Local adaptation of Daphnia pulicaria to toxic cyanobacteria

We quantified within-species variation in the tolerance of the large, lake-dwelling daphnid, Daphnia pulicaria, to toxic cyanobacteria in the diet. Juvenile growth rates on diets consisting of 100% Ankistrodesmus falcatus (a nutritious green alga) or 100% Microcystis aeruginosa (toxic) were compared for D. pulicaria clones isolated from lakes expected to have low and high levels of bloom-forming cyanobacteria during summer. Growth rates of clones isolated from high-nutrient lakes (range of total phosphorus, 31–235 mg L21) were higher, and showed less relative inhibition, on the cyanobacterial diet compared to clones isolated from low-nutrient lakes (range of total phosphorus, 9–13 mg L21). Our results suggest that D. pulicaria populations exposed to high cyanobacterial levels over long periods of time can adapt to being more tolerant of toxic cyanobacteria in the diet. A well-established tenet in limnology holds that the taxonomic composition of summer phytoplankton assemblages shifts with phosphorus enrichment toward greater dominance by cyanobacteria (Smith 1986; Trimbee and Prepas 1987; Watson et al. 1997). It is also widely accepted that harmful species of cyanobacteria (temperate species within the genera Anabaena, Aphanizomenon, Microcystis, and Oscillatoria) are more likely to attain bloom densities in freshwaters that are nutrient rich (Paerl 1988). One important consequence of this shift toward cyanobacteria with eutrophication is that summer phytoplankton assemblages in eutrophic lakes are relatively resistant to zooplankton grazing (Porter 1977; Lampert 1987; DeMott 1989, 1999; Sarnelle 1993). The grazing resistance of filamentous and colonial cyanobacteria, although not absolute, has been suggested as a reason for escape from grazer control of summer phytoplankton biomass in enriched lakes (Carpenter 1989; Faafeng et al. 1990; Brett and Goldman 1997; Carpenter et al. 2001; Ghadouani et al. 2003). In addition, many laboratory assays have demonstrated that various species of bloom-forming cyanobacteria can inhibit the feeding, growth, and reproduction of zooplankton, and in particular members of the genus Daphnia, as well as being of relatively low food quality for herbivores in general (Lampert 1982, 1987). Limited recent observations, however, suggest that zooplankton populations may adapt to tolerate bloom-forming cyanobacteria in their diets. The seminal observation was perhaps made by Gilbert (1990), who found that a toxic