, and water clarity. We found that fish biomasses ranging from 132-142 kg ha -1, had little effect on zooplankton biomass or size structure, and no statistical relationship with chlorophyll-a concentrations; but that there was a significant negative correlation between fish biomass and Secchi depth. Chlorophyll-a size fraction data showed that in the fish-free enclosures, the phytoplankton community was dominated by large algal cells and in the fish-treatment enclosures small cells were more abundant. Because this pattern could not be attributed to the direct effects of zooplankton grazing, we investigated alternative explanations and identified several confounding factors including: (1) direct additions of phosphorus by living fish, (2) small additions of phosphorus by dead fish, (3) nutrient transport by the fish from the sidewalls to the limnetic region of the enclosures and nutrient absorption by periphyton on the sidewalls, (4) amelioration by fish-phosphorus additions, of the potential effects that pulsed nutrient additions had on algal cell size structure, and (5) selection by strong nutrient competition, of small algal cell sizes in the fish-treatment enclosures. We note that these confounding factors are common to most biomanipulation experiments and that they make unequivocal interpretation of results extremely difficult.