Grazer control of bacterial abundance in a freshwater pond community
Creators: name: Macinnis, Maura Jan
Publisher: name: University of British Columbia
Year: 2009

description: Metazoan grazing of bacteria represents a potential pathway for the transfer of bacterial production to higher trophic levels. Freshwater cladocerans of the genus Daphnia are able to reduce bacterial abundance in lakes, but many experiments have been restricted to the summer months. It is therefore necessary to test the generality of Daphnia'?, role across seasons and across a broad range of food web configurations. The impact of mechanical grazing inhibition of Daphnid bacterivory, a potential outcome of algal blooms, has also not been addressed. I tested the ability of each of three crustacean zooplankton species, Daphnia pulex Leydig, Bosmina longirostris (O.F.M.), and Skistodiaptomus oregonensis (Lillj.), and a mixed rotifer community, to control bacterial abundance in 80 litre enclosures suspended in a freshwater pond. Experiments were conducted in August 1995 and some treatments were repeated in a second experiment in October 1995 to test for seasonal differences in grazer impact. Bacterial cell abundances at the end of the summer experiment were found to be significantly lower in Daphnia enclosures (1.87 x 10⁶ cells ml⁻1) than in B. longirostris (3.91 x 10⁶ cells ml"') and S. oregonensis (4.⁶9 x 10⁶ cells ml⁻1) enclosures, using repeated measures ANOVA. Bacterial abundances were also low in the absence of macrozooplankton in both summer (2.47 x 10⁶ cells ml⁻1) and fall (1.19 x 10⁶ cells ml⁻1). In contrast to the results observed in summer, Daphnia enclosures sustained high bacterial abundances in the fall. Daphnid grazing of bacteria appears to have been influenced by seasonal shifts in algae composition. The presence of a bloom of Elakatothrix sp. coincided with significantly higher bacterial cells abundances in Daphnia enclosures (2.76 x 10⁶ cells ml⁻1 and 3.65 x 10⁶ cells ml⁻1), while in both seasons, grazing by Daphnia reduced rotifer and ciliate abundances. Daphnid grazing of bacteria appears to be more susceptible to changes in grazing behaviour than other components of the food web. Thus, the presence of Daphnia can be expected to have a detectable effect on bacterial abundance, but the direction of impact may differ seasonally as algal composition changes. Smaller zooplankton are not able to reduce bacterial abundance, but the absence of macrozooplankton can also result in low bacterial abundances, due to the loss of indirect influences of macrozooplankton on the microbial food web. An experiment conducted to determine the impact of suspended particles on Daphnid grazing of bacteria resulted in an increase in bacterial abundance when grazing was inhibited by the presence of glass fibre filaments. The filaments, also resulted in a modest increase of bacterial abundance in the absence of macrozooplankton grazers. Mechanical interference with Daphnia grazing may mitigate Daphnia's potential for top-down control of the microbial food web. Suspended inorganic filaments were able to increase bacterial cell abundances in the absence of any substrate additions, indicating that the increased spatial heterogeneity and complexity afforded by suspended particles can cause a detectable enhancement of the microbial food web.