Size distri bution of primary production in a whole-lake biomanipulation experiment under hypertrophic conditions

Abstract

The size distribution of planktonic primary production (PP) was measured in order to estimate the top-down impact exerted by Daphnia galeata on the size distribution of PP under hypertrophic conditions in a whole-lake biomanipulation experiment (Bautzen reservoir, Germany). The most important characteristics of the experimentally changed food web are strong top-down control by high intensity of piscivory but weak nutrient limitation due to high external and internal P and N loadings. With the exception of a summer depression (June to August), high biomasses of D. galeata (up to 13 mg wet weight L -1 ) were observed. In summer and fall, large colonies of Microcystis spp. dominated the phytoplankton which was not a result of grazing by daphnids. The quota of non-edible size fractions in the total particulate PP was 76 ± 24 % (annual mean) and as much as 99 ± 0.4 % in September and October when the calculated filtration rate of D. galeata was very high. It was found to be impossible to control total PP by top-down pressure alone. But the edible fraction can be strongly controlled by grazing if a certain threshold of the filtration rate is exceeded. Before and during the summer depression of D. galeata, PP of edible phytoplankton reached relatively high values revealing that food limitation was not the reason for low biomass of the daphnids during this depression period. At low values of edible PP due to high grazing pressure exerted by Daphnia, exudation by mainly non-edible phytoplankton was much higher than edible PP indicating that a relatively important carbon flow via bacteria to Daphnia is possible.