Food web ecology of zooplankton communities in lakes




Matthews, Blake

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I used natural abundances of stable isotopes (δ13C and δ15N) to examine the food web structure of lake zooplankton communities. I focused on modeling isotopic variation with respect to trophic variation (δ15N) and to variation in dietary carbon sources (δ13C). The isotopic patterns suggest that zooplankton food webs have reticulate connections between food chains, and a large diversity of interactions between consumers and their resources. Variation in the δ13C of zooplankton depended on taxonomic identity, body composition, and habitat specialization. In Sooke Lake Reservoir, seasonal variation in the δ13C of zooplankton was mainly related to variation in lipid content and the δ13C of lipids. This has significant consequences for interpreting the pathways of terrestrial carbon through plankton food webs. In Council Lake, variation in the δ13C of zooplankton among taxa was related to habitat specialization, and indicates taxon- specific exploitation of allochthonous resources. Using a cage experiment, I confirmed that δ13C can indicate habitat specialization of zooplankton. Among lakes, my data suggest that zooplankton communities can readily exploit carbon produced below the epilimnion. Large inter- and intra-lake variation in the δ15N of zooplankton suggests significant trophic variation within zooplankton communities. In a year-long study, annual averages of taxa specific δ15N matched our expectations about the feeding ecology of zooplankton. However, short-term variation in the δ15N of herbivorous zooplankton (like Daphnia) was decoupled from seasonal variation in the δ15N of invertebrate predators. This suggests there are multiple food chains within the plankton community (i.e. grazing chain, microbial chain), and that the strength of each food chain may vary among lakes or seasonally within a lake. This seasonal variation in the food web structure of zooplankton has significant consequences for how we model and consider the trophic position of individual fish.



zooplankton, stable isotopes