Fish effects on reservoir trophic relationships

To study the relationships between the biomass of reservoir communities and to examine the differences between reservoirs with different fish assemblages, we used limnological and fish data for 31 Argentinean reservoirs. Despite conspicuous systemic differences with lakes, reservoirs dominated by visual zooplanktivorous fishes have had phytoplankton biomass drastically higher than reservoirs with more balanced fish assemblages where piscivorous fish biomass were more noticeable. Contrary td earlier results for a more inclusive set of lakes and reservoirs (Quiros, 1998a), our results show macrozooplankton biomass was dramatically higher for reservoirs with planktivores not controlled by piscivores, when nutrient or chlorophyll concentrations were held constant. Macrozooplankton was not strongly suppressed by planktivores, but these fish may be suspected controlling macrozooplankton size. All the biological standing stocks were higher for reservoirs with planktivores but without piscivores when nutrients were held constant, with the exception of total fish. For the latter variable, there were not significantly differences between both types of reservoirs. Our results indicate that changes in fish assemblage composition will have dramatic effects on trophic relationships in reservoirs. Heavy biased introductions and stocking of facultative planktivores may conduct to unbalanced fish assemblages with undesired effects on water quality.